# The CMS Phase-1 Pixel Detector

## Abstract

The original CMS pixel detector has been replaced with an upgraded pixel system in the LHC winter shutdown 2016/2017. The design of the upgraded CMS pixel detector allows to cope with the higher instantaneaous luminosities that have been achieved by the LHC after the first long shutdown of the accelerator. The new upgraded detector has higher tracking efficiency and lower mass with four barrel layers and three forward/backward disks to provide a hit coverage up to absolute pseudorapidities of 2.5. This paper describes the design and construction of the CMS Phase-1 pixel detector as well as its performance during collision data-taking.

## General information

• Contact persons: Lea Caminada, Will Johns
• Author list: full tracker author list
• Target journal: JINST

## Reviewers

• Anadi Canepa (chair), Ulrich Husemann, Andrea Venturi, Katja Klein (ex-officio)

### First version of paper draft

• Comments by Katja, 22.3.2019: pdf
• Comments by Ulrich, finished on March 18:

• Generally the paper reads very well, there are no significant language issues.
• What is the group of readers this paper is targeted to? In my opinion this should be a physicist interested in HEP instrumentation in general, not necessarily from the LHC. Most sections are ok for this target group, but some are written for specialists only and need much more explanation to be accessible.
• There is some imbalance in the depth in which topics are treated. For example, the module assembly is dealt with on 1 page (despite several assembly and test procedures that have been developed, different bump bonding techniques, ...) On the other hand, each test procedure is described in gory details.
• At least in my group, a lot of details that go beyond this paper are documented in PhD (and master) theses. We should make these resources available by citing them wherever appropriate.

