Brian Backer, Zoran Ninkov, and
Massimiliano Corba
Chester F. Carlson Center for Imaging Science
Rochester Institute of Technology
54 Lomb Memorial Dr., Rochester, NY 14623
e-mail: bsb5301@cis.rit.edu
ABSTRACT
Charge coupled devices have been the dominant solid
state detector array in the visible due to their relatively simple
design and easy implementation. With recent advances in lithographic
techniques, arrays having smaller photosite dimensions and an
increased number of pixels have become available. Further advances in
large format CCDs have been limited by charge transfer efficiency
(CTE) of photoelectrons to the readout amplifier. The increased
number of pixel transfers in large arrays can degrade image quality
and MTF unless even higher CTEs are achieved.
Multiplexer designs that remove the need for
thousands of charge transfers can bypass these CTE limitations. One
such focal plane architecture is the CID or charge injection device.
This paper presents results obtained with one particular CID based
system. The array is housed in a dewar capable of liquid nitrogen
operation. The output signal from the array is amplified with a
nearby low noise preamplifier before digitization. Results on
injection efficiency, readout noise, and other pertinent CID
parameters, are presented obtained from this device preamplifier as
well as specific experiments.
Keywords: Charge Injection Device (CID), device
characterization, injection, low noise techniques.
1. INTRODUCTION
Most applications of solid state sensors to low light
imaging have used CCDs. An example is the near dominance of CCDs in
astronomical imaging. However there are many situations where an
alternate focal plane architecture such as that of the CID has
distinct advantages over the CCD. CIDs have been proposed for
application in space environments since the device is inherently
radiation hard due from the high resistivity of the P-channel process
used to create the imager. In addition, since no charge transfer
between pixels is needed for readout, defective pixels do not result
in more that a single pixel being corrupted. In addition to this
application, CIDs have been designed and used in spectroscopy and
medical imaging.
CIDs use row and column electrodes to select a single
pixel and sense the stored photoelectrons from the perimeter of the
light sensitive area. In this way there is only one charge transfer
(from a storage well to a sense well) per pixel for each pixel on the
array. In addition, CIDs have advantages of non-destructive readout
for noise reduction, individual pixel injection, and multiple
subarray definition and readout. Due to the minimal amount of
electrical structure on the surface, the array has good blue
wavelength response extending into the UV.
2. CID STRUCTURE AND OPERATION
The array and pixel layouts of a CID-38 are shown in
Figure 1. This front illuminated device has 512x512 square pixels on
28mm pitch. Each pixel consists of a pair of orthogonal polysilicon
electrodes that creates two MOS capacitors in n-doped silicon for
storage and sensing of photogenerated charge. These electrodes also
connect the rest of the pixels on the column or row to the scanners
on the periphery.
The CID-38 has a preamp per row (PPR) architecture.
All pixels along each row are buffered by a single FET and hence the
PPR architecture requires calibration of the row FETs to minimize
nonuniformities between them. A thin metal strip is layered on top of
the row polysilicon to reduce readout noise. A drawback to this metal
structure is a small amount of obstruction of the incoming light.
Further reduction of read noise will be achieved using improved row
amplification and with preamp per pixel (PPP) structures currently
being manufactured by CIDTEC.
Figure 1: Structure of a CID-38 (a) and of an
individual pixel (b). Pixel boundaries are created by the field oxide
which reduces the electric field in the epitaxial region under the
polysilicon runs and creates pixels.
Integration of photogenerated holes occurs in the positively biased epitaxial region. Since the substrate is grounded, a reverse biased p-n junction is created inside of every pixel. This provides excellent antiblooming protection when overexposed since excess holes outside the well are swept through the junction into the substrate. Negative or slightly positive voltages on the column and row electrodes create depletion wells for storage of holes. In PPR devices such as the CID-38, columns are biased more negatively and holes collect under this electrode called the "collection pad".
Figure 2: Photon collection and the readout scheme of a single CID-38 pixel.
