Speaker
Description
Summary
The ILC physics program requires detectors with unprecedented jet
energy resolution. To achieve this goal, the detectors will need
highly granular calorimeters and, for the electromagnetic calorimeter, the
use of a silicon-tungsten calorimeter has been favored. The granularity
and readout requirements of such a calorimeter are closely interrelated.
Detailed simulations show that a pixel size of 50x50 microns results in
most pixels being only hit once per event. Thus we can employ a simple
binary readout using a comparator instead of an analogue measurement. We
have designed and fabricated such a CMOS Monolithic Active Pixel Sensor
(MAPS) using a novel "INMAPS" process.
The first prototype chip (TPAC1) comprises 168x168 pixels with a total
of over 8 million transistors. TPAC1 consists of 4 sub-arrays of 84x84 pixels
implementing two distinct pixel architectures, with two variants of each.
Each pixel contains four N-well diodes for charge collection, analogue
front-end circuits for signal pulse shaping, a comparator for threshold
discrimination, and digital logic for per-pixel threshold trim adjustment
and pixel masking.
For readout, pixels are served by shared row-logic which stores the location
and time-stamp of pixel hits in local SRAM, and was designed to target the
189 ns beam bunch crossing rate of the ILC. The sparse hit data are read
out from the columns of logic in the quiet time between bunch trains.
The INMAPS process is a standard 0.18 micron CMOS image-sensor technology
but includes a high energy deep p-well implant. A conventional MAPS design
will allow charge absorption by any PMOS active devices in the pixel. Hence,
the signal charge is shared between the N-well collection diodes and the
rest of the circuit, dramatically reducing the efficiency of the pixel.
By implanting the deep p-well in the regions of the pixel containing the
PMOS active devices, charge deposited in the epitaxial layer is reflected
and conserved for collection only at the exposed N-well collection diodes.
The charge collection performance of pixel test structures on the chip has
been evaluated using a focused IR laser and clearly demonstrates the
improvement achieved by the deep p-well implant. These results will be
compared with device simulations. The performance of the main sensor pixels
has been evaluated in terms of gain, noise and pixel uniformity. A Fe55
radioactive source is used to calibrate the pixel gain, and the laser is
used to evaluate per-pixel gain uniformity. Further tests include cosmic
rays, and alpha and beta radioactive sources. The status of the project,
including latest results on the sensor performance, will be reported.
This is an exciting CMOS technology that offers a competitive alternative
to the challenges of large-scale silicon detector systems both in
performance and cost. The INMAPS process which has been developed under
this program would also be applicable to a wide range of other applications.
