Test of Low Doped SBRC256x256 InSb array with cryogenic operational amplifiers in IRATEC

G. Finger, M. Meyer and G. Nicolini

European Southern Observatory

Karl Schwarzschild Str. 2, 85748 Garching b. München, Germany.

Abstract

This test was is performed to investigate the feasibility of CMOS amplifiers (TLC 27481) at cryogenic temperatures for use with InSb FPA's. The advantage of cryogenic amplifiers is the possibility to place them close to the FPA. The video signal is amplified directly at the detector. In ISAAC the long lines (~ 50 cm) passing the video signal through two radiation shields to the area of room temperature which is required for bipolar amplifiers are susceptible to noise pick-up since lines for stepper motors are close to the video line.

Two main problems have to be solved for using cryogenic amplifiers. First, the power dissipation of 180 mW is sufficient to heat up the detector from 30 K to 60 K because of the extremely small cooling power available at the detector (~ 1 mW). Second, the CMOS amplifiers will freeze out at temperatures below 40 K. Additional disadvantages of the cryogenic amplifiers are higher noise and smaller bandwidth.

The concept to overcome these problems was to introduce a high thermal impedance between detector and detector board using 10 cm long and 80 mm thick manganin wires.

1. Cryogenic configuration

Many cooldowns have been performed attempting to reach the required operating temperature (around 30 K) for the InSb detectors.

Using the original ISAAC detector mount was not possible to cool the detector to the operating temperature. The following reasons have been identified for the poor cryognic performance of the detector in ISAAC:

1. Little cooling power available at the cold finger (also due to copper wiring of the temperature control)

2. Low thermal impedance between the detector board and the cold structure (due to the copper flex boards for the detector)

We tried to solve the problems in three ways:

3. Increasing the thermal coductivity between the cold finger and the detector

4. Increasing the thermal ipedance between the detector board and the cold structure. Manganin hand made extension cables (AKA manganin spaghetti) have been used to extend the copper flex boards. The copper wireing of the temperature control was replaced by manganin.

5. Increasing the thermal ipedance between the detector board and the detector socket. 80 micron diameter and 10 cm long manganin wires (AKA Manganin thin wires) have been used between the socket pins and the detector board pads.

In two special cooldowns the detector temperature was measured for exactly the same conditions the only difference being the temperature control wireing which was connected in the first cooldown and disconnected in the second. The detector temperature was reduced from 110 K to 64 K by disconnecting the temperature control.

With a combination of 3) and 5) we have obtained the best results. Only used pins of the socket are wired. The detector board temperature was measured at the fixation of one flexible board connecting the board with the connector ring which is at instrument temperature. The flexible boards between this ring and the vacuum connectors were not thermalized.

If the cryogenic amplifier is switched on the temperature of the detector board is increased from 100 K to 125 K (third curve from bottom in figure 1). The temperature of the on-chip detector temperature sensor increases from 27 K to 35 K and the temperature of the cold finger increases from 24.6 K to 27 K. The power dissipation of all cryogenic amplifiers is 180 mW.

In order to get a feeling for the photon flux at the detector emitted by different parts of the instrument being at different temperatures the photon flux is calculated for typical temperatures and field of views as shown in table 1. The wavelength range l=1 to 5.45 mm is the sensitive range of the InSb 256x256 array which has a pixel size of 30 mm. The case f/0 is equivalent to a detector field of view of 2p sterradians, f/4.72 is the objective L2 of ISAAC, f11 is the Offner camera of IRATEC and f/10000 is a radiation leak of 100 mm diameter in a distance of 1 m from the detector.

The temperature increase of the detector board from 100K to 125K in figure 1 was caused by switching on the cryogenic amplifiers. If the detector would receive radiation from the detector board from a solid angle of 2p the photon flux would increase from 1462 photons/sec to 3.66E5 photons/sec and for an f/11 beam from 3 photons/sec to 755 photons/sec as can be read from table 1. Radiation leaks of 100mm diameter looking at room temperature targets generate a photon flux of several hundred photons or more. If the detector board remains at a temperature of 125 K it has to be baffled to better than f/100.

2. Pixel transfer function

The PTF was calibrated by the usual shot noise measurement. The flux observed in filter K is varied by scanning the blackbody temperature from 10C to 120 C. The signal is plotted versus the variance of the signal. The conversion factor electrons/ADU.

