The Sensor Test for CMS pixel upgrade

Introduction

The Compact Muon Solenoid, CMS, is a general purpose detector for the Large Hadron Collider, LHC, accelerator at CERN. The CMS tracker, the innermost part of the CMS detector, is composed of a pixel detector with three barrel layers at radii between 4.4cm and 10.2cm and a silicon strip tracker with 10 barrel detection layers extending outwards to a radius of 1.1m. Each system is completed by endcaps which consist of 2 disks in the pixel detector and 3 plus 9 disks in the strip tracker on each side of the barrel, extending the acceptance of the tracker up to a pseudorapidity of |eta|< 2.5.

After 2-3 years of operating at LHC the performances of the innermost barrel layer sensors will degradate significantly due to radiation damage. For this reason a replacement of the whole pixel detector is scheduled for 2013 in conjunction with the shutdown for Phase I upgrade.

The sensors for the CMS-pixel detector adopt the so called n-on-n concept. The pixels consist of high dose n-implants introduced into a high resistance n-substrate. The rectifying pn-junction is placed on the back side of the sensor surrounded by a multi guard ring structure. Despite the higher costs due to the double sided processing this concept was chosen as the collection of electrons ensures a high signal charge at moderate bias voltages (< 600V) after high hadron fluences. Furthermore the double sided processing allows a guard ring scheme keeping all sensor edges at ground potential.

The sensor quality is tested through the leakage current vs. bias voltage measurement. The current-to-voltage curve has an obvious charactersistic. For the voltages below full depletion the current increases proportional to the square root of the bias. When the space charge reaches a structured backside, an additional surface contribution arises. After full depletion the IV-curve displays a plateau region in which the current increase is very small before electrical breakdown occurs at very high voltages. Almost all possible problems in the sensor production process lead to a deviation of the curve from the expected shape.

the Instruments Setup

• a PC, communicate with the Keithley source meter and the prober station
• a Keithley 2410 source meter, supply the voltage
• a prober station SUSS PA200, has a 3-groove chuck fixing the sensors and two needles touching the sensors

the Labview VI

The move_meas.vi is designed to integrate the chuck movement and IV measurement. Its front panel looks like,

The values of x, y, z and sensor No. should be set before mesurement. The (x,y,z) means chuck's position, which should be precisely pre-tested to make sure the two needles touch the small square pads on the sensor.

If the groove has sensor, choose "on"; otherwise choose "off".

The data is saved in the given "path".

On the bottom the error-outs show the error information for initialization, load back, movements and I-V measurements.

The default voltage setup is from 0~-600v with each step -5v and waiting time 1s. The alarm current limit is 100uA. These values can be changed in the subVI sensor_IV_measurement.vi .

Tips

The first chuck point's z value is better to be larger than 1500um before fix the needles.

The z relative values can be obtained by adjusting the chuck to microscope's focus distance while fixing scope height.

-- XiaWan - 04-May-2011