• Abstract:
• Start with more context: The original silicon pixel detector of the CMS experiment at the CERN Large Hadron Collider has been replaced...
• Use consistent naming for the upgraded detector. In the abstract there are two already, "upgraded CMS pixel detector" and "CMS Phase-1 pixel detector" (where Phase-1 has not been defined before)
• Chapter 1:
• L2: at the CERN Large Hadron Collider (LHC)
• L3: Suggest to reorder these sentences: The pixel detector is a key component of the CMS experiment and is indispensable for high-precision charged-particle tracking close to the interaction point and for vertex reconstruction. The pixel detector is located in a particularly harsh radiation environment characterized by a high track density.
• L13: the phrase "pixel Phase-1 upgrade" refers to the upgrade process, not to the upgraded system. Suggest to just call it "upgraded pixel detector" and "upgraded pixel detector system" if also the services are addressed"
• The new geometry has not been explained yet, better start like "The radial distance to the interaction point of the innermost sensitive layer has been moved closer to the IA point ...; therefore, faster FE electronics had to be developed...
• L27: In this paper, the design... is reviewed and its performance ... is presented. (a paper cannot do anything actively)
• L34: Reference to chapter 9 (nuclear interactions) missing
• Chapter 2:
• L41: to have four-hit coverage
• L43: suggest to also mention redundancy from fourth layer
• L45: give number of pixels per sensor module, e.g.: Each module consists of a sensor with 16\times4160 pixels with a nominal size of ...
• L49: upgraded pixel detector system
• L51: supporting eight detector modules
• L56: upgraded pixel detector system
• Fig. 1: upgraded pixel detector
• L64: suggest to introduce jargon word "services" here (why do you mentioned power cables and fibers separately, are they not part of the services?)
• L65: upgraded pixel detector has 1.9 times ... than the original pixel detector
• L67: The upgraded pixel detector system
• L69: readout and power systems (plural)
• L79: why do you only mention ionizing radiation, but not NIEL fluence (which is very relevant for the sensors)?
• L81: The upgraded pixel detector maintains... and overcomes limitations of the original pixel detector at higher luminosities.
• Chapter 3:
• L84: remove
• L87: array of 2x8
• L102: this section is very difficult to understand for non-experts on silicon sensors, see detailed comments below
• L105: suggest to delete: as described...
• L106: pixels along the chip boundaries have twice the area and those at the corners have four times the area compared to a standard pixel...
• L108: explain n-in-in, e.g.: follow the n-in-n approach, with strongly n-doped (n+) pixelated implants on an n-doped silicon bulk and a p-doped backside. In a reverse-bias configuration, the n+ implants collect electrons, which is advantageous as their mobility is higher compared to holes.
• L110: need mini-introduction to radiation damage to understand "trapping" (plus point to 3.1.4?), e.g.: The charge collection in a silicon sensor is impeded by radiation damage: charge carriers may be trapped for a certain time such that they do not contribute to the charge signal. Another advantage of collecting electrons is that they are less prone to charge trapping than holes.
• L116: "requires a double sided sensor process" is obscure for non-specialists, maybe: requires that photolithography processes must be applied to both sides of the sensors (double-sided process)
• L118: the concept of guard rings may also not be familiar to the reader, please add a small explanation
• L120: "an n-side isolation" is difficult for non-specialists, suggest to explain that electron accumulation below the Si oxide layer that would short-circuit the pixels and that therefore the pixels must be isolated from each other.
• L129: The small gaps ... also facilitate the implementation of punch through bias structures, the bias dots.
• L130: Suggest to break this massive pile of information down in smaller pieces, e.g.: The bias dots provide a highly resistive connection to each pixel. This can be used to apply bias voltage to the sensor prior to any further processing. This in turn allows sensor quality assurance measurements, such as the current-voltage (IV) characteristic.
• L133: explain FZ, e.g.: wafers from silicon monocrystals produced in the float-zone (FZ) process.
• Fig. 5: More information in caption, e.g.: Photographs of four pixel cells on a BPIX sensor (a) and on an FPIX sensor (b)>
• L141: reverse bias voltage.
• L142: the FPIX sensor subsection is much shorter than its BPIX counterpart. Suggest to extend to give the same level of detail. B.t.w.: why did you avoid the names of the sensor manufacturers?
• L154: wouldn't it be more natural to argue with the band model rather than the E field when talking about radiation damage?
• L156: is available within the 25-ns bunch crossing rate of the LHC
• L157: suggest to mention that the threshold of the readout is due to noise
• L160: This fluence corresponds to
• Fig. 6: the quality of the plot seems not adequate for a paper in 2019 (too small axis titles, no units in legend), suggest to "beautify" and then refer to it as "after [5]"
• Fig. 6 caption: chemical element not in italics
• L164: full depletion of what?
• L165: A higher bias voltage was not...
• L166: Motivate why 10000 electrons are sufficient, e.g. by moving the information from L170 earlier
• L172: During the second long shutdown of the LHC (LS2)
• L175: spatial resolution
• L180: "significantly higher flux" compared to what?
• L182: higher than a few tens
• L189: and in FPIX
• L191: suggest to add a reference to the column drain mechanism
• L213: "a counter running behind" sounds strange to me, maybe: a counter delay with respect to the bunch crossing counter...
• L215: this is the first time the readout token is mentioned. Suggest to introduce TBM and its role briefly in the beginning of Section 3.2
• L223: spell out acronym ADC the first time you use it
• L224: spell out acronym FIFO the first time you use it
• L225: spell out acronym PLL the first time you use it
• L227: cross talk between what?
• L227: time walk of what?
• L228: below 1800 e^-
• L233: shows excellent performance
• L238: data losses ... outer BPIX layer are less
• L241: faster compared to what?
• L258: could be mitigated (conditional) or were mitigated (past tense)?
• L272: a reset signal (to whom?)
• L273: "found and fixed" is jargon, maybe: identified and corrected
• L278: does the time-walk optimization also refer to the revised version of the PROC600?
• L280: cross talk between what?
• Fig. 8: unit on x axis should be cm^2 (superscript missing)
• L282: First state what the TBM actually does, before going into the details and comparing it to the old version
• L292: "same header and trailer timing" remains obscure for non-experts, please explain better
• L297: what is meant by "trigger stack" in this context?
• L299: $N$ in math mode
• L306: what is the TBM stack? Is it the same as the "trigger stack"?
• L313: has there been an outline of the LS2 consolidation work before in this paper?
• L330: I believe it would be of great interest to the read to expand a bit on bare module production: reasons for multiple BB vendors, new BB processes developed at DESY (solder-ball jetting) and KIT, bare module probe stations, differences between indium, AgSn, SnPb, different UBMs, cleaning procedures, quality control, ...)
• L354: the µ should be in upright, suggest to use siunitx package.
• L354: the cap was made of Kapton = polyimide, not polyamide
• L358: the section on FPIX module construction is strangely different from BPIX, with details a places where I would find them irrelevant. The two sections should be made similar in the level of (relevant) detail, examples:
• L365: if you give details on the encapsulant, please give the type. Alos, Sylgard is a trademark.
• L366: LabView is a trademark
• L369: the "primary issues" mentioned here seem a strange choice. Of course, excess glue is something each production center faced at some point. At least at KIT the reason for this was found and eliminated during preproduction.
• L383: the section suggests that module testing has only performed once after production. On the contrary, key to high-quality modules was to test as early as possible. Therefore part of the tests have been performed first on bare modules.
• L384: A suite of tests was performed
• L388: damage (singular, mass noun)
• L389: pXar is specific to the community. Is the name really relevant? If yes, is there a reference (e.g. anything from Urs Langenegger or Simon Spannagel’s PhD thesis)?
• Fig. 11: plot as it comes out of the box, to be made more pretty
• L408: If the number of signals sent to the pixel and the number of signals read out from the pixel is the same.
• L429: suggest to add erf parameterization to introduce threshold and noise parameters
• L447: the BB test was much more subtle than is conveyed here. The capacitance of the device depends critically on the bump height, which was much smaller for indium than for solder bumps. This was especially important during bare module probing.
• L460: that there is no hidden sensor damage (mass noun) that manifests itself
• Fig. 12: need unified plot style. Add legend to upper plots. Is there a Gaussian fit involved for the upper plots? If so, explain in the caption.
• L476: metallic films or foils or plates?
• L477: you mean the material (or more precisely: chemical element) of the foil, right?
• Fig. 14: x axis title should be: Hit rate [MHz/cm^2]
• Fig. 15: x axis title: pulse height
• Fig. 15: “Calibration … in electrons” sounds strange to me, maybe: Clibration of one ROC on a pixel detector module with characteristic X-rays. The pulse height in units of the internal calibration signal V_cal shows a linear dependence on the number of electrons expected to be collected from different target materials. From a straight-line fit to the data points, a calibration constant of ??? e^-/V_cal is extracted. (Either label the points in the plots or add to the caption which data point is which material)
• L486: good timing settings found?
• L492: if I remember correctly, the ratio of leakage currents at 100 V and 150 V was part of the criteria
• L494: add white space between 1088 and (96)
• L499: The number of produced BPIX modules over time is presented…
• L503: damage (mass noun)
• Fig. 16: redo plots in same style, label the dashed line in upper plot, add a similar line in lower plot
• Fig. 16 caption: this just shows the number of modules vs. time, not the yield (= percentage of detector-grade modules)
• Chapter 4:
• L529: spell out acronym CCU?
• L530: spell out acronym PIA?
• L547: explain meaning of "balanced" in this context
• L556: different implementations of what?
• L564: four humidity sensors
• L566: stack of three connector boards
• L571: two DOHs
• L573: electrical signals
• L573: Kapton (trademark!) → polyimide
• Fig. 18: will the updated drawing contain labels?
• L581: seven
• L582: seven
• L583: four
• Fig. 19: add labels to drawing
• L622: "The regulation, i.e. the delivery..." sounds strange. If you need the term "regulation" later on: The delivery of the output voltage, called (voltage) regulation, ...
• L628: passives → passive components
• Fig. 21: add labels to photographs?
• L714: Delete "At the time of writing..." (Run 2 already over)
• L716: eliminate the leakage current problem?
• L720: CO2 cooling as the technology
• L726: two-phase accumulator
• Fig. 22: drawing to small, suggest to move phase diagram below cooling system sketch.
• Fig. 22 caption: add short description of what is shown in the drawing and diagram
• L744: this may be a matter of taste, but I would prefer the requirements (w/o the paragraph on redundancy) before the cooling concept
• L759: To ensure continuous operation
• Fig. 23: labels too small
• Fig. 24: labels too small
• L772: backup
• L773: eight cooling loops
• L784: backup
• Fig. 25: these screen shots are not very instructive for non-experts. They are also too small for anything to be read off the plots.
• Fig. 25 caption: describe what is shown on the plots are these time series of temperature readings?
• Fig. 25 caption: Performance of (mass noun)
• Fig. 27: The measurements are
• Chapter 5:
• L826: half shell underwent
• L849: use math mode for variable z
• L857: this chapter has some redundancy with Section 4.3. Refer to that chapter rather than repeating.
• L864: are joined with
• L869: use math mode for variable z
• Fig. 29: label Segments A, B, C on photograph
• L873: use math mode for variable z
• L876: is VCR the Swagelok trademark or the acronym for something? In either case, indicate what it is.
• Fig. 30: labels way too small
• L919: "identical to Phase 0" is not very useful if you do not know that design. Add a reference?
• Footnote 4: Dow Corning
• L939: Cooling lines
• L939: This section only describes the cooling supply/return, but not how the cooling is distributed inside the disks (only in 5.2.6). Suggest to at least add a hint here.
• L953: Kapton → polyimide
• Fig. 33: indicate where the test heaters are located that you mention in the caption
• L974: why has the radiation hardness only been tested using gammas and not protons and/or neutrons?
• Fig. 34: all labels way too small
• Chapter 6:
• L1029: "could be" (conditional) or "were" (past tense)?
• L1045: In this paragraph you are mixing present tense and past tense inconsistently. Things that happened once should be in past tense10
• L1053: delete "neatly"
• L1064: can "thermal cycle" really be used as a verb?
• L1072: inside CMS → into a fixed position inside CMS?
• L1076: joined
• L1086: comma after 2017
• L1089: "operating the detector cold" is jargon, maybe: operating the detector at -20C (cold test).
• L1092: leading to short circuits
• L1095: detector was ready for installation
• L1109: High quality modules is not well defined here
• L1112: flex cables
• L1122: why "having ... installed"?
• L1135: flex cable
• L1146: up to the point where a test?
• L1147: I am surprised that the problem with abrupt cool-down was only found on the half cylinder and not during previous power cycles. Can you motivate this better (different cycle frequency?)
• L1168: delete "in any case"
• L1180: where joined
• L1181: out of the transport box directly into the final position?
• Chapter 7:
• L1258: I would start this section by pointing back to the module tests described in Chapter 3.
• L1259: "Pixel thresholds are..." starts too quickly. Maybe "The lowest possible value that can be chosen for the detection threshold of a given pixel is an important performance parameter..."
• L1278: The thresholds for L1
• L1281: threshold and noise values per ROC
• Fig. 44: use the same abbreviation of "layer" as in the rest of the text, e.g. L1 or Layer 1

