- Email: [email protected]
Comprehensive measurements of GaAs pixel detectors capacitance
Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
Comprehensive measurements of GaAs pixel detectors capacitance M. Cariaa,b, L. Barberinia, S. D’Auriac, A. Laib, P. Randaccioa, S. Cadeddua,b,* a
Dipartimento di Fisica, Universita" di Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy b INFN Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy c Department of Physics and Astronomy, University of Glasgow, UK
Abstract We have studied GaAs pixel detectors on semi-insulating wafers with Schottky contacts. We performed comprehensive measurements on the inter-pixel and capacitance to back plane. Being semi-insulating, the behaviour is totally different with respect to other common semiconductors, such as high resistivity silicon. Non-homogeneities are also an issue, due to both the contacts and the crystal bulk. In order to detect them and their influence on capacitance, we undertook systematic measurements with different configurations of the measuring electrodes. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Gallium arsenide; Pixel; Capacitance; Imaging; Semiconductors
1. Introduction Pixel detectors made from materials alternative to high resistivity silicon are relatively little known. Large communities are now undertaking the design of radiation detection systems making use of pixel detectors for a wide variety of applications, from medical imaging apparatus to space and nuclear particle detectors [1]. A material of more and more widespread use for this purpose is gallium arsenide. Being relatively new in the field of radiation detectors, there are no standardised processes for bulk growth and contact implantation. The characteristics of these materials may vary greatly. In particular, the leakage current and the capacitance to bulk and inter-pixel, which in *Corresponding author. E-mail addresses: [email protected] (M. Caria), [email protected] (S. Cadeddu).
turn depend on the bulk composition, may be variably affected. We studied pixel detectors processed on semi-insulating (LEC) gallium arsenide with Schottky contacts [2,3]. In order to implement the detectors in full imaging devices, one needs electronic Read Out Integrated Circuits (ROIC). The design of these is crucially dependent on the electrical characteristics of the detectors which must be known with good accuracy. Among the characterisation measurements, the one on inter-pixel and bulk capacitance has never been performed systematically due to experimental difficulties and limited reliability of current technology. Such measurements are essential for read-out scheme performances. In an amplifier, read-out mode of the low noise-type being designed for imaging applications [4–6], the interpixel capacitance will be a contribution to the noise at the input.
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 7 9 2 - 2
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
Besides standard IV characterisation, we collected measurements on the inter-pixel and bulk capacitance. Being semi-insulating, the behaviour is totally different with respect to more standard high resistivity silicon. The tests are also meant to identify the relevance of contact and bulk nonhomogeneity. In order to detect that and its influence on capacitance, we undertook systematic measurements in different configurations of the measuring electrodes. We found dependence on the measuring set-up and position. Pixels behave differently if they are on the border or in the middle of the matrix. We quantified these variations and their spread so as to allow suitable design of the analogic circuitry. This is the first time that a pixel detector on GaAs is fully characterised in terms of its capacitance characteristics [7,8]. We show that devices built with GaAs are of limited reliability in the construction of imaging devices due to relatively immature technology, but the results are encouraging and allow the design and construction of a full system with integrated electronics.
2. Detector devices We characterized custom-designed devices processed by Alenia (Alenia Marconi Systems (AMS), Italy). These GaAs devices were originally developed for medical physics applications, such as X-ray detectors. The process of fabrication is intrinsically double-sided, with silicon nitride ‘‘passivation’’ on the side where the electrode is and a polyamide protection on the ohmic side. The Schottky contact is made of Ti/Pt/Au layers deposited on the bare GaAs surface. Electroplated Au was used for the bonding pads. Fig. 1 shows the matrix consisting of 8 8 pixels 200 mm 200 mm on a 280 mm thick substrate.