The gain of the signal train is determined by keeping the reset switch of the detector unit cell permanently closed. Applying uncorrelated sampling the signal is taken as function of VDDUC which is the supply voltage of the unit cell source follower of the detector pixel. 1 ADU corresponds to 26 mV at the integrating node capacitance as can be seen in figure 3.

The full range of the ADC is +/- 2.5 Volt. The gain of the acquisition chain including the gain of the on chip source follower is . Since the gain of the cryogenic amplifier is 4 the gain of the two stage on chip source follower is concluded to be 0.73 which is a reasonable value.

The capacity of the integrating node can be determined by to be 61 fF which is exactly the same value determined in a completely different setup previously.

Figure 3: Gain calibration : 25.92 microV/ADU. Gain is 2.943. Gain of on chip source follower is 0.73

3. Analog bandwidth CMOS operational amplifier at cryogenic temperatures

Since the risetime and falltime of the instrumentation amplifier on the ADC board is 300 nsec and 1 msec for the cryogenic amplifiers the bandwidth is limited by the detector to 2.1 microsec. The shortest read-out time for uncorrelated sampling will be limited by the detector to 60 msec,

4. Quantum efficiency

The overall efficiency including the entrance window and the optical efficiency of the camera but excluding the filter efficiency is 70%. The measured signal flux is plotted versus calculated photon flux in figure 5.

5. Darkcurrent

The darkcurrent was measured for three conditions. The upper curve in figure 6 was taken with the on chip temperature sensor switched on, the lower curve with the temperature sensor switched off. A third measurement taken after the power for the detctor was switched off for 12 hours did not show a reduction of darkcurrent.

Figure 6: Darkcurrent: Upper curve: diode for on-chip temperature measurement on. Lower curve: diode for on-chip temperature measurement off.

6. Noise

The rms noise was measured for simple double correlated sampling using the dark position in the filter wheel. 32 integrations of 265 msec have bee taken to calculate an rms noise of 61 e rms. There is probably still emf pick-up.

Figure 7: Noise for double correlated sampling with filter dark and dit=265 msec. Rms noise 61 electrons.

7. Cutoff wavelength

The MONOSPEC 27 monochromator was used to determine the cutoff wavelength of the detector. The monochromator was calibrated by the l=632.8 nm HeNe laser line in 0 to 12th order. The accuracy of the wavelength calibration is 4 nm show as bar on the right side of the transmission peak of figure 8.

Figure 8: Transmission of narrow band filter M in IRATEC measured with monochromator (black curve) and with Fourier Transform interferometer (red curve)

A blackbody at 150 C was the radiation source illuminating the entrance slit of the monochromator. The exit slit was re-imaged to the telescope focus of the test camera. To check the calibration the transmission curve of the narrow band M filter of the test camera was measured by stepping the grating across the spectral bandpass of the narrow band filter in steps of Dl = 10 nm. The signal was the integrated intensity at the exit slit taking the difference with the shutter between the blackbody and the entrance slit in the open and closed position. A cold neutral density filter having a transmission of t=0.01 reduced the flux. The integration time was 75 msec and the read-out mode was uncorrelated sampling. The transmission spectrum is shown as solid line in figure 8 and compared with the transmission spectrum measured with the Fourier spectrometer (dashed line). The quality of the wavelength calibration is sufficient

The cutoff wavelength of the detector was measured in a similar way but using a neutral density filter of t=0.001 to further reduce the flux, since the open position in the second filter wheel has to be used. The cutoff wavelength is l c = 5.45 microubm.

Figure 9: Cutoff wavelength lambdac of InSb at 30 K. Lambdac is 5.45 mirometer.

The monochromator can be controlled by observing templates using the command:

gpioCmd "set grating 2.42"

set wvelenght to 2.42 µm

The wavlength is written in the fits keyword INS.MONOCHRO in units of nanometers (use command gpioCmd("get status"))

8. Conclusions

All copper wireing of the InSb detector has to be replaced by manganin wires including also the wireing of the temperature control. Thermal heatsinking of the detector has to be improved by minimizing the number of mechanical interfaces and carefully annealing the materials used.

Cryogenic amplifiers have been demonstrated to work for InSb arrays. A careful tradeoff has to be made for the decision which concept to implement for the ALADDIN arrays.

It would still be the best solution to use bipolar amplifiers at room temperature. Therefore, the mechanical and cryogenic design has to focus on the detector requirements. Sufficient cooling power has to be provided and the mechanical design has to reduce the cable length to a minimum which is required to reach the room temperature amplifier. For service purposes the amplifier should be placed outside the vessel.