• Fig. 44: x axis label should not only show units but the quantity that is plotted (left: threshold (electrons), right: noise (electrons))
• Chapter 8:
• L1311: "a few additional calibrations" sounds too colloquial to me
• L1313: The alignment of the pixel detector deserves more than just five lines! Suggest to add a section to Chapter 7 (and call that chapter "Calibration and Alignment") following the following conference proceedings article: https://www.sciencedirect.com/science/article/pii/S0168900218305953?via%3Dihub
• L1320: Suggest to put material on the number of working channels as a separate section
• Fig. 45: The distributions for L1
• L1327: fixed is too colloquial → repaired, solved, corrected ...
• L1357: comma after 46,
• L1357: plateaus
• L1357: Fig. 46 does also show the plateaus for L3 and L4
• L1384: link this section to the section on the DC-DC crisis
• L1395: remind reader that 2019/2020 is LS 2 and will lead to Run 3
• L1409: the definition of "in-time threshold" is not very exact. It's not the increase in threshold, but rather the sum the threshold and the time walk.
• L1414: fixed → corrected
• Fig. 47: align plots and consider choosing the same x and y axis ranges for both
• L1425: pixel charge → charge collected in a single pixel
• L1436: suggest to move explanation of charge trapping in the sensor from L1437 up here.
• L1440: "production depth" is not well defined here, do you mean sth. like the distance from the sensor surface (or implants) at which the charge has been produced in the substrate? Please define.
• Fig. 48 caption: explain that the curves have been fit with a Landau (+Gauss?) and that the most probable value (MPV) is given for each layer/disk
• Fig. 49 caption: mention that the profiles are for different years of data taking, different HV settings, and different annealing times
• Fig. 50: spell out luminosity in x axis title
• L1469: More meaningful title? Performance of what?
• L1484: that is → which in turn depends on
• L1504: Gaussian or a t-student function → Gaussian or a Student's t distribution
• L1507: radial position of the layer
• L1509: amount of radiation is too unspecific. Do you refer to total dose or fluence?
• L1519: do you really want to explain radiation damage with electric fields only? Usually one would go back to mechanisms that change the effective doping concentration.
• Table 2 caption: in volts
• Table 2: The table style is rather ugly, but I do not have a great alternative idea to fit all this information in a single table
• L1544: in Figs. 53 and 54
• L1549: "used by CMS pixel can deal..." sounds too colloquial, how about: used in the CMS pixel detector are designed to cope with these signals
• Fig. 5: all labels way too tiny
• L1551: this section should close the loop to the thermal mockup in 4.3.5. If there is no other public write-up of the results, suggest to refer to Julia Hunt's Master thesis (https://ekp-invenio.physik.uni-karlsruhe.de/record/49071)
• Chapter 9:
• L1572: precision
• Fig. 57 caption: Suggest to put "Hadrography" in quotes
• Fig. 57 caption: of the CMS tracking system
• Fig. 57 caption: first barrel layer
• Fig. 58: I found it confusing that the units in the plot are in cm, but beam spot coordinates you give the caption is in mm. Suggest to use cm throughout.
• L1627: proper minus sign for -1.76 mm
• L1638: "summarizes the final results" is promising too much, the table merely shows the reconstructed rail positions

• the paper is overall very pleasant to read and complete. It will be a valuable document for the future.
• However Sec 1 to 3 (and section 6.3) would benefit from some consolidation. The same piece of information is repeated in various places, leading to redundancy and possible confusion. Table with specs and layout details would be beneficial.
• Also the language is at times too colloquial and not appropriate for a paper. It would good if you could review carefully these sections and ensure that statements are always quantitative and/or supported by references.
• Figures tend to lack units and the format is quite difference from fig to fig. The labels can be fixed (even in the pdf file), it would be good if plots were remade with a common format. I know that the latter is challenging.
• Please review the tense used in the paper (sometime it is past, sometime it is present tense). I expect the CLE to make this remark as well, but it would be quicker if you fixed this while you implement the reviewers’ comments.