3. Measurement set-up and configuration The detectors were characterised using a shielded probe station and a Keithley CV590
427
Fig. 1. Photograph of the 8 8 pixels 200 mm 200 mm. Double pads on the sides are for wire bonding, pads visible on the matrix are for probing.
analyser and a Keitley 2400 voltage source. Data were collected by automatic acquisition systems via a GPIB interface. All measurements were performed at 2 V bias and 100 kHz, but at 1 MHz slight deviations were observed as expected. This was the range of operation of the CV analyser and it was adequate for our measurements as the detector was fully depleted. It should also be the running condition for an imaging device with a low-noise ROIC. The probing signal to the pixel was 20 mV rms. For each pixel, there are eight neighbours that contribute to the inter-pixel capacitance: four of them along the diagonal, the other four along the closest distance. Their contribution should be disentangled separately as well as the contribution from the adjacent pixels and from the border and also from the different behaviour of the more central pixels with respect to the ones at the periphery. In order to do that we performed measurements configuring the polarization and the sensed pixel in geometries indicated as ‘‘T’’ and ‘‘L’’. These are illustrated in the sketches in Fig. 2. The ‘‘L’’ geometry should enhance the contribution from outermost pixels, i.e. along the diagonal, while the ‘‘T’’ should disentangle the contribution of those versus the closer ones.
428
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
Fig. 2. Sketches of the ‘‘T’’ and ‘‘L’’ measurement configuration, indicating the pixel probed, the pixels polarised and the ones left floating.
4. Results of measurements The curves in Fig. 3 show the variation in capacitance of the pixel versus its position in the ‘‘L’’ configuration. All measurements represent half of the detector pixels spanned over the diagonal, covering the area of the triangle, which is half of the area of the detector. The remaining symmetric portion of the detector is expected to behave similarly. This was verified in several locations outside the fully measured half triangle. The curves represent the entire set of measurements and from them it can be seen that there are variations among pixels in the same column or in same row. This indicates different material quality in different positions. Given the fact that the metalisation appears to be of the same quality under close visual inspection, we ascribe this to non-homogeneous bulk and/or contacts. As expected, these influences capacitance values and our measurements are sensitive to them. Surface states are likely to influence these values when the detector is depleted, while the bulk crystal nonhomogeneities are due to impurities during the growth phase.
The change of the capacitance values with the polarisation configuration is summarised in Fig. 4 where two curves of a series of pixels, probed in ‘‘L’’ and ‘‘T’’ are reported. The length to be taken into account facing the grouped pixel as a single electrode along the side of the pixel is twice the pitch for the ‘‘L’’ and three times the pitch for the ‘‘T’’ configuration. Taking into account these scaling factors, the capacitance values should be the same. However, there are variations between pixels in the same configuration, such as the ones in pixel B2, which is also repeated in the ‘‘T’’ and ‘‘L’’ configurations, although with different magnitudes, which are much bigger than the variations between ‘‘T’’ and ‘‘L’’ measurements. The magnitude of the variation between pixels in the same configuration amounts to less than 0.100 pF (Fig. 3) and that between configurations at a fixed column for different rows (Fig. 4) is about 0.030 pF. Considering an average value of 0.040 pF per 200 mm, this gives about 2 pF/cm with some values at a minimum of 1 and up to 4 pF/cm. By scaling the measured values by the length of the effective electrodes, one obtains an average value of 1 pF/cm.
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
429
Fig. 3. Curves for different pixel columns and rows in the ‘‘L’’ configuration. Letters indicate the columns and numbers indicate the rows of the detector matrix.
It should be stressed that the measurements performed are very delicate and present serious implications in their experimental systematic uncertainties. In probing the pads, both the needles and the pads should be very carefully touched and handled and kept in good conditions. Tiny variations in the positioning of the probe tips and their repositioning from one measurement to another, even after the offset subtraction, lead to significant variations in the capacitance of the measured pixel itself. The repositioning of the probe needles was accurately checked with manual turns of the manipulator. The accuracy of the operation is close to one micron, which is still enough to affect the measured capacitance values given the high sensitivity of our system. In order to evaluate the accuracy of measurements, i.e. to quantify the sensitivity of the system including systematic errors, we collected the measurements shown in Fig. 5. The same pixel was probed in subsequent time intervals, each indicated as a measurement series for each pixel.
Fig. 4. Curves of capacitance values for the same column for different rows in the ‘‘T’’ (top, full circles) and ‘‘L’’ (bottom, triangles) configurations. Letters indicate the columns and numbers indicate the rows of the detector matrix.
