# Presentation: 805466

## Proton Radiation Damage in P-Channel CCDs Fabricated on High-Resistivity Silicon.

- C. Bebek, D. Groom, S. Holland, **A. Karcher**, W. Kolbe, J. Lee, M. Levi, N. Palaio, B. Turko, M. Uslenghi, M. Wagner, G. Wang
*Lawrence Berkeley National Laboratory*

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**Notes:**
Our group has fabricated p-channel CCDs on high resistivity silicon at LBNL.
The names listed work both on manufacture, development and on evaluation of CCDs

## Overview

- High-Resistivity CCDs
- Performance Parameters
- Irradiation
- Results
- Comparison to Conventional CCDs
- Conclusion

## High-Resistivity CCDs

**Notes:**
All CCD’s are based on the same principle:
Charge is collected and confined in pixels created by channel stop implants in one direction and the electrostatic potential of the vertical clocks in the other dimension.
During readout the Vertical clocks move an entire row of charge into the horizontal register. The horizontal clocks move the charge so that it can be read pixel by pixel at the output transistor.

## High-Resistivity CCDs

**Notes:**
The difference between our CCDs and conventional CCDs is not merely the fact that we use n-type silicon (while conventional CCDs use p-type), but the fact that we use high resistivity silicon.
While the conventional CCD relies on the inherent potential of the junction to create a depletion zone of 20 um or less, we are able to supply an external bias voltage to fully deplete a 300 um thick CCD.
This does not only have advantages in production yield (since we don’t have to thin down a chip to less than paper thickness), but also in detection efficiency in the infrared.
Red light just passes through the active volume of the conventional CCD mostly undetected, while it is absorbed and detected in the high resistivity CCD.

## Performance Parameters

- Charge Transfer Efficiency (CTE)
- CTE becomes critical in large CCDs.
- Smaller CCDs would mean higher complexity in mosaic cameras.
- Radiation damage reduces CTE, making radiation tolerant CCDs essential for extended space missions.
- Dark Current
- Dark current can limit the usefulness of long exposures.
- It is a volume effect, stemming from thermal generation in the bulk silicon.
- High purity silicon and low operating temperatures along with a gettering process reduce dark current.

**Notes:**
After we have seen the inherent advantages of the high resistivity CCD, we will now look at the performance and especially it’s degradation due to radiation damage.
Firstly there is the charge transfer efficiency. This is the fraction of charge transferred successfully from one pixel to the next during readout.
For large Devices CTE should deviate from 1 only in the 4rth or 5th place to give a high contrast image after 
thousands of pixel shifts.
Resorting to smaller CCDs would mean much higher complexity, especially in large area mosaic imagers.
Since CTE is strongly affected by radiation damage, radiation tolerant CCDs are essential for extended space missions.
Secondly there is dark current.
Dark current can limit the maximal useful exposure length, since sqr(n) noise reduces S/N ratio.
Dark current on modern CCDs is mostly a volume effect, stemming from thermal generation in the bulk silicon. Since the depleted volume on our CCDs is much bigger measures had to be taken to minimize dark current.
High purity silicon, lower operating temperatures and a proprietary gettering process reduce dark current to levels as good or better than conventional CCDs

## Irradiation

- 2 sets of 4 CCDs each were irradiated.
- One set incorporated an additional channel implant, reducing lateral charge movement to reduce CTE degradation.
- Applied doses of 5x109, 1x1010, 5x1010 and 1x1011 protons/cm2 at 12 MeV.
- The highest dose corresponds to 300 years in a high earth orbit!
- Devices were irradiated at room temperature without power.

**Notes:**
We irradiated 2 sets of 4 CCDs each at the 88” cyclotron at LBL.
One set incorporated an additional channel implant which reduced the amount of space accessible to low-signal charge packets within a pixel. This is a proven method to reduce CTE degradation, since the charge packets do not see damage outside the implant region.
We irradiated the CCDs using 12 MeV protons. We chose doses of 5x109, 1x1010, 5x1010 and 1x1011 protons/cm2 to cover even severe radiation exposure, such as encountered during extended space missions.
The devices were exposed to radiation at room temperature without any applied voltages.