• L4 I find this statement a bit vague. I suggest you state the requirements or remove the statement

• L5 Likewise (it is of course a matter of taste) I find the words “key”,” indispensable” not appropriate for a paper. I would be factual and state how the pixel info is used.

• L8 (and other cases) I don’t think the interaction point is a reference in the CMS system, so you would not be able to define a distance with respect to that. I suggest you add “instantaneous” in front of luminosity

General comments for Sec 2. Having a high level introduction is very useful, however the risk (as in every paper) is that we use terms that not defined (for example ladder on L51). I would streamline this section. For example I would build the section starting from sensor, then modules, ladder, large structure.

• L44, 45 are the hybrid modules the same as the modules? the sentence may be mis-leading to non expert readers

• L46 the bonding is not electrical per se, I suggest you remove “electrically”

• L65 While I understand what you mean, I think the correlation between “fitting in the same envelope” and “need for larger bandwidth” is not clear. I suggest you reword it.

• L78-92 is a more precise description of the scope of the upgrade, with respect to L22-26. As I mentioned above, I think making sure that the statements in the introduction are more quantitive (for example the requirements in L78-92 could be moved to the introduction) and Sec 2 is consolidated (we don’t add qualitative statements when you have a proper rigorous statements in the following section) will help.

Given that the ROC will differ depending on the layers, it would help if you added a table with the requirements for BPIX (for each layer) and FPIX: dose, hit rate, instantaneous luminosity, etc.

• L84 spurious Fig?

• Fig 3 could you make the Fig with the same x axis? If there is a reason for having different x-axis, can you explain?

• L85-ff Likewise above, I think it is better to introduce definition and descriptions only once. Here for example you mentioned the pixel size that was already mentioned in the introduction, and you quote the size of the sensor which is then presented again on L104. I strongly encourage you to go through the paper and make sure there is no redundancy (otherwise the paper becomes hard to read) - it would help make the paper more compact.

Another example of redundancy that could be resolved is on L100 and L122. You state twice that the designs are different but you don’t provide the details till L124/143. Please try to consolidate.

• L106-107 the sentence does not flow nicely grammatically

• L117 given that you don’t provide cost and schedule for the project, I don’t think we have to add this piece of information

Title of 3.1.4 I find ‘height’ to be a bit jargon. I would simply state "signal". I find this section a bit too pedagogical for a peer reviewed publication. I suggest you remove the explanation of why the signal degrades (I would actually remove the description about the benefit of n-n silicon for the same reason) and give the technical requirements, specifications, description of the CMS specific sensors. Here for example what do you mean by ‘small signal’, can you provide the spec of the chip?

• Fig 6 is a bit hard to read. There are more lines that documented in the legend. Can you explain why (adding the explanation to the caption) or fix the plot?

• L167 This is the first time you write about the actual bias voltage. It would be useful to quote the planned range of bias voltages in the silicon sensor section to place this discussion into a context.

• L169 what do you mean by “probably”? I suggest you are quantitative. What is the impact of having on average 6k signals on hit finding?

• L171 is 3000e the threshold for Phase1? The number is different on L228. What ROC is used here?

• L172 This piece of information is important but should be introduced ahead, when you discuss the expected range of bias V (in the sensor section)

• L173 how did you estimate 10k from Fig 6?

• L181 Another example of redundancy, you have already stated what your requirement is (600MH/cm2). and the need for a new ROC. Please consider consolidating, you could for example remove L179-185.

• L185 citation for the ROCs? please add here where each chip is used.

• L227-239 by the time the reader reaches this section, they should know what the requirements are, to place the test into a context

• L239 It is not clear why you chose 600MHz/cm2 as the boundary? From Fig 7 one would infer that PROC600 would be needed already for ~150MHz/cm2.

• L251 As I wrote above, it would be helpful to have a table of requirements on L71 to help understand with what margin the specs are met

• L253 can you be quantitative? What is the degradation?

• L316-317 A table summarizing the various options and the number of modules per layer would help guide the reader

• L321-322 Are you going to describe how the modules differ? If so please reference the section. If not, I suggest you remove the sentence (or add a detailed explanation) otherwise the reader does not learn much

I don’t fully appreciate why the discussion about the production schedule is presented in the paper. I would remove L323-325 and L328. If however you would like to keep it, please add the details of the L1 vs L2/3/4 schedules, what was the schedule driver, etc. Our objective is to have the reader learn about past experiences with detector construction.

• L323 The “test” itself did not contact the ROC, please reword this sentence (for example the “goodness of the bump bonding was verified using …”)

• L337 how many centers? Please be specific. Also what made a given center capable of that operation? Why not sending all faulty modules to the centers capable of reworking the assembly?

• L349 Do you have an exploded view of the module to help understand the mounting procedure better? Fig 4 is too compact.

• L359 made of —> instrumented with

• L373 Was any of the issues reported in FPIX encountered in BPIX? If so, please specify in the previous section

• L385 - 388 This test is described together with the others in the following part of the section. Why did you single it out here? I would move whatever relevant information you have here to the 7. IV Test to avoid redundancy and possibly confusion

• L389 do you have a reference for the SW?

