- Email: [email protected]
A 4.2K CMOS optical detector
PII: S0011-2275(98)00080-0
Cryogenics 38 (1998) 943–945 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0011-2275/98/$—see front matter
A 4.2K CMOS optical detector E.A. Gutie´rrez-D*†, S.V. Koshevaya† and M.J. Deen‡ †National Institute for Astrophysics, Optics and Electronics INAOE, P.O. Box 216 and 51, Z.P. 72000, Puebla, Mexico ‡School of Engineering Science, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada
Received 3 April 1998 An integrated CMOS (Complementary Metal-Oxide-Semiconductor)-compatible optical detector for liquid-helium-temperature LHT (4.2K) operation is presented. The optical detector, which responds to 0.768- and 1.1-m wavelength light sources, is built in the n-well of a 0.7 m CMOS integrated circuit technology. The optical detection is based on the photogeneration of carriers in the frozen n-well that changes its output resistance at a ratio of 1.79 M⍀ per mW of optical power. The cryo-optical system has been tested with and without a built-in preamplification, and has been proved to respond to 50 MHz-optical pulses. 1998 Elsevier Science Ltd. All rights reserved Keywords: silicon; optical detector; low temperature
Introduction
The structure and experimental results
The electronics for cryogenic applications have been traditionally applied for infrared detection systems, and the CMOS technology has been used for implementing the readout electronics due to its excellent performance at cryotemperatures1. For infrared detection, Schottky-barrier detectors implemented with CMOS-compatible silicide materials, like CoSi2, and PtSi have been used2. This extensive research and development has given CMOS infrared detection systems a leading edge in the world of infrared detection. However, no similar effort has been made in the direction of visible-and-near IR-wavelength detection at 4.2K. Here, a full CMOS-compatible 4.2K optical detector, that operates in the 0.768–1.1 m wavelength, is introduced. No photodiodes or Schottky barrier detectors are used to detect light. Full advantage is taken of the excellent properties of the n-well as a photoconductor. The doping level used for the n-well allows good light absorption, and a time response in the order of 1 ns at 4.2K. Moreover, the n-well is interconnected to a 0.7 m n-MOS transistor that serves to drive the signal for pre-amplification. The cutoff frequency of the 0.7 m n-MOS transistor at room temperature is 7 GHz3, and estimated to be in the order of 12 GHz at 4.2K4 due to the increase in carrier mobility. This gives the chance to operate this system at a clock frequency of 1 GHz at 4.2K.
Experiments are described in which free electrons in the frozen n-well of a CMOS process, cooled down to 4.2K, are generated by photo-ionization of phosphorous donors. A semiconductor laser diode of wavelength = 0.768 m (red light) and output optical power Po = 3 mW (at a laser current Ilaser = 43 mA) is used to excite electrons from their frozen state to the conduction band. Under illumination, the photogeneration of electrons causes the current in the nwell resistor to increase from its dark value Idark = 1 pA up to about 2 A, giving a Iph/Idark ratio of 2 ⫻ 106. This also causes the value of the n-well resistance Rn to change from 15 M⍀ at Po = 1.2 mW, to 0.68 M⍀ at Po = 9.2 mW. We take advantage of the photoconductive properties of doped silicon at 4.2K and the low temperature operation of MOS transistors to create a CMOS-based optical detector, as shown in Figure 1. While this structure has not been optimized, it has proved to work efficiently for = 0.768 and 1.1 m wavelength light sources. The technological parameters of the structure are the following: the n-well junction Xjnwell = 1.8 m, the n-well doping Nnwell = 1 ⫻ 1017 cm−3, the gate oxide thickness Tox = 10 nm, and the field oxide, the one on top of the n-well region, Tfo = 650 nm. The current–voltage I–V characteristics of the n-well resistor isolated from the NMOS transistor were first measured, the results are shown in Figure 2. Note the excellent linearity of the I–V curves in the 0.0–0.4 V range which shows that the n-well behaves as an ohmic device. The onset of impact ionization occurs at the breakdown voltage VB ⬇ 0.68 V which gives an ionization electric field EI = 680 V cm−15. Nine different illuminations levels (96, 138, 188, 235, 282, 352, 480, 612, and 765 mW) were used to
*To whom correspondence should be addressed.
