Exploring the quality of latest sensor prototypes for the CMS Tracker Phase II Upgrade

Abstract The luminosity of the LHC will be increased by a factor of five to seven after the third long shutdown (LS3) scheduled in the mid of the next decade. The significant increase in luminosity along with the limitations of the current Tracker require a complete renewal of the CMS Outer Tracker, the Tracker Phase-2 Upgrade, during the LS3. New types of modules called PS and 2S modules are foreseen offering enhanced functionality and radiation hardness. Milestones in sensor R&D for the 2S modules as well as first characterization results are presented. AC-coupled silicon strip sensors of two vendors, produced on 6-inch as well as on 8-inch wafers, are considered which both are in n-on-p technology. Global as well as single strip parameters were measured providing insights into the quality of the sensors.


Introduction
The current CMS Tracker will be operational until LS3 at an integrated luminosity of 300 fb −1 . Beyond LS3, the performance of the Tracker will significantly decrease mainly due to radiation damage causing an increase in leakage current which cannot be compensated by an appropriate temperature decrease of the cooling system anymore [1]. This fact alone makes a replacement of the Tracker during LS3 compulsory. For the high luminosity period after LS3, an integrated luminosity of up to 3000 fb −1 in total and fluences of up to 1.5 × 10 15 n eq cm −2 ( Figure 1) are expected which pose several challenges to the new Tracker in terms of radiation hardness and pile-up.
In particular, the new modules must withstand ∼ 10 years of high luminosity data taking which makes more radiation hard sensors one of the most important aspects of the Tracker Upgrade. An estimated pile-up of 140 collisions per bunch crossing in average, three times more than currently observed, requires a higher granularity of the Phase II Tracker to keep the occupancy below 1% [1]. The resulting large amount of data in combination with bandwidth limitations and the compliance with the L1 trigger upgrade represents another major issue for the upgrade. To overcome this issue, the modules for the Phase II Upgrade will offer an enhanced functionality.

Trigger capabilities
The design of the new modules allows to distinguish between high and low transverse momenta (p T ) of incident particles on * Corresponding author Email address: axel.koenig@oeaw.ac.at (A. König) Figure 1: One quarter of the CMS Tracker showing particle fluences in n eq cm −2 for different regions corresponding to an integrated luminosity of 3000 fb −1 [1].
module level and therefore contributing to the L1 trigger decision at a bunch crossing rate of 40 MHz. The p T discrimination is achieved by using two stacked and closely separated silicon sensors. If a particle is passing through the sensors, the hit position on the first and second sensor is correlated giving information on the particle's p T . High p T particles will only slightly be deflected by the magnetic field of CMS and will hit both sensors nearly on the same position (such an event is called "stub") whereat low p T particles will be deflected more resulting in considerable different hit positions ( Figure 2). Depending on the window size ("stub window") where for the second hit is searched, specific transverse momenta can be discriminated. The whole procedure is called stub finding logic and is implemented in the new read-out chip of the modules. The read-out chip is called CMS binary chip (CBC) providing only binary data suitable for the above described p T discrimination and the demands of high luminosity data taking.

PS and 2S modules
Two types of modules called PS and 2S modules are foreseen for the CMS Tracker Upgrade. The PS module incorporates one macro pixel sensor and one strip sensor which are stacked and closely separated. The 2S module follows the same concept of stacked sensors but incorporates two identical silicon strip sensors with parallel strip orientation. The distance between the sensors will be 1.8 mm to 4 mm depending on where they are located inside the Tracker. Generally, the PS modules will be closer to the interaction point due to the required higher resolution there.  Figure 3 shows an exploded view of a 2S module. Each front end (FE) hybrid hosts 8 CBCs which are wire bonded to the strips of the sensors (the strips are segmented at the center, Section 3.2). The signals of the CBCs are fed by a CIC concentrator chip to the service hybrid which transmits the data via a multi-gigabit transceiver. The spacing of the sensors is defined by aluminum carbon fiber (Al-CF) spacers.

Silicon strip sensors for 2S modules
The 2S modules for the Tracker Upgrade will incorporate sensors very different to the currently used ones. In particular, they will have p-type bulk material showing a better performance concerning charge collection after irradiation [2]. Furthermore, non gaussian noise was observed for p-on-n sensors causing random fake hits [1]. Besides the change of the polarity of the base material, thinner sensors are preferred due to a reduced leakage current and material budget and a lower trapping probability after high irradiation. In addition, a better annealing behavior was observed for thinner sensors [1].