The procedure was repeated for several pixels. The probe tips were lifted to the same height but kept in the same position in the plane. Besides the
430
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
5. Conclusions
Fig. 5. Variation of capacitance values in the same pixel for measurements performed at subsequent times, indicated from 1st to 4th in temporal order.
We performed, for the first time, a systematic analysis of the behaviour in terms of inter-pixel and bulk capacitance of commercial GaAs devices in view of imaging applications. The values are a crucial ingredient for the design and realistic simulation of ROIC amplifiers, especially in photon counting circuits. The systematic variations and the dependence of capacitance on the measurement configuration were disentangled and quantified. The influence of the material quality has also clearly been addressed. On average, we measured a value of 1 pF/cm, which allows a comfortable design for low-noise integrated circuits.
References relative inaccuracy caused by repositioning, these measurements also suffer from the impossibility of re-applying a perfectly homogeneous force. Fig. 5 represents a summary of the possibility of reproducing the measurements. In certain cases, the variation amounts to a factor of two. However, given the small value, the results range between 1 and 3 pF/cm in all cases. The indications given above are, therefore, consistent and sufficient for our conclusions, on the average: Cinterpixel ¼ ð1:0070:03Þ pF.
[1] [2] [3] [4] [5] [6] [7] [8]
M. Caria, Nucl. Instr. and Meth. A 435 (1999) 233–241. S. D’Auria, Nucl. Instr. and Meth. A 386 (1997) 177–185. R. Bates, et al., Nucl. Instr. and Meth. A 410 (1998) 6–11. S. Cadeddu, et al., Nucl. Instr. and Meth. A 419 (1998) 270–275. S. Cadeddu, et al., Nucl. Instr. and Meth. A 422 (1999) 357–362. M. Campbell, et al., IEEE Trans. Nucl. Sci. 45 (3) (1998) 751. R. Irsigler, et al., Nucl. Instr. and Meth. A (1998) 427–434. V. Bonvicini, et al., Nucl. Instr. and Meth. A 365 (1995) 88–91.
Comprehensive measurements of GaAs pixel detectors capacitance M. Cariaa,b, L. Barberinia, S. D’Auriac, A. Laib, P. Randaccioa, S. Cadeddua,b,* a
Dipartimento di Fisica, Universita" di Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy b INFN Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy c Department of Physics and Astronomy, University of Glasgow, UK
Abstract We have studied GaAs pixel detectors on semi-insulating wafers with Schottky contacts. We performed comprehensive measurements on the inter-pixel and capacitance to back plane. Being semi-insulating, the behaviour is totally different with respect to other common semiconductors, such as high resistivity silicon. Non-homogeneities are also an issue, due to both the contacts and the crystal bulk. In order to detect them and their influence on capacitance, we undertook systematic measurements with different configurations of the measuring electrodes. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Gallium arsenide; Pixel; Capacitance; Imaging; Semiconductors
1. Introduction Pixel detectors made from materials alternative to high resistivity silicon are relatively little known. Large communities are now undertaking the design of radiation detection systems making use of pixel detectors for a wide variety of applications, from medical imaging apparatus to space and nuclear particle detectors [1]. A material of more and more widespread use for this purpose is gallium arsenide. Being relatively new in the field of radiation detectors, there are no standardised processes for bulk growth and contact implantation. The characteristics of these materials may vary greatly. In particular, the leakage current and the capacitance to bulk and inter-pixel, which in *Corresponding author. E-mail addresses: [email protected] (M. Caria), [email protected] (S. Cadeddu).
turn depend on the bulk composition, may be variably affected. We studied pixel detectors processed on semi-insulating (LEC) gallium arsenide with Schottky contacts [2,3]. In order to implement the detectors in full imaging devices, one needs electronic Read Out Integrated Circuits (ROIC). The design of these is crucially dependent on the electrical characteristics of the detectors which must be known with good accuracy. Among the characterisation measurements, the one on inter-pixel and bulk capacitance has never been performed systematically due to experimental difficulties and limited reliability of current technology. Such measurements are essential for read-out scheme performances. In an amplifier, read-out mode of the low noise-type being designed for imaging applications [4–6], the interpixel capacitance will be a contribution to the noise at the input.