Two types of readout techniques are shown in Figure
2. During readout, a "zero level" is sensed on the sense pad along
the row by allowing the pad to float and digitizing. The sole charge
transfer from the collection well to the sense well is performed by
driving the column high and all stored charge moves to the sense
well. The amount of collected charge sensed on the row electrode
modulates the drain-source current of the output FET amplifier. In
non-destructive readout (NDRO) the low potential on the collection
pad is reestablished and accumulation continues. In destructive
readout (DRO), the pixel is injected.
Pixels along a row are read by shifting the column
register until the last pixel has been read. The column register is
then reset and the row register shifted to the next row. This row
read operation is repeated for all of the rows. This method of
readout differs from "true" random access. Any single CID-38 pixel
can be randomly read but the addressing of that pixel is achieved by
quickly shifting the row and column shift registers to get to the
pixel. A true random access array with multiplexing, the CID-35, is
under development and will be tested at RIT soon.
Pixel injection can be performed with two different
methods. The entire array can be cleared in one step with "global"
injection. This is done by simultaneously driving all columns and
rows to Vinj, the
prescribed injection voltage. Each pixel in the array then has both
pads biased such that holes are injected through the junction. The
other injection method removes only the charge in a subarray in a
method similar to array readout. In "subarray" injection, each pixel
is addressed sequentially as during readout and the injection voltage
is applied to the row electrode. Horizontal shifting during subarray
injection (0.2mm/pixel) is substantially faster than readout
(62mm/pixel).
The commercial CID control electronics, or SiCam, generate the readout and injection clocking sequences with a sensor control board. These clocks drive the shift registers on the periphery of the CID and produce the pixel readout and injection results described above. The electronics also perform dual slope sampling of the analog input and interface the digitized data to a PC.
3. PREAMPLIFIER DESCRIPTION
A modified preamplifier scheme was implemented to
further reduce readout noise. A two channel preamp design was
designed since the CID-38 has output FETs for both even and odd rows.
Each channel consists of three low noise operational amplifiers. The
first amplification stage consists of a current-to-voltage converter.
The second gain stage is a simple inverting amplifier configuration.
A third op-amp buffers the biasing voltage applied to the CID output
FETs. Feedback capacitors on the gain stages filter high frequency
above the maximum clocking frequency. Large ground planes and input
guard shielding were used in the design and construction of the
amplifier board.
The preamplifier design was validated with SPICE analysis. The DC Gain was simulated at 623 and the gain at 16.5kHz readout frequency is 580. The -3dB point for high frequency cutoff was determined to be about 100kHz.
4. EXPERIMENTAL SETUP
The layout for all of the experiments is shown in
Figure 3. An incandescent white light source is powered with a
stabilized voltage supply and focused onto a pinhole. The spatially
filtered light is wavelength bandpassed into the integrating sphere
which produces a uniform illumination field on the detector array. An
external shutter on the dewar provides control of integration times.
A close up of the devices inside the evacuated dewar is shown in the
insert. The CID package makes thermal contact against a cold finger
which keeps the array near liquid nitrogen temperatures (77K). The
pins of the CID make electrical connection to the fanout board which
is also held at
LN2 temperatures
to reduce thermal conductivity to the array. External clocks and bias
voltages are fed to the fanout board through fine wires and a
connector not shown in the figure. The fanout board contains no
active components.
The preamplifier is placed inside the dewar to reduce the length of the wires between the CID output pins and amplifier input. The op-amps are thermally connected to the dewar wall with metal based adhesive tape. The odd and even row output pins of the CID are connected to the input of the preamplifier with low resistance fine wire. The amplifier outputs are fed through a military connector in the dewar wall and supplied to the SiCam electronics for further amplified and digitization. The digital data is then transferred to a PC and displayed with modified commercial imaging software.
Figure 3: Physical layout of the experimental optics and electronics.
5. RESULTS
a. System and preamplifier noise and gain.
