The CMS Phase-1 Pixel Detector

• 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", 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.