- Email: [email protected]
biological discrimination and detection
Materials Science in Semiconductor Processing 5 (2002) 17–22
Colour sensor for (bio)chemical/biological discrimination and detection Daniel Puiu Poenar*, Tse Man Siu, Tan Ooi Kiang Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Block S2, Nanyang Avenue, Singapore 639798, Singapore
Abstract Life science is a field of dynamic development and can benefit from the usage of microelectronics in numerous applications. Various devices for separation of different (bio)chemical components from a mixture could be miniaturized in silicon, but they need detectors at their output to identify and characterize the separated elements. The colour sensor is such a detector, and it was preferred because other classical approaches typically used in chemistry or biology employ IR or UV-based analysis, for which it is more difficult to design, optimize and fabricate a silicon-based sensor. Unlike classical detection (using three different filters placed on separate detectors) the proposed device is based on an entirely different approach, using vertically stacked detectors within a single structure that can be fabricated using CMOS-compatible processing. The main requirements for the design of such a vertically stacked multi-junction structure are presented, together with details regarding the most critical processing steps and process parameter values obtained after simulation which were used in the manufacturing of the first version of the device, including some optical design aspects. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Colour sensor; Active optical filtering
1. Introduction Miniaturization is an important driving force for the modernization of many devices into integrated microanalysis systems for various applications [1–2], particularly for those which can be implemented in silicon [3]. The characterization of the elements synthesized or separated in these microsystems require suitable detectors at their output. The principles they use can range from either simple resistance/capacitance measurement or frequency changes in surface acoustic waves for electronic noses [4,5], to thermal detection in chromatographs [6] and spectrometric identification in bio/
*Corresponding author. Tel.: +65-790-5237; fax: +65-7920415. E-mail address: [email protected] (D.P. Poenar).
chemical analysis [7,8]. In these latter cases, the particular optical sensors employed were simple photodiodes and CCD imagers, respectively. However, instead of using this more complex approach, which requires first splitting the light into spectral components and then detecting them, it may be more convenient to identify directly the light’s spectral characteristics using a colorimetric methodology. Typically, this is done by using 3 different filters (usually made of dyed polymers), one for each fundamental colour, red (R), green (G) and blue (B), deposited on independent and separate detectors horizontally placed in a mosaic-like arrangement. However, this approach may have two disadvantages: (i) One complete colorimetric sensing unit (‘pixel’) needs an area three times larger than that of a single detector, and
1369-8001/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 5 1 - 3
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
18
(ii) In some cases the polymer’s properties degrade due to aging, leading to an uncontrollable and unreproducible variation in time of the detector’s spectral selectivity. A completely distinct device can be based on the different penetration of various wavelengths into silicon at different depths. Thus, more sensors (e.g. simple pn junctions) can be incorporated in a single solid-state structure, with each of them ‘tuned’ properly to extract the carriers photo-generated at the depths corresponding to different colour ranges. Because the device is based only on silicon’s inherent absorbing properties, its spectral tuning and reliability can be better controlled. Furthermore, both the ‘filters’ and the detectors are integrated together and fabricated using CMOS-compatible processing, thus being potentially attractive for any industrial manufacturer. Moreover, this also provides flexibility in controlling the overall spectral responsivity of the detector.