Figure 4: Photon transfer curve for the CID-38. Both noise and gain measurements were made at a pixel read rate of 16kHz.
The gain and noise of the system were characterized
with the photon transfer technique. Two frames of equal exposure are
subtracted and the resultant frame has no bias shift and any pattern
noise is removed. The average exposure level is plotted vs. the
variance read noise of a subarray in Figure 4.
The system gain, 1/g, and the read noise Nr can be determined with the equation below.
The gain was graphically determined with the
reciprocal of the regression slope. The read noise was determined
from the system gain and the intercept of the regression fit to the
data points. From the figure, the system gain and read noise are ~20
e-'s/ADU and
~170e-'s
respectively. This read noise figure is a 15% improvement over
previously published read noise figures of
~200e-'s5.
When not in operation, the output of the CID-38 can
be controlled with a simple hardware circuit and a software driven
TTL pulse. A pin on the package can be used to turn the output FETs
on and off. This enables temperature control of the op-amps by
reducing their power consumption. These modifications were determined
to be unnecessary since temperatures fluctuated by only 1K with the
op-amps thermally attached to the inside of the dewar wall.
b. Injection Efficiency.
The efficiency of an injection cycle depends on three key parameters shown in Figure 5. One variable is the injection voltage applied to the row and column electrodes. Voltages less than the positive biasing of the n-doped epitaxial region do not completely remove the depletion wells and not all of the charge is injected. Exploiting this procedure is called "pixel knockdown" and can be used to increase the effective dynamic range of the array.5 The second parameter for injection is the duration of the injection pulse. Longer pulses give holes more time to diffuse to the junction and be removed from the pixel. The third parameter is the number of injection pulses that are applied to the pixel. If a short pulse duration is chosen, many pulses can be applied to the pixel to fully clear the accumulated charge.
Figure 5: Representation of a complete injection
cycle. The delay time (~4ms) and minimum pulse width (~1ms) are
limiting factors due to the SiCam electronics and delays in software
processing.
A series of measurements were made to determine the efficiency of an injection cycle. The array is exposed to a level near full wells, S, and the digital values readout. The array is injected and another readout, sinj, is performed. The efficiency is determined by the following ratio.
The experiment was repeated for two different injection voltages and are shown in Figure 6 and Figure 7. From the plots, higher injection voltages remove charge much more efficiently than lower injection voltages as expected. Higher injection voltage is also more effective parameter in removing charge than longer injection pulses.
Figure 6: Plot of the injection efficiency for Vinj = 8.15V. The 'Injection Cycles' axis is not a direct correlation to injection time since the pulse duration and delay times vary for each of the curves.
Figure 7: Plot of the injection efficiency for
Vinj = 8.15V.
Again the 'Injection Cycles' axis in not a direct correlation to
injection time since the pulse duration and delay times vary for each
curve. The efficiency scale for both graphs is the same to show the
effects that injection voltage has on efficiency.
One goal of the experiments was to determine the best
combination of injection parameters for the fastest global injection
of the array. For example, Figure 6 shows that one 640ms injection
pulse completely clears the array. However, seven 2ms pulses (and 6
delay times,
tdelay) also
clears all of the charge and this procedure only takes a total of
38ms. A trade off situation exists between the number of pulses and
the pulse duration.
A table of the total injection times obtained by
experiment is shown below. From the table, the fastest way to totally
clear the array at 8.15V is with three 4ms injection pulses; the
total injection time then becomes 20ms. Similar experiments with a
8.10V injection voltage produced a best injection method of nine 3ms
pulses and a total injection time of 59ms. No final injection time is
available for
Vinj=8.10V:1ms
pulse duration since the array wasn't fully injected after many
cycles.
|
pulse duration |
640 |
320 |
160 |
80 |
40 |
20 |
10 |
5 |
4 |
3 |
2 |
1 |
|
8.15V Total Injection time |
640 |
320 |
160 |
80 |
40 |
44 |
24 |
23 |
20 |
24 |
26 |
61 |
|
8.10V Total Injection time |
640 |
644 |
324 |
248 |
172 |
116 |
80 |
68 |
60 |
59 |
62 |
--- |
The main limitation to the above measurements is the delay time
between pulses and the minimum pulse duration of 1ms. Both of these
are related to hardware limitations that are currently being
modified. It is thought that the total injection time at 8.15V can be
as small as 1ms with a modified system that removes these
limitations.
c. Low Light Level Linearity.