• Fig 11 If the z axis is the efficiency why doesn’t reach 100% Please explain the figure in the text. Missing units on axis
• L402 and caption of Fig 11 please use subscript where appropriate (thr, Comp, Del)

• Fig 12 would it be possible to make these plot more inform? For example the x axis labels? Also for BPIX, what is he difference between the dark and light color? Please add the legend. Missing units

• Fig 14 please fix the label Hitrate [MHz/cm2] —> Hit rate [MHz/cm^2] . Caption: please explain why the 50 o 120 range is used to determine the efficiency.

• L487 please specify what you mean by ‘defective pixel’ (i.e. what test the pixel failed)

• L488 is 600 the threshold for both chips?

• Fig 18 you have a note “update picture”. Is the picture up to date and the text needs updating or the other way around?

Both Fig 18 and 19 would be more useful if you added a description of the relevant components to the caption.

• L590 any lesson learnt from this test that is worth documenting?

• Sec 4.2.1 given the issue with the DC-DC converters, I think it would be very useful if you went into the details of what QC tests were made during the R&D, preproduction production phases. It is important to document what tests were carried out which - unfortunately - did not spot the issue that would emerge later on during data taking.

• Fig 22 Too small, please move the sub fig on the right to the bottom

• Fig 25 the Fig is a bit hard to read. Please add a detailed explanation to the caption

• L807 what was the conclusion of this test? Again, it would be useful if you documented whether or not the test was successful and reliable for future references.

• L829 I am not sure you refer to Fig 18, as Fig 18 is not labelled

• L890 was the yield 100%?

• L962 is the sag within spec?

• L972 As mentioned above it would be important to expand on QC and explain what (if) issues were encountered or all parts met the specs

• Fig 34 is too small, please expand it. Please explain the Fig in the caption. What is ref 1/2/3? If those are preirradiated samples, why is the variation so large in some cases? For the irradiated samples, the araldite depends strongly on irradiation, please explain.

• Sec 5.2.7 Was any material used by FPIX tested for rad hardness?

• L1009 a reader may wonder why the DC-DC converter issue did not manifest itself in the pilot system. Please add a short explanation about it

General comment to Sec 6.3: the language is frequently too colloquial; technical details ought to be added to actually provide valuable information to the reader. Please have a look at this section again. I also give my line-by-line comments below.

• L1110 what does “the wire bonds were pulled” mean?

• Sec 6.3.1 Please specify the temperature and duration of the baking

• L1122, 1123 “heavy’, ‘bulky’ too colloquial. Please report the details of the pigtails (size, etc)

• L1136 ‘inadvertent’ colloquial as well. Please write that cables did not meet specs. Also explain why the issue was not identified ahead of time. Add if any other issue originate from the cables being shorter than needed.

• L1137 “went more smoothly” again too colloquial

• L1138 in this case as well it would be important to be specific: what flex cables, how many broke, etc.

• L1141 please specify what 2% refers to

• L1147 ‘abrupt’ too colloquial. Please specify DeltaTem/DeltaTime and explain in detail what issues were observed (including % of bump bonds lost). What was the new procedure? What was the improvement?

• L1152-54 I would have this statement only if it can be made fo FPIX and BPIX. Also, in order to be informative, it should be more quantitative: how many people (students RA senior engineers tech), how many shifts etc?

• L1280 please quantify, indicate how much the thresholds were changing till we collected 10/fb

• Fig 44 still missing FPIX?

• Fig 45 unfortunately the colors are really hard to distinguish

• L1317 how long?

• L1320 Previously it was stated that only 0.4% of the BPIX channel only 2% of the overall FPIX were lost. Could you please explain how you get to 98.4 and 96.1%?

• L1343 what reduced the number from 100% to 96.7% after the DC-DC fix?

• L1376-1384 The language is too colloquial (mistake, unpleasant, periodically,…). Please review this part. Provide additional details about what transistor was actually causing the problem. Add the length of the power cycle and the corresponding down time.

• L1384 I assume that a study of efficiency was made, in comparing the two approaches: reset during fill and in between fills. Can you please support the decision taken in a quantitative manner?

• L1390 It would be useful to have a table with the threshold (spec, during data taking) for the 2 chips. On L228 we mention 1800e for the PSI46dig, on L171 we have 3000e (not clear for which one of the two chips) and here we state that instead of 2000e we lower the threshold to 1300e for PROC600. I assume the final numbers are provided on L1405-6. A Tab like Tab 2 for HV would be useful

• L1382 why is Fig 51 here? Some issues with the numbering of the Fig (it should be 47)

• L1418 please reword “sudden jump’

• Fig 47 please align the plots

• Fig 48 please explain the shape of L1

• Fig 50/52 is this the latest plot?

• Fig 55 is vey had to read. Please produce it again with a different format or include a description of the markers in the caption. Why is data/sim better in 2018?

• Fig 56 likewise for the start of 2017. Please enlarge this fig (it uses anyway the full page)

• Comments by Andrea, started to be filled on April 4th (STILL IN PROGRESS: last update on April 5th 14h00):

The paper is very nice and full of useful information. It is long but at this round of comment I will not try to suggest way to make it shorter. This does not mean we should not try later...

#### Paper organization (last update on April 4th)

• The sub-sub-section 3.1.1, 3.1.2, 3.1.3, 3.1.4 are not needed. Their content fits very well one after the other without the need of a sub-sub-title. Minor changes are needed around rows 103, 124, 143 and 153 to compensate for the absence of the titles. The text in rows 96-101 should be, eventually re-checked to avoid unnecessary duplications with what is described below (for example rows 104-107). There is no need of a new paragraph between row 97 and 98

• The sub-sub-sections 3.2.1 and 3.2.2 are not needed. The fact that we have two ROCs is well explained in rows 185-187 and it is enough to start the paragraphs about PSI46dig (row 189) and the paragraphs about PROC600 (row 241) in the proper way (as it is now it could be already ok) to make clear that we are talking of one or the other ROC

• No need to break and start a new paragraph between row 277 and 278

• There is no real need for sub-sub-section 3.6.1, 3.6.2 and 3.6.3. These three parts fit very well one after the other we no need for a dedicated title.