2S prototype sensors of two manufacturers
In total, 45 float-zone p-type base material wafers with a crystal orientation of <100> were ordered at two different manufacturers, Hamamatsu Photonics K.K. (HPK) and Infineon Technologies AG (IFX). HPK produced 20 6-inch wafers with incorporated 2S sensors having the baseline design whereat IFX produced 25 8-inch wafers with an elongated version of 2S sensors called "2S long" representing the worlds first AC-coupled silicon strip sensors produced on 8-inch wafers [3]. The concept of 2S long sensors is currently under evaluation for the application in the outer regions of the Tracker which would provide major cost advantages.

Basic Sensor Properties
HPK processed 320 µm thick wafers with a resistivity of 3 kΩ cm. The active thickness is confined to 240 µm using the deep diffusion technique where the backside implant is deeply diffused into the bulk enabling a smaller active area. Sensors of IFX are produced on 200 µm thick wafers (physical and active thickness) with a resistivity of 7 kΩ cm. Both 2S and 2S long are AC-coupled and biased via polysilicon resistors. Due to the oxide charge induced n-type inversion layer a p-stop respectively p-spray layer serves for strip separation. Each strip spans over the half length of the sensor. Both sensor types have 2032 strips with a width to pitch ratio of 0.25. The basic physical dimensions of the 2S and 2S long sensors are displayed in Table 1.

Electrical sensor characterization
The prototype sensors of HPK and IFX were extensively characterized providing insights into their overall quality. In particular, 15 out of 25 2S long sensors of IFX were electrically characterized. The other sensors of IFX were hold back for prototype module construction and more detailed investigations at the manufacturer and in the laboratory. Measurement results of only three 2S sensors of HPK are shown in the following. These samples serve as reference samples since very similar observations were made on the remaining sensors. All measurements were performed at 23 • C and below 30 % relative humiditiy.

Global parameters
The measured current-voltage characteristics (IV) and capacitance-voltage characteristics (CV) of sensors of HPK can be found in [4]. The sensors exhibit a full depletion voltage of ∼ 200 V, are stable up to 1 kV and show very low dark currents in general. Figure 5 and 6 show the IV and CV behavior for 15 2S long sensors of IFX. The CV curves are well overlapping and indicate a full depletion voltage of ∼ 70 V which is, as for HPK, in agreement to the specified resistivity and thickness. However, the global IV characteristics are very different. Most of the sensors show breakthroughs right after 100 V and draw significantly more current than sensors of HPK.

Single strip parameters
Four major single strip parameters were measured on sensors of HPK and IFX. The single strip current I strip , the polysilicon resistance R poly , the coupling capacitance C ac and the current through the dielectric layer I diel . Figure 7 shows the measured single strip currents per cm strip length. Sensors of both vendors exhibit currents below 2 nA/cm fulfilling the preliminary CMS specifications 1 . Figure 8 shows the measured polysilicon resistances. According to the specifications R poly values of (1.5 ± 0.3) MΩ are required. Sensors of HPK exhibit values within this range 1 These specifications might change for a later mass production.   The measured currents through the dielectric layer are displayed in Figure 9. Sensors of both vendors meet the specifications of I diel < 1 nA. The values measured for sensors of HPK are slightly smaller which correlates to the smaller strip length of a 2S sensor compared to a 2S long.  Figure 10 shows the measured coupling capacitance per cm strip length. Again all measured strips of sensors of HPK meet the specifications of > 26 pF/cm. On the other hand, only a negligible number of strips of sensors of IFX do meet the specifications. Generally, the coupling capacitance is too small and shows a too large spread in case of IFX. This might result in serious issues concerning the signal which is directly proportional to the coupling capacitance of the strip and demands for a more detailed investigation.

Summary
The prototype 2S sensors of HPK show excellent results in the electrical characterization and fulfill all of the current CMS specifications. 2S long sensors of IFX, representing the worlds first AC-coupled silicon strip sensors processed on 8inch wafers, show promising results whereat some issues need to be resolved for a later mass production. Especially the too small coupling capacitance in general but also its large spread must be resolved. This topic is currently under investigation at the manufacturer and in the collaboration. Another issue represents the too small polysilicon resistances of sensors of IFX which will be resolved by machine calibration in future batches.
The presented results show that prototype sensors of two possible vendors for a later mass production are of a high quality in general. The concept of 2S long sensors is depending on the availability of other manufacturers offering thin 8-inch wafer processes where Infineon demonstrated the feasibility of this concept. Finally, CMS foresees to launch the market survey for sensors for the CMS Tracker Phase II Upgrade in Q1 2016.