Cryogenics 1998 Volume 38, Number 9 943
A 4.2K CMOS optical detector: E.A. Gutie´rrez-D et al.
Figure 1 The structure of the CMOS optical detector. The nwell depth xj is 1.8 m, the n-well doping concentration Nwell is 1 ⫻ 1017 cm−3, the n-well length L is 10 m, the n-well width W is 70 m, the silicon oxide on top of the n-well is 600 nm thick, the gate oxide thickness Tox of the NMOS transistor is 10 nm, the gate length Lg is 0.7 m, the gate width Wg is 70 m, and the threshold voltage VT at T = 4.2K is 0.9 V
Figure 4 Measured output voltage Vo at the drain terminal of the NMOS transistor as a function of the laser optical power Po
For the Po range used here, the product n varies from 8 ⫻ 1014 to 9 ⫻ 1015 [1 cm−1V−1 s−1 ], and the photocurrent Iph as a function of Po changes as 1.409 A mW−1. Based on the measured ID –VD characteristics of the NMOS transistor operated at 4.2 K, RG = 0.68 M⍀, and a RD = 50 ⍀ were used. The measured output voltage Vo as a function of Po is shown in Figure 4. Note the good Vo –Po linearity in the 2–6 mW power range. The n-well resistance Rnw, the voltage drop at the gate VG, and the drain current ID through the MOS transistor, are shown in Figure 5 as a function of the optical power Po. The Rnw –Po relationship is not linear, but the Vo –Po relationship is linearized in the 2–6 mW range through the VG dependence of ID (see Equation (1)). Vo = (VDD − ID(VG )·RD ) Figure 2 Experimental photocurrent I versus voltage V ( V2 – V1 ) for different levels of light illumination. The continuos lines are linear fittings. Plot 1 corresponds to the lowest illumination level while plot 9 corresponds to the highest one
(1)
radiate the n-well and this caused the value of the n-well resistor Rn to change from 15 M⍀ to 0.68 M⍀. We can determine from the linear part of I–V in Figure 2 the resistance Rn, from which the product ( xn) carrier mobility ( ) and electron concentration (n) is calculated. Since the product ( n) is a linear function of the laser optical power Po, then this suggests the carrier mobility is constant and independent of the photogeneration ratio G (see Figure 3).
Once the detector configuration was proven to operate at dc, a pulsed voltage Vlaser was applied to the laser with a frequency fL = 50 MHz, and the results are shown in Figure 6. From these results, we extracted a response time in the order of 15 ns (fC = 66 MHz). However, this is the frequency response of the circuit and not the frequency response of the n-well detector itself fD. The frequency response of the circuit is determined by the distributed shunt capacitance Cp of the wiring connections, the ac value of the detector resistance Rn, and the resistance RG. The cut-off frequency fT of the MOS transistor is in the order of 7 GHz for a 0.7 m transistor3 at 300K, and of about
Figure 3 Experimental detector resistance R and product carrier mobility–electron concentration ( n) versus optical power P
Figure 5 Measured n-well resistance Rnw, gate voltage VG, and drain current ID, versus output optical power Po
944
Cryogenics 1998 Volume 38, Number 9
A 4.2K CMOS optical detector: E.A. Gutie´rrez-D et al.
Conclusion A 4.2K CMOS compatible optical detector is proposed. The structure is proven to detect light in the 0.768 to 1.1 m wavelength region at a speed of 15 ns, response time that, according to Equation (2), can be improved down to about 1 ns for the particular case of a this 0.7 m CMOS technology. The photodetection response of the n-well resistance is linearized through the current-to-voltage conversion of the nMOS transistor that acts as a buffer. This makes this structure fully compatible with CMOS integrated circuit technology. This research is significant because to date, there has not been any work done on fully CMOS compatible optical detectors for 4.2K operation. Figure 6 Measured output voltage Vo as function of time t with the laser voltage VL pulsed at a frequency fL = 50 MHz
12 GHz at 4.2K4. By using a properly designed preamplifier, the internal capacitances can be reduced and the circuit frequency can be increased up to the point when it becomes limited by the response time of the n-well detector D. This D time depends on the excess carrier lifetime , and is inversely proportional to the number of ionized impurity atoms which can be lowered to about 1.0 ns6 giving a possible frequency response up to 1 GHz. A model for the response time is given by the following equation6.