• Section 3.6.1 text could be better organized. The text in rows 388-392 could be moved earlier, about row 384 and replace the sentence about “testing procedure is described below”. The details about the different temperatures given in row 387 are a repetition of what is written in row 459. There is no need to write them in the introduction to the tests (unless ALL the tests are done at two temperatures but in this case the fact that only in row 459 this is reminded is misleading). The preliminary tests described in rows 394-404 and the tests described in rows 406-461 should not appear as “something” different and presented with different styles. In particular the “titles” of the tests in rows 406-461 should be removed (and, if needed, the text at the beginning of the description of each test adjusted). Moreover, the sentence “Before starting the testing procedure all modules were tested…” does not sound OK. There is no need to split between pre-test and test in a public paper.

• The sentence about rejected modules in row 414 has to be moved where the module grading is described.

• Since, I guess, the threshold trimming is done by using the results of the S-curve, the paragraph in rows 416-422 and the one in rows 424-431 have to be coordinated better. One possibility is to swap them and explain that the iterative process for the trimming consists of repeating the S-curve measurements. In any case it is not clear in the trimming paragraph what is the observable used to iterate.

• The details on how the threshold and the noise is extracted from the S-curve have to be moved from section 7.4 to the paragraph in rows 424-431. Section 7.4 can refer to this section later.

• The gain and pedestal measurement described in rows 438-446 is repeated also in section 7.5. Can you move here the description or make an explicit forward reference to section 7.5 (as of now I am not able to judge which solution would be better)

• Section 4 has to be removed since the statement of rows 510-512 is, correctly, already in the introduction. Sub-sections 4.1, 4.2 and 4.3 have to be upgraded to be sections.

• Section 5 has to be moved before subsection 4.1 and 4.2 (not before 4.3, though): having the mechanics and service cylinders already described help a lot the description of the readout and power system.

• Rows 899-900 do not contain a good introduction to the whole FPIX mechanics sub-section since they describe only the FPIX disks (!). No introduction was present in section 5.1 . Think about removing all the sub-sub-section 5.1.x and 5.2.x. In particular 5.2.3 is really a short sub-sub-section.

• The text of sub-sub-section 5.2.5 has to be merged where the service cylinder mechanics is described since it completes its description

• The text of the sub-sub-section 5.2.6 has to be moved where the disk mechanics is described: it completes its description.

• The Pilot system deserves its own section. Not a subsection of section 6 (it does not even match with the title, to some extent).

• Sub-Sub-sections 6.2.1, 6.2.2, 6.2.3 can be removed: the three parts fit nicely in a single section with, maybe, a bit of rewording at the beginning of each part.

• The description of the TBM hub address should be moved where the TBM chip is described. (rows 1023-1027).

• Section 6.3: the sub-sub-section titles can be removed and the time evolution of the FPIX integration, now described in row 1097-1104, can be distributed later: after line 1119 for the module integration to disks, after line 1153 for the integration of the service cylinder and the shipment (somehow rows 1149-1150 already repeat what is at the beginning of section 6.3 and that I propose to remove)

• What is described in rows 1261-1267 was (partly) already described around row 402. A strategy on where this has to be described and proper (forward/back-)ward references have to be established

• Rows 1268: the threshold trim is already described in row 205: refer to that (or write the sentence assuming that reader know already what the trim is.

• Section 7.5 has to be coordinated with section 3.6.1. For example the sentence about the use of the average slope per column (ropws 444-445) can be moved in section 7.5 (since it is more related to the detector operation and reconstruction). Instead what is presented in rows 1304-1308 should be anticipated as much as possible (section 3?) because it helps to convert Vcal into electron and many results before section 7.5 are presented as a function of Vcal. Moving this part is not a problem because the X-ray test is described in section 3.

• The content of rows 1310-1319 can be moved later, when the results with the collision data are discussed (time alignment and resolution). Section 8 can start with the summary of the good channel fraction (starting from row 1320), followed by the description of the DCDC converter issue, the repair campaign and is consequences. A subsection whose title refers to the fraction of active channels could be introduced

• The text of subsection 8.1.3 is not needed. The issue with the present PROC600 is described in the section where PROC600 is described, the issue of the cross talk noise and the consequences on the thresholds are described in section 8.2.1 and the measurement of the efficiency is presented in section 8.3.1 . Remove the text of rows 1386-1396 and, if needed, add more details in the other sections where this is already described

• Section 8.2.1 describes facts and results which are not obtained with collision (or cosmic) data and, therefore, there is no need to delay it so much. This part has to be moved in section 7.4 so that we have described in a single place and in a compact form everything about the thresholds: how they are tuned, measured, trimmed, how they evolved with radiation and the issue with PROC600.

• Section 8.1 contains sub-sub-sections which have little to do among each other: time-alignment (which is done once) and the procedure to mask and recover bad channels. These two sub-sub-sections have to be de-coupled.

• Section 8.1.1 should be completed with the statements about the alignment (now in rows 1310-1317) and its title should changed into something like “Detector calibration (or tuning) with first collisions”. It fits well after the part about the fraction of active channels (see comment above)

• Section 8.1.2 is the ONLY section which is purely about routine operation. Since it describes how unresponsive channels are masked dynamically (actually it does not but it has to be expanded (just a bit) to describe the Soft Error Recovery procedure) and, eventually, focus on the “stuck-TBM” issue as an annoying example (which requires a special treatment), this section can be moved “AS IT IS” at the end of the DAQ session (now 4.1) because it describes issues already introduced (TBM sensitivity to SEU) and that are addressed only with DAQ/power tools. In this way we avoid long range forward/backward references

• Section 8.2.2 and 8.2.3 can be moved into section 8.3 (whose title could be changed slightly into something like “Detector response and performance monitoring”).

• Section 8.3.3 should be moved as early as possible in the section about detector response and performance monitoring. Being before 8.2.3 helps to understand the LA dependence on the luminosity. The title of section 8.3.3 is not very meaningful. The sensor bias does not evolve by itself. We are showing the results as a function of the bias voltage at different level or irradiation.

• Remove the break between lines 1528 and 1529

• Add a paragraph break in row 1538 at the sentence “The sensor properties…”.

