Y. Mei

A Low-Noise CMOS Pixel Direct Charge Sensor, Topmetal-II-

We report the design and characterization of a CMOS pixel direct charge sensor, Topmetal-II-, fabricated in a standard 0.35µm CMOS Integrated Circuit process. The sensor utilizes exposed metal patches on top of each pixel to directly collect charge. Each pixel contains a low-noise charge-sensitive preamplier to establish the analog signal and a discriminator with tunable threshold to generate hits. The analog signal from each pixel is accessible through time-shared multiplexing over the entire array. Hits are read out digitally through a column-based priority logic structure. Tests show that the sensor achieved a < 15 e analog noise and a 200 e = minimum threshold for digital readout per pixel. The sensor is capable of detecting both electrons and ions drifting in gas. These characteristics enable its use as the charge readout device in future Time Projection Chambers without gaseous gain mechanism, which has unique advantages in low background and low rate-density experiments.

A Low-Noise CMOS Pixel Direct Charge Sensor Topmetal-IIa for Low Background and Low Rate- Density Experiments

We present the design and characterization of a CMOS pixel direct charge sensor, Topmetal-IIa, fabricated in a standard 0.35µm CMOS process. The sensor features a 45 × 216 pixel array with a 40µm pixel pitch which collects and measures external charge directly through exposed metal electrodes in the topmost metal layer. Each pixel contains a low-noise charge-sensitive preamplifier to establish the analog signal, which is read out through time-shared multiplexing over the entire array. Compared to the earlier Topmetal-II- chip, the analog readout noise of Topmetal-IIa is reduced by 10.8% from 13.9e- to 12.4e-, and the DC voltage variation noise is reduced by 21% from 1.2mV down to 0.946mV. The sensor is capable of detecting both electrons and ions drifting in gas. These characteristics enable its use as the charge readout device in future Time Projection Chambers without gaseous gain mechanism, which has unique advantages in low background and low rate-density experiments.

Characterization of the column-based priority logic readout of Topmetal-II− CMOS pixel direct charge sensor

We present the detailed study of the digital readout of Topmetal-II- CMOS pixel direct charge sensor. Topmetal-II- is an integrated sensor with an array of 72X72 pixels each capable of directly collecting external charge through exposed metal electrodes in the topmost metal layer. In addition to the time-shared multiplexing readout of the analog output from Charge Sensitive Amplifiers in each pixel, hits are also generated through comparators with individually DAC settable thresholds in each pixel. The hits are read out via a column-based priority logic structure, retaining both hit location and time information. The in-array column-based priority logic is fully combinational hence there is no clock distributed in the pixel array. Sequential logic and clock are placed on the peripheral of the array. We studied the detailed working behavior and performance of this readout, and demonstrated its potential in imaging applications.