2. Basic principle The detector’s structure is designed starting from just one fundamental principle: the variation of silicon’s optical properties with the wavelength l: As it is wellknown, each material can be characterized by its optical admittance N* ¼ n ik; where n is the refractive index and k the extinction coefficient. Both n and k are wavelength dependent, and in this particular case we are interested in the dependence k ¼ kðlÞ for monocrystalline silicon, which is shown in Fig. 1 [9]. It can be seen that k decreases about 2 orders of magnitude throughout the entire visible range, causing the violet (the short wavelength range) to be strongly absorbed in a very thin
layer at the very surface of the wafer, whereas red (the long wavelength range) penetrates to a considerable depth and requires a much broader region in which it is fully absorbed. Using these data it is possible to calculate the position and width of each specific detector as required for the detection of a certain hue, i.e. the depth at which a certain pn junction is placed and the necessary width for its depleted region required in order to place the selectivity peak at the desired wavelength. The exact results are presented in Fig. 2, which shows both the values for the depths where the depletion regions should be situated and their corresponding widths, as well as the resulting spectral selectivity curves for these values. Each curve i corresponds to the absorption taking place in a depleted region of width wi situated at a depth Di ; respectively. The complete structure of such a detector is shown in Fig. 3. The top-most n2þ –p junction must be extremely shallow and with the smallest width of its depleted region (with the values of D1 and w1 ; respectively, as indicated in Fig. 2) in order to detect the violet–blue range, because the shortest wavelengths will be absorbed very rapidly in silicon. As the value of the extinction coefficient k dramatically decreases for longer wavelengths, the next p2n junction should be situated deeper and with a wider depleted region (having the values of D2 and w2 ; respectively, in Fig. 2) in order to detect the green–yellow range. Similarly, the deepest junction with the broadest depletion region (junction depth and depleted region widths values of D3 and w3 ; respectively, in Fig. 2) will respond only to the long wavelengths of the visible spectrum, i.e. the orange–red range. Consequently, this vertically stacked multi-junction structure ensures that both the filter and the detector are
70 60
1
Absorption [%]
Extinction coefficient k
10
0
10
50
Colour sensor selectivity for the component junctions the depths Di & depletion widths wi [nm]: D1=50; w1=200; D2=650, w2=2000; D3=3000; w3=12000 1
2 3
40 30 20
-1
10
10 0 350 400 450
-2
10
-3
10
350
400
450
500
550
600
650
700
750
800
Wavelength [nm]
Fig. 1. Variation of monocrystalline silicon’s extinction coeffi* with the cient (imaginary part of the optical admittance N) wavelength.
500 550 600 650 Wavelength [nm]
700
750 800
Fig. 2. Simulated spectral selectivity curves for the three separate detectors that are supposed to be stacked vertically in a colour sensor. Curve 1 corresponds to the top-most junction (for the detection of the violet–blue spectral region), curve 2 represents the absorption in the middle junction (for the detection of the green–yellow range), and curve 3 shows the spectral selectivity for the third and deepest junction (for the detection of the orange–red range), respectively.
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
AR coating
n
p
GND or -V3
+V2
GND
n++
p+
n+
Depleted region (G detection)
n epi Depleted region (R detection)
~ ~
~ ~
p+ substrate
+V1
‘Active’ sensing region
Fig. 3. Schematic representation of the cross-section through the ideal structure of a complete colour sensor. The values of the junction depths and depletion region widths for each junction are those shown in Fig. 2.
Active sensing areas
AR coating Silicon Oxide (FOX)
Contacts
n+ shallow n (~15 nm deep)
p+ p substrate
Fig. 4. The simplified first version of the sensor, sensitive in the violet–blue range of the visible spectrum.
integrated together, in a single solid-state device which can deliver three R–G–B signals. Due to various practical constraints we were forced to split the realization of such a device into smaller subsequent stages. The first stage dealt with the design and implementation of the first (top-most) and most difficult junction, resulting in a modified and simplified version of the sensor, as shown in Fig. 4. The next section details the realization of this element of the colour sensor.