• Section 8.3.4 can be merged into section 8.3.3 (and moved together with it). In this way some statements about FLUKA and the Hamburg model can be unified.

• Section 9 has to be removed. I guess that 90% of what is described in this section (motivations, strategy, analysis technique, performance,…) are already in the Nuclear Interaction paper. The only parts which have to be taken are: the fact that we have repeated the analysis with the 2018 data, that we have obtained the results shown in figure… (choose only two figures, my suggestion is figure 57 and the left plot of figure 58) and that the beam pipe position is … as expected after the adjustement of his position, that the tight tolerance between L1 and the beam pipe is visible and that the two halves of BPIX are nicely overlapping. All this can be moved at the end of section 6.4 as a conclusion of the installation. This is because, presently, the results of the Nuclear Interaction analysis are not providing anything more than a confirmation of the positions of the different parts and this matter mostly for the installation.

#### What is missing and has to be added to the paper (last update on April 5th)

• CMS coordinates system has to be described as soon as possible in the paper, as it is done in the CMS papers. This will allow us to continue to refer to it in the rest of the paper (as we already do)

• Module local coordinate system has to be defined as soon as possible when the modules are described. This will allow is to continue to refer to it in the rest of the paper.

• Every time we explain why the number of layers and disks have been increased by one, we have to remind that this is done to increase the redundancy of the detector (in particular for the HLT). For example in row 17, 42, 43

• Around row 57 it has to be reminded that this scheme is needed also to allow detector maintenance and refurbishment also during the short shut down during the Winter break (this will be proven when the replacement of the DCDC converters done in 2017-2018 will be discussed)

• In table 1 the titles of the columns in the BPIX section are missing

• In table 1 the z and RADIAL position of each FPIX INNER and OUTER disks should be quoted. The FPIX table should have six rows, one for each inner and outer disk, and one column more with the radial position.

• Somewhere in the paper (for example in the introduction of section 3) it has to be reminded that the FPIX modules have an important difference with respect to the previous pixel detector: the modules are tilted mostly in the rz plane to achieve charge sharing and that the pixel orientation is with the long side along r, different from the previous detector. This can be done around row 89 where (implicitly) only the BPIX module orientation is explained.

• In row 151 we have to give at least a hint about the specifications for the FPIX sensors. Is there a reference? Can we refer to the Turkish paper?

• Something has to be said or a reference has to be added to remind about the effective type inversion when the sensors are irradiated. This should happen either when the comment about the n-in-n approach ensure high E field close to the pixels after irradiation (row 111-112) or when the signal loss is described (subsubsection 3.1.4)

• Close to line 157 an approximated value of the detection threshold(s) should be provided to guide the reader

• In section 3.2.1 the expected hit rate in BPIX L2 (assuming that this is the largest experienced by PSI46dig, otherwise FPIX inner disk) should be reminded (and compared to the figure quoted in row 235, with the usual caveat that X-ray hit rate is equivalent to a more pessimistic particle hit rate)

• Explore the possibility to add a statement about the ROC and TBM chip wafer probing and the resulting yield. It would make the paper more complete and it can fit in one sentence.

• Around lines 210-212 we are missing the description of the fact that there is a CMS level 1 trigger decision which is transmitted to the modules. The word “trigger” is introduced all of a sudden and used in a jargon style. To be seen if it is relevant to explain what happens IF a trigger does not arrive, eventually.

• In rows 298 and 300 we have to write what is the setting actually used during Run 2: enabled or disabled?

• In section 3.5 nothing is said about the bump-bonding. See also the comment below about references about bump-bonding techniques. We should also remind that FPIX bare modules are not tested.

• As already suggested above, section 3.6.1 should already contain the information about the Vcal to electron conversion factor because: it is measured with the X-ray test which is in section 3.6.1, and it is important to understand the results about the threshold and the noise which are also in section 3.6.1. Also its dependence on radiation should be described briefly because it may be relevant when the signal and threshold evolution vs luminosity are shown.

• Figure 16: does the BPIX trend plot include also layer 1 modules? From the position of the dashed line (lower than 1184) I would say that it does not while, instead, it has to.

• In rows 535-537 it has to be said that FED, pxFEC, tkFEC are FC7 boards with different, custom, mezzanines. A reference to FC7 boards (L1 trigger upgrade or reference therein?) has to be added.

• Rows 580-582: are we missing gatekeepers and LCDS among the components which populate the FPIX port card? Or do they not exist for FPIX? Sorry for my ignorance.

• Section 4.2 is missing the description of the HV part: we have to remind at least the granularity, maximum possible HV (and remind that we are planning to upgrade it to 800V) and maximum current per channel

• Section 4.2 is missing the description of the LV power system for the auxiliary electronics and the control rings. Briefly we have to remind about it, the LV value, the power supply models (because we do that for the main power system).

• In row 693 we have to write explicitly if we have run with the slow sensing enabled or not, eventually.

• Add in row 698 the luminosity integrated at that time (about 30 fb-1 if I recall correctly) and the dose accumulated by the converter at that time (about 1 Mrad if I am not mistaken).

• In the introduction of section 4.3 (row 720) it has to be described what are the advantage of the 2-phase CO2 cooling: low mass, efficient heat exchange because it is two phase, simple: temperature is controlled only by the pressure in the accumulator,…

• In section 4.3.2 we have to remind which pressure values operating at -23 C and at 15 C correspond to. This will make more clear to reader statements which appear later in the paper about the fact that the system has to run at “high pressure”.

• In section 4.3.2 (or in any case somewhere in the cooling section), it has to be reminded the target vapor quality value (range) at which we would like to operate the detector and we have to remind the flow values at which we have operated the detector cooling loops.

• Around rows 805-806 we can say that based on the results of that test during 2018 run we have reduced the flow in some BPIX cooling loop and gained 0.x C in temperature.

• In section 5.1.1 we should remind that the ladders are alternated at smaller and larger radii values compared to the average values. This could be useful to understand how critical is the rate and radiation effects on the innermost L1 ladders. A picture/drawing like the one in the TDR could be useful.