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 7 9 2 - 2
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
Besides standard IV characterisation, we collected measurements on the inter-pixel and bulk capacitance. Being semi-insulating, the behaviour is totally different with respect to more standard high resistivity silicon. The tests are also meant to identify the relevance of contact and bulk nonhomogeneity. In order to detect that and its influence on capacitance, we undertook systematic measurements in different configurations of the measuring electrodes. We found dependence on the measuring set-up and position. Pixels behave differently if they are on the border or in the middle of the matrix. We quantified these variations and their spread so as to allow suitable design of the analogic circuitry. This is the first time that a pixel detector on GaAs is fully characterised in terms of its capacitance characteristics [7,8]. We show that devices built with GaAs are of limited reliability in the construction of imaging devices due to relatively immature technology, but the results are encouraging and allow the design and construction of a full system with integrated electronics.
2. Detector devices We characterized custom-designed devices processed by Alenia (Alenia Marconi Systems (AMS), Italy). These GaAs devices were originally developed for medical physics applications, such as X-ray detectors. The process of fabrication is intrinsically double-sided, with silicon nitride ‘‘passivation’’ on the side where the electrode is and a polyamide protection on the ohmic side. The Schottky contact is made of Ti/Pt/Au layers deposited on the bare GaAs surface. Electroplated Au was used for the bonding pads. Fig. 1 shows the matrix consisting of 8 8 pixels 200 mm 200 mm on a 280 mm thick substrate.
3. Measurement set-up and configuration The detectors were characterised using a shielded probe station and a Keithley CV590
427
Fig. 1. Photograph of the 8 8 pixels 200 mm 200 mm. Double pads on the sides are for wire bonding, pads visible on the matrix are for probing.
analyser and a Keitley 2400 voltage source. Data were collected by automatic acquisition systems via a GPIB interface. All measurements were performed at 2 V bias and 100 kHz, but at 1 MHz slight deviations were observed as expected. This was the range of operation of the CV analyser and it was adequate for our measurements as the detector was fully depleted. It should also be the running condition for an imaging device with a low-noise ROIC. The probing signal to the pixel was 20 mV rms. For each pixel, there are eight neighbours that contribute to the inter-pixel capacitance: four of them along the diagonal, the other four along the closest distance. Their contribution should be disentangled separately as well as the contribution from the adjacent pixels and from the border and also from the different behaviour of the more central pixels with respect to the ones at the periphery. In order to do that we performed measurements configuring the polarization and the sensed pixel in geometries indicated as ‘‘T’’ and ‘‘L’’. These are illustrated in the sketches in Fig. 2. The ‘‘L’’ geometry should enhance the contribution from outermost pixels, i.e. along the diagonal, while the ‘‘T’’ should disentangle the contribution of those versus the closer ones.
428
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
Fig. 2. Sketches of the ‘‘T’’ and ‘‘L’’ measurement configuration, indicating the pixel probed, the pixels polarised and the ones left floating.
4. Results of measurements The curves in Fig. 3 show the variation in capacitance of the pixel versus its position in the ‘‘L’’ configuration. All measurements represent half of the detector pixels spanned over the diagonal, covering the area of the triangle, which is half of the area of the detector. The remaining symmetric portion of the detector is expected to behave similarly. This was verified in several locations outside the fully measured half triangle. The curves represent the entire set of measurements and from them it can be seen that there are variations among pixels in the same column or in same row. This indicates different material quality in different positions. Given the fact that the metalisation appears to be of the same quality under close visual inspection, we ascribe this to non-homogeneous bulk and/or contacts. As expected, these influences capacitance values and our measurements are sensitive to them. Surface states are likely to influence these values when the detector is depleted, while the bulk crystal nonhomogeneities are due to impurities during the growth phase.