=
1 BTND − AI(ND − NA )
(2)
where BT is the coefficient for the rate of capture of electrons by the empty donor centers, AI is the coefficient for the rate at which impurity centers are ionized by impact of free carriers, ND is the donor doping level, and NA is the acceptor doping level. BT and AI have values of 10−10 cm3 s−1, and 10−8 cm3 s−1, respectively. ND in our case is the n-well doping level, which is equal to 1017 cm−3, and NA is about 1016 cm−3. Substituting these values back into Equation (2), one gets a response time t in the order of 1 ns.
Acknowledgements E. A. Gutie´rrez-D., wishes to acknowledge the Interuniversity MicroElectronics Center IMEC, Leuven, Belgium for providing the device samples used, and the National Council for Science and Technology CONACyT-Mexico, for providing the financial support for this work.
References 1. Seinaeve, J., Dierickx, B., Scheffer, D. and Alaerts, A., Performance of the cryogenic readout amplifiers for FIRST’s stressed Ge:Ga array. In Electrochemical Society Proceedings, Low Temperature Electronics and High Temperature Superconductivity IV, Montreal, Canada, May 4–11 1997, Vol. 97-2, pp. 358–368. 2. Akiyama, A., et al., 1040 ⫻ 1040 infrared charge sweep device imager with PtSi Schottky-barrier detectors. Optical Engineering, 1994, 33(1), 64–71. 3. Murphy, R.S.A., Prospects of the MOS transistor as a high frequency device. PhD dissertation, INAOE, Puebla, Mexico, 1997. 4. Gutie´rrez-D, E.A., Claeys, C., Simoen, E. and Koshevaya, S.V., Perspectives of the cryo-electronics for the year 2000. In IEEE Workshop on Low Temperature Electronics WOLTE, San Miniato, Italy, 24–26 June 1998, pp. 315–320. 5. Gutie´rrez-D, E.A., Deferm, L. and Declerck, G., Selfheating effects in silicon resistors operated at cryogenic ambient temperatures. SolidState Electronics, 1993, 36(1), 41–52. 6. Putley, E.H., Far infra-red photoconductivity. Physical States of Solids, 1964, 6, 571–614.
Cryogenics 1998 Volume 38, Number 9 945
Cryogenics 38 (1998) 943–945 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0011-2275/98/$—see front matter
A 4.2K CMOS optical detector E.A. Gutie´rrez-D*†, S.V. Koshevaya† and M.J. Deen‡ †National Institute for Astrophysics, Optics and Electronics INAOE, P.O. Box 216 and 51, Z.P. 72000, Puebla, Mexico ‡School of Engineering Science, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada
Received 3 April 1998 An integrated CMOS (Complementary Metal-Oxide-Semiconductor)-compatible optical detector for liquid-helium-temperature LHT (4.2K) operation is presented. The optical detector, which responds to 0.768- and 1.1-m wavelength light sources, is built in the n-well of a 0.7 m CMOS integrated circuit technology. The optical detection is based on the photogeneration of carriers in the frozen n-well that changes its output resistance at a ratio of 1.79 M⍀ per mW of optical power. The cryo-optical system has been tested with and without a built-in preamplification, and has been proved to respond to 50 MHz-optical pulses. 1998 Elsevier Science Ltd. All rights reserved Keywords: silicon; optical detector; low temperature
Introduction
The structure and experimental results
The electronics for cryogenic applications have been traditionally applied for infrared detection systems, and the CMOS technology has been used for implementing the readout electronics due to its excellent performance at cryotemperatures1. For infrared detection, Schottky-barrier detectors implemented with CMOS-compatible silicide materials, like CoSi2, and PtSi have been used2. This extensive research and development has given CMOS infrared detection systems a leading edge in the world of infrared detection. However, no similar effort has been made in the direction of visible-and-near IR-wavelength detection at 4.2K. Here, a full CMOS-compatible 4.2K optical detector, that operates in the 0.768–1.1 m wavelength, is introduced. No photodiodes or Schottky barrier detectors are used to detect light. Full advantage is taken of the excellent properties of the n-well as a photoconductor. The doping level used for the n-well allows good light absorption, and a time response in the order of 1 ns at 4.2K. Moreover, the n-well is interconnected to a 0.7 m n-MOS transistor that serves to drive the signal for pre-amplification. The cutoff frequency of the 0.7 m n-MOS transistor at room temperature is 7 GHz3, and estimated to be in the order of 12 GHz at 4.2K4 due to the increase in carrier mobility. This gives the chance to operate this system at a clock frequency of 1 GHz at 4.2K.