• In section 5.2.1 the explicit information about how many cooling loops are present per FPIX disks (either the inner+outer disk pair or inner and outer disks separately) should be added. I think that it is trivially one cooling loop per half disk, both for inner and outer.

• Around line 973 the power dissipated in the test figure 33 refers to, has to be indicated.

• In section 6.1 it has to be added that the pilot system was used also to test the novel DCDC converter based power system (noise, SEU sensitivity, …) and that some DCDC converters were connected to dummy loads (and some moduled were NOT powered with DCDC converters)

• Around row 1028 we should add that thermal grease was used for the layer 1 module installation

• Around line 1202 we have to write that, eventually we had no issue during the 2017 because of the leak reported above and that the leak was not observed again when the cooling connections were tested again after the 2018 installation (I guess…)

• At the end of section 6.4 it has to be said that the same detector insertion procedure and verification was adopted before the 2018 run after the DCDC converter refurbishment and no problem was experienced, confirming that the procedure is well established and reproducible with very little unknowns (or something like that).

• In line 1208 if we want to keep the comment that the tests are done at +17C before lowering the temperature set point we have to explain why: to operate a low temperature the Tracker volume has to be sealed that this does not allow us to intervene on the patch panels if the tests reveal problems that can be fixed in that way.

• Can we add a trend plot of the fraction of (in-)active channels at the end of section 8 (around line 1343)? I would give a try to see how it looks in the paper. Not a strong suggestion, though.

• Around lines 1381-1382 we have to remind that in 2017 the TBMs were power cycled by disabling the DCDC converters and that in 2018 we decided not to do it anymore to avoid to break them

• In section 8.2.1 the logic behind the threshold choice and setting is not described. Nothing is said about what happens when the threshold is set too low, for example. If we want to avoid too many details the CRAFT paper can be referred to, since it contains all the details (assuming that the process remains conceptually the same).

• In what is now section 8.2.1 (but that has to be moved and merged in section 7.4) I would add very briefly about the successful attempt, done at the end of the 2018 run, to reduce the L1 thresholds by xxx electrons (I don’t remember the exact value: 500-1000) once we have reduced by software one of the main source of the cross talk noise (no need to go in more details than that)

• Around lines 1524-1528 it has to be said that the observables as a function of the bias voltage are shown at different value of the integrated luminosity.

• Related to figure 53, the concept of the annealing has to be anticipated with respect to line 1547 and better explained: otherwise it is impossible to understand why the curves at different values of the integrated luminosity do not behave monotonically. It looks like the old module behavior measured at 51 fb-1 is less affected by the radiation than that of the new modules after 31.5 fb-1 and the reason is, I guess, the annealing of the old modules. When the annealing is introduced it has to be said very briefly that it usually happens in a (more or less) controlled way during the Winter technical stop by stopping the cooling

• In section 8.3.4 (or wherever the text which is presently there will be moved) it has to be reminded that the leakage current can be reduced with the annealing (see comment above on when the annealing can be done) but we cannot exaggerate because of the reverse annealing. References are welcome. Highlight that the effect of the annealing is visible in the plots of figure 56.

• Around lines 1567-1568 we have to say something about the possibility to run during Run 3 without any issue related to the leakage current. It is enough to say that given the limits of the power supplies we see no problem to collect ~300-400 fb-1 during Run 3 (we can agree on a sensible value but it is not critical and the sentence can be turned into “no problem is expected during Run 3 when at most 300 fb-1 will be collected”)

• In figure 58 left (that I suggest to keep and move at the end of section 6.4) a few elements should be indicated with an arrow and a legend and among them the module cable footprints in order to get rid of the sentence presently in line 1623-1624

• Missing references to be added
• A reference to the TDR has to be added in the introduction. It has to be used to justify why a new detector is needed: higher hit rate: increase the bandwidth and reduce the dynamic inefficiency, more complex events because of the luminosity: increased redundancy (one extra layer), extension of the lifetime, …
• A reference for the n-in-n approach has to be added around row 108. At least we can refer to the CMS detector paper (even if in this case this reference can be anticipated ALSO to row 96)
• In section 3.4 (and 3.5) we should really try to find sensible references to the different bump-bonding technologies used by the different manufacturers
• Row 389: is there a reference to pXar? Otherwise it is a bit pointless to refer to it by name.
• In section 3.6.1 (and later in section 7) reference to old papers where the module calibration procedures are described, should be added. For example the CRAFT paper, CFT-09-001, contains a good description of gain and threshold measurements. It is useful to remind the similarity of some but not all the procedures with the old detector. I don’t know if there are other papers/proceedings/public notes which describe these testing procedures, even if they refer to the old detector and modules but are still valid for the upgraded one.
• Also in section 3.6.2 a reference to the X-ray measurements done for the old detector could be useful (to show similarities and differences w.r.t. the old detector). The CRAFT paper can be used of the reference [18] in that paper.
• A reference to microTCA has to be added in row 533
• When the detector alignment is referred to (now at the beginning of section 8) the paper(s) that describe the alignment has to be cited. Even if they refer to the old detector the technique is the same
• When the detector alignment is referred to (now at the beginning of section 8) we should remind briefly that the alignment procedure had to be repeated once more at the beginning of 2018, after the re-installation, at least for the large structures, the new modules and the modules that were recovered.
• Around line 1428 a reference to the paper(s) were the pixel local reconstruction is described should be added. Probably TRK-11-001 is ok.
• References to previous papers where the Lorentz Angle measurement is described have to be added in section 8.2.3. In addition to reference [42] (THERE IS A MISTAKE, REFERENCE [41] AND [42] ARE SWAPPED!!) also CFT-09-001 (the CRAFT paper) and TRK-10-001 (the first tracker performance paper) have to be added.
• Around lined 1472-1473 remind that the pixel detector is used also for the track seeding and put a reference to TRK-11-001
• Around line 1499 put a reference to TRK-11-001 and to BTV-16-002.
• Around lines 1509-1513 references which describe the change of the electric field with the radiation and the increase of the trapping have to be added.

KatjaKlein - 2019-03-22

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