The change of the capacitance values with the polarisation configuration is summarised in Fig. 4 where two curves of a series of pixels, probed in ‘‘L’’ and ‘‘T’’ are reported. The length to be taken into account facing the grouped pixel as a single electrode along the side of the pixel is twice the pitch for the ‘‘L’’ and three times the pitch for the ‘‘T’’ configuration. Taking into account these scaling factors, the capacitance values should be the same. However, there are variations between pixels in the same configuration, such as the ones in pixel B2, which is also repeated in the ‘‘T’’ and ‘‘L’’ configurations, although with different magnitudes, which are much bigger than the variations between ‘‘T’’ and ‘‘L’’ measurements. The magnitude of the variation between pixels in the same configuration amounts to less than 0.100 pF (Fig. 3) and that between configurations at a fixed column for different rows (Fig. 4) is about 0.030 pF. Considering an average value of 0.040 pF per 200 mm, this gives about 2 pF/cm with some values at a minimum of 1 and up to 4 pF/cm. By scaling the measured values by the length of the effective electrodes, one obtains an average value of 1 pF/cm.
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
429
Fig. 3. Curves for different pixel columns and rows in the ‘‘L’’ configuration. Letters indicate the columns and numbers indicate the rows of the detector matrix.
It should be stressed that the measurements performed are very delicate and present serious implications in their experimental systematic uncertainties. In probing the pads, both the needles and the pads should be very carefully touched and handled and kept in good conditions. Tiny variations in the positioning of the probe tips and their repositioning from one measurement to another, even after the offset subtraction, lead to significant variations in the capacitance of the measured pixel itself. The repositioning of the probe needles was accurately checked with manual turns of the manipulator. The accuracy of the operation is close to one micron, which is still enough to affect the measured capacitance values given the high sensitivity of our system. In order to evaluate the accuracy of measurements, i.e. to quantify the sensitivity of the system including systematic errors, we collected the measurements shown in Fig. 5. The same pixel was probed in subsequent time intervals, each indicated as a measurement series for each pixel.
Fig. 4. Curves of capacitance values for the same column for different rows in the ‘‘T’’ (top, full circles) and ‘‘L’’ (bottom, triangles) configurations. Letters indicate the columns and numbers indicate the rows of the detector matrix.
The procedure was repeated for several pixels. The probe tips were lifted to the same height but kept in the same position in the plane. Besides the
430
M. Caria et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 426–430
5. Conclusions
Fig. 5. Variation of capacitance values in the same pixel for measurements performed at subsequent times, indicated from 1st to 4th in temporal order.
We performed, for the first time, a systematic analysis of the behaviour in terms of inter-pixel and bulk capacitance of commercial GaAs devices in view of imaging applications. The values are a crucial ingredient for the design and realistic simulation of ROIC amplifiers, especially in photon counting circuits. The systematic variations and the dependence of capacitance on the measurement configuration were disentangled and quantified. The influence of the material quality has also clearly been addressed. On average, we measured a value of 1 pF/cm, which allows a comfortable design for low-noise integrated circuits.
References relative inaccuracy caused by repositioning, these measurements also suffer from the impossibility of re-applying a perfectly homogeneous force. Fig. 5 represents a summary of the possibility of reproducing the measurements. In certain cases, the variation amounts to a factor of two. However, given the small value, the results range between 1 and 3 pF/cm in all cases. The indications given above are, therefore, consistent and sufficient for our conclusions, on the average: Cinterpixel ¼ ð1:0070:03Þ pF.
[1] [2] [3] [4] [5] [6] [7] [8]
M. Caria, Nucl. Instr. and Meth. A 435 (1999) 233–241. S. D’Auria, Nucl. Instr. and Meth. A 386 (1997) 177–185. R. Bates, et al., Nucl. Instr. and Meth. A 410 (1998) 6–11. S. Cadeddu, et al., Nucl. Instr. and Meth. A 419 (1998) 270–275. S. Cadeddu, et al., Nucl. Instr. and Meth. A 422 (1999) 357–362. M. Campbell, et al., IEEE Trans. Nucl. Sci. 45 (3) (1998) 751. R. Irsigler, et al., Nucl. Instr. and Meth. A (1998) 427–434. V. Bonvicini, et al., Nucl. Instr. and Meth. A 365 (1995) 88–91.