## CTE Measurement

**Notes:**
We measure CTE by irradiating the CCD with X-rays from an 55Fe source.
To calculate parallel CTE a stacking plot is used: the x-ray peak height vs row number is plotted, so that all x-rays from a single row are “stacked” into one column on the plot.
The most common x-ray charge as a function of row number can easily be seen. We obtain the CTE value from the slope of the fitted line.

## CTE Results

- CTE is measured using the 55Fe X-ray method at 128 K. The readout speed is 30 kHz, the X-ray density is 0.015/pixel.

**Notes:**
This graph shows the result of the CTE degradation for the set of four CCDs that did not have the additional radiation hardening implant.
Data was taken at 128 K, with a readout speed of 30 kHz, and a X-ray density of 0.015/pixel.
The dependence on these parameters is discussed later. 

## Improved Radiation Tolerance on “notch” implant devices

**Notes:**
Here we can see the parallel CTE from the last page compared to the improved CTE due to the “notch” implant.
An improvement in radiation tolerance of about 75% can be seen.
Only the parallel CTE was plotted since the vertical register remains unchanged, and therefore the serial CTE is not expected to change.
However since we don not yet have data on large number of CCDs at the same dose, we do not know how much individual devices vary in radiation tolerance.

## CTE Dependence on Temperature

**Notes:**
The CTE vs Temperature plot for the highest dose CCD is shown, since the features are most clearly visible. The line was added to guide the eye.
While the CTE is a function of temperature, readout speed and X-ray density, Only the serial CTE changes appreciably over the operation regime of our CCDs (from 130 to 160K). 
At high temperatures the detrapping time is much shorter than clock overlap, giving very good CTE. 
At low temperatures the traps remain saturated in the serial register. This effect is directly dependent on the X-ray density (about 1 every 70 pixel).
The parallel transfer is too slow to benefit from trap saturation.
This plot shows the CTE degradation in pessimistic light.
Faster readout, or a background charge (which is present in any sky image) would improve CTE.

## Dark Current Degradation

**Notes:**
The dark current did increase with radiation exposure as expected, however for 1e10 protons/cm^2 and below no change could be observed. Only at the two highest doses a marked increase in dark current was visible.
Even at the highest dose the dark current remained low enough so that the shot noise contribution for a 30 minute exposure does not exceed the read noise.

## Dark Current vs Temperature

**Dark Current vs Temperature**

**for CCD after 5x10**

**9**

** ****protons/cm**

**2**

- 0.1

- 1

- 10

- 100

- 1000

- 10000

- 100000

- 50

- 60

- 70

- 80

- 90

- 100

**1/kT (eV)**

**Dark Current (e**

**-**

**/h)**

**e****-0.609 eV/kT**

- 208 K

- 158 K

**Notes:**
The graph clearly shows the exponential increase of dark current at warmer temperatures.
At very low temperatures bulk dark current does not seem to be the main contribution, we therefore observe a saturation effect.
The activation energy for midgap levels can be extracted by fitting a line to the higher temperature points.

## Comparison to Conventional CCDs

- [1]L.Cawley, C.Hanley, “WFC3 Detector Characterization Report #1: CCD44 Radiation Test Results*,” *Space Telescope Science Institute Instrument Science Report WFC3 2000-05, Oct.2000
- [2] T. Hardy, R. Murowinski, M.J. Deen, “Charge transfer efficiency in proton damaged CCDs,” IEEE Trans. Nucl. Sci., 45(2), pp. 154-163, April 1998

**Notes:**
To compare the CTE degradation in our CCDs to results found in literature, obtained at different proton energies we choose the non ionizing energy loss (niel) as a measure of damage. The niel data for various proton energies can be found on the NIST website using the PSTAR calculator.
We observe a change in CTE of:
CTE=2.5x10-13 g/MeV (Standard LBNL devices)
CTE=9.6x10-14 g/MeV  (“Notch” implant LBNL devices)
In Literature we find:
CTE=1.9x10-12 g/MeV
for current CCD designs [1].

## Conclusion

- P-channel high-resistivity CCDs show remarkable radiation tolerance against CTE degradation.
- Dark current remains low even after proton doses equivalent to decades in space.

**Notes:**
The considerable radiation tolerance of the high resistivity p-channel CCDs has been demonstrated.
We also found that the dark current is low enough, to allow any reasonable exposure time, even after extensive irradiation.