The linearity of a CID in low light situations was determined as follows. Incremental time exposures of low intensity were given to the array and the bias subtracted signal level is plotted vs. the exposure time in Figure 8.
Figure 8: A plot representing low light level linearity of CID
arrays. The lack of response in the ultra low exposure times is
classically referred to as reciprocity failure. The array was
determined to be >99% linear in the midrange of the A/D.
The array shows minimal reciprocity failure at ultra low light
levels. To find the optimal injection settings necessary to reduce
this effect, a series of measurements varying the three injection
variables were made. Three injection voltages, 8.15V, 8.125V, and
8.1V were used. Plots from these experiments were created and the
intercept of the regression line drawn from the linear region was
determined. From the regression intercepts, the "precharge" levels,
or number of photons necessary to overcome the insensitivity, were
determined and these values are shown in the table below.
|
Voltage |
8.15V |
8.15V |
8.15V |
8.15V |
8.15V |
8.125V |
8.125V |
8.10V |
|
Pulse Duration |
640ms |
320ms |
160ms |
4 ms |
1ms |
640ms |
320ms |
640ms |
|
Number of cycles |
1 |
1 |
1 |
5 |
20 |
1 |
1 |
1 |
|
precharge |
1671.177 |
1602.241 |
1718.562 |
1238.2 |
648.681 |
1836.417 |
2167.712 |
3521.274 |
Lower injection voltages have higher precharge levels. Different pulse durations used with a 8.15V injection voltage have little effect on the precharge level. The main improvement comes from using multiple injection pulses. The 300% improvement with multiple injection reduces the precharge level from ~1600 photons to ~650 photon. Again the limitation to more precharge reduction lies in our inability to create smaller pulse durations and delay times. These are hardware and software programmable and are being modified for further investigation.
6. CONCLUSION AND FUTURE WORK
All of the injections studies described above were performed with
global injection techniques. It is important to characterize the
performance of subarray injection as well. To do this and perform
different CID clocking schemes, a high speed Pulse Instruments based
array testing system has been acquired. The system supplies
electronics for biasing voltages, A/D conversion and a VMEbus
backplane interface. A DSP board with two 40Mhz C-40 DSPs is used to
produce the clocks. Preliminary tests with this system are under
way.
An application using the CID-38 to correct for the random image
motion of stellar objects through the atmosphere has been discussed.
A subarray on the CID is defined in software and used for object
tracking. Error signals are derived from motions of the object
centroid and sent to a fast steering mirror which compensates the
random motion induced by the atmosphere. The rest of the array
continues integration until the exposure is complete. The resultant
point spread functions of the objects in this field are sharper as a
result of the image correction.
A second application uses a CID as the imager in a photon counting
system. Different regions of the field of view contain both bright
and dim objects and a separate subarray is defined for each.
Subarrays containing brighter objects can be read faster than the
subarrays with dimmer objects. This type of control effectively
increases the dynamic range of the system. In addition to the Pulse
Instruments electronics described above, event detection and event
centroids are calculated by a custom VMEbus centroider board.
7. ACKNOWLEDGMENTS
The authors acknowledge support received from a NSF Support for
the Center for Electronic Imaging Systems grant. We would like to
thank Joe Carbone, Steve VanGordon and Jeff Zarnowski at CIDTEC and
Bob Slawson at RIT for their support with the above research and data
collection.
8. REFERENCES