The CMS Phase-1 Pixel Detector


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
  • Target journal: JINST


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

UH comments on v0

Stay tuned, I will add my comments here...

General comments:
  • 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.

Line-by-line comments:

  • 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:
    • L38: upgraded pixel system
    • 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
    • Table 1: add header row for BPIX: Layer | Radius | Number of ladders | Number of modules
    • 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
    • L85: upgraded pixel detector
    • L87: array of 2x8
    • L94: "upgrade modules" sound strange, maybe: upgraded (pixel) detector modules?
    • 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
    • L161: Has the signal height been measured before and/or after radiation? Please add this information.
    • 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?
    • L321: four readout links
    • 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)

Paper draft

KatjaKlein - 2019-02-27

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