Experiments are described in which free electrons in the frozen n-well of a CMOS process, cooled down to 4.2K, are generated by photo-ionization of phosphorous donors. A semiconductor laser diode of wavelength = 0.768 m (red light) and output optical power Po = 3 mW (at a laser current Ilaser = 43 mA) is used to excite electrons from their frozen state to the conduction band. Under illumination, the photogeneration of electrons causes the current in the nwell resistor to increase from its dark value Idark = 1 pA up to about 2 A, giving a Iph/Idark ratio of 2 ⫻ 106. This also causes the value of the n-well resistance Rn to change from 15 M⍀ at Po = 1.2 mW, to 0.68 M⍀ at Po = 9.2 mW. We take advantage of the photoconductive properties of doped silicon at 4.2K and the low temperature operation of MOS transistors to create a CMOS-based optical detector, as shown in Figure 1. While this structure has not been optimized, it has proved to work efficiently for = 0.768 and 1.1 m wavelength light sources. The technological parameters of the structure are the following: the n-well junction Xjnwell = 1.8 m, the n-well doping Nnwell = 1 ⫻ 1017 cm−3, the gate oxide thickness Tox = 10 nm, and the field oxide, the one on top of the n-well region, Tfo = 650 nm. The current–voltage I–V characteristics of the n-well resistor isolated from the NMOS transistor were first measured, the results are shown in Figure 2. Note the excellent linearity of the I–V curves in the 0.0–0.4 V range which shows that the n-well behaves as an ohmic device. The onset of impact ionization occurs at the breakdown voltage VB ⬇ 0.68 V which gives an ionization electric field EI = 680 V cm−15. Nine different illuminations levels (96, 138, 188, 235, 282, 352, 480, 612, and 765 mW) were used to
*To whom correspondence should be addressed.
Cryogenics 1998 Volume 38, Number 9 943
A 4.2K CMOS optical detector: E.A. Gutie´rrez-D et al.
Figure 1 The structure of the CMOS optical detector. The nwell depth xj is 1.8 m, the n-well doping concentration Nwell is 1 ⫻ 1017 cm−3, the n-well length L is 10 m, the n-well width W is 70 m, the silicon oxide on top of the n-well is 600 nm thick, the gate oxide thickness Tox of the NMOS transistor is 10 nm, the gate length Lg is 0.7 m, the gate width Wg is 70 m, and the threshold voltage VT at T = 4.2K is 0.9 V
Figure 4 Measured output voltage Vo at the drain terminal of the NMOS transistor as a function of the laser optical power Po
For the Po range used here, the product n varies from 8 ⫻ 1014 to 9 ⫻ 1015 [1 cm−1V−1 s−1 ], and the photocurrent Iph as a function of Po changes as 1.409 A mW−1. Based on the measured ID –VD characteristics of the NMOS transistor operated at 4.2 K, RG = 0.68 M⍀, and a RD = 50 ⍀ were used. The measured output voltage Vo as a function of Po is shown in Figure 4. Note the good Vo –Po linearity in the 2–6 mW power range. The n-well resistance Rnw, the voltage drop at the gate VG, and the drain current ID through the MOS transistor, are shown in Figure 5 as a function of the optical power Po. The Rnw –Po relationship is not linear, but the Vo –Po relationship is linearized in the 2–6 mW range through the VG dependence of ID (see Equation (1)). Vo = (VDD − ID(VG )·RD ) Figure 2 Experimental photocurrent I versus voltage V ( V2 – V1 ) for different levels of light illumination. The continuos lines are linear fittings. Plot 1 corresponds to the lowest illumination level while plot 9 corresponds to the highest one
(1)
radiate the n-well and this caused the value of the n-well resistor Rn to change from 15 M⍀ to 0.68 M⍀. We can determine from the linear part of I–V in Figure 2 the resistance Rn, from which the product ( xn) carrier mobility ( ) and electron concentration (n) is calculated. Since the product ( n) is a linear function of the laser optical power Po, then this suggests the carrier mobility is constant and independent of the photogeneration ratio G (see Figure 3).
Once the detector configuration was proven to operate at dc, a pulsed voltage Vlaser was applied to the laser with a frequency fL = 50 MHz, and the results are shown in Figure 6. From these results, we extracted a response time in the order of 15 ns (fC = 66 MHz). However, this is the frequency response of the circuit and not the frequency response of the n-well detector itself fD. The frequency response of the circuit is determined by the distributed shunt capacitance Cp of the wiring connections, the ac value of the detector resistance Rn, and the resistance RG. The cut-off frequency fT of the MOS transistor is in the order of 7 GHz for a 0.7 m transistor3 at 300K, and of about
Figure 3 Experimental detector resistance R and product carrier mobility–electron concentration ( n) versus optical power P
Figure 5 Measured n-well resistance Rnw, gate voltage VG, and drain current ID, versus output optical power Po
944
Cryogenics 1998 Volume 38, Number 9
A 4.2K CMOS optical detector: E.A. Gutie´rrez-D et al.
Conclusion A 4.2K CMOS compatible optical detector is proposed. The structure is proven to detect light in the 0.768 to 1.1 m wavelength region at a speed of 15 ns, response time that, according to Equation (2), can be improved down to about 1 ns for the particular case of a this 0.7 m CMOS technology. The photodetection response of the n-well resistance is linearized through the current-to-voltage conversion of the nMOS transistor that acts as a buffer. This makes this structure fully compatible with CMOS integrated circuit technology. This research is significant because to date, there has not been any work done on fully CMOS compatible optical detectors for 4.2K operation. Figure 6 Measured output voltage Vo as function of time t with the laser voltage VL pulsed at a frequency fL = 50 MHz
12 GHz at 4.2K4. By using a properly designed preamplifier, the internal capacitances can be reduced and the circuit frequency can be increased up to the point when it becomes limited by the response time of the n-well detector D. This D time depends on the excess carrier lifetime , and is inversely proportional to the number of ionized impurity atoms which can be lowered to about 1.0 ns6 giving a possible frequency response up to 1 GHz. A model for the response time is given by the following equation6.
=
1 BTND − AI(ND − NA )
(2)
where BT is the coefficient for the rate of capture of electrons by the empty donor centers, AI is the coefficient for the rate at which impurity centers are ionized by impact of free carriers, ND is the donor doping level, and NA is the acceptor doping level. BT and AI have values of 10−10 cm3 s−1, and 10−8 cm3 s−1, respectively. ND in our case is the n-well doping level, which is equal to 1017 cm−3, and NA is about 1016 cm−3. Substituting these values back into Equation (2), one gets a response time t in the order of 1 ns.
Acknowledgements E. A. Gutie´rrez-D., wishes to acknowledge the Interuniversity MicroElectronics Center IMEC, Leuven, Belgium for providing the device samples used, and the National Council for Science and Technology CONACyT-Mexico, for providing the financial support for this work.
References 1. Seinaeve, J., Dierickx, B., Scheffer, D. and Alaerts, A., Performance of the cryogenic readout amplifiers for FIRST’s stressed Ge:Ga array. In Electrochemical Society Proceedings, Low Temperature Electronics and High Temperature Superconductivity IV, Montreal, Canada, May 4–11 1997, Vol. 97-2, pp. 358–368. 2. Akiyama, A., et al., 1040 ⫻ 1040 infrared charge sweep device imager with PtSi Schottky-barrier detectors. Optical Engineering, 1994, 33(1), 64–71. 3. Murphy, R.S.A., Prospects of the MOS transistor as a high frequency device. PhD dissertation, INAOE, Puebla, Mexico, 1997. 4. Gutie´rrez-D, E.A., Claeys, C., Simoen, E. and Koshevaya, S.V., Perspectives of the cryo-electronics for the year 2000. In IEEE Workshop on Low Temperature Electronics WOLTE, San Miniato, Italy, 24–26 June 1998, pp. 315–320. 5. Gutie´rrez-D, E.A., Deferm, L. and Declerck, G., Selfheating effects in silicon resistors operated at cryogenic ambient temperatures. SolidState Electronics, 1993, 36(1), 41–52. 6. Putley, E.H., Far infra-red photoconductivity. Physical States of Solids, 1964, 6, 571–614.
Cryogenics 1998 Volume 38, Number 9 945