3. Design and implementation of the simplified sensor The most delicate element of the sensor is the very shallow n-type region. This was realized using an indirect phosphorus doping by diffusion through a thin oxide layer, followed by a short annealing and the growth of another thin oxide film. Such an approach was preferred instead of a more straightforward implantation through an oxide at very low energies
because of the damage which this latter method could cause to the silicon. The subsequent annealing has to be carried out at relatively low temperatures in order to achieve the desired shallow junction, which means that probably only a partial crystalline network recovery and possible incomplete activation of all the dopant atoms would take place. Both these factors would result in increased generation-recombination, diminished carrier lifetimes and other undesired effects that would adversely affect the photodiode’s performance. Simulations carried out using the TSUPREME4 software helped in determining accurately the values of the critical processing parameters, especially the dopant’s drive-in time and temperature, as well as those for the subsequent thermal oxide growth step. The optimal outcome was given by a short initial oxidation (8501C for 25 min) followed by the phosphorus drive-in step (9501C for 45 min), which resulted in a junction depth of about 0.12 mm. Fig. 5 shows the layout of the manufactured device. It can be noticed that a single large area device was tested, with multiple interdigitated teeth, in order to ease the detection of the generated photocurrents that otherwise would have been much reduced in magnitude if a small area device (as those used in high resolution arrays) had been employed. Additionally, an antireflectant (AR) multiplayer coating has to be deposited on top of any optical sensor in order to maximize its detection efficiency. Obviously, the simplified sensor shown in Fig. 4 would be sensitive only in the violet–blue part of the visible range. Therefore, the AR coating necessary to maximize the light absorption efficiency within the detector should be designed to have a transmissivity peak into silicon for this shortwavelength region. A simple two-layer (nitride on oxide) design was adopted for this initial simplified sensor, and Fig. 6 presents the simulated characteristic curves as well as the values for the physical parameters of each layer.
NWELL P+CONT PBASE NSURF N+CONT
~1.1 mm
Depleted region (B detection)
19
~1.1 mm
Fig. 5. The layout of the manufactured device, with a blow-up showing a detail of a quarter of the chip, highlighting the structure with interdigitated teeth.
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
20 λ 0=425 nm 100
T
R
95
19
90
14
85
9
80
125 105 85
4
Wavelength [nm]
65
0
75 400 Violet
450 Blue
500
550
Green
600 Orange
650
700
Current [[µ µA]
145 Reflectance [%]
Transmittance [%]
165
24
750
45
800
Red
25
Yellow l
Fig. 6. Simulated transmittance and reflectance for the AR coating designed for the fabrication of the simplified detector ( SiO2 and 210 A ( Si3N4). (100 A
-3
-1.4
-0.6
55 -5
-0.2
1
Voltage [V]
(a)
10-4
4. Measurements and results
10-5 Current [A]
Fig. 7a shows the current–voltage characteristic measured on one of the chips, and which was repeatable across the wafer. Although the general shape of the curve clearly demonstrates that the device is indeed behaving like a diode, two important features can be noticed from the measurement. Firstly, if the forward bias region of the I–V curve is plotted using a logarithmic scale for the y axis as shown in Fig. 7b, one can notice the deviation from ideality of the diode. A simple model of a resistor connected in series with an ideal diode was considered in order to account for this behaviour, which we believed, would be mainly due to the high resistance typically associated with the sensor’s thin n-type regions. A simulation was performed to find the best fit to the experimental data, from which the characteristic parameter values were extracted. The simulation ultimately provided the values that are also shown in Fig. 7b: for the diode, the nonideality factor m ¼ 1:643 and IS ¼ 16:86 nA using IF ¼ IS expðVF =mVT Þ as the description of the diode’s static characteristic (with VT the thermal voltage VT ¼ kT=q ¼ 25:875 mV at room temperature, i.e. T ¼ 300 K), whereas the series resistor has the value RD3:1 kO. All these values reflect the intrinsic attributes of the fabricated device, as detailed below. The very shallow junction was obtained after diffusion through an oxide, and this explains the slightly larger value of the non-ideality factor. Remembering that an ideal abrupt junction has m ¼ 1 and that a linearly graded junction is characterized by mD3; it becomes clear that the slope of the doping gradient finally obtained in silicon is intermediary between these two cases. This high resistance value is also to be expected for such a shallow junction realized with long fingers in a large area device. Of course, this simple model can be improved upon if a better understanding of the device’s functioning is
-2.2
D
10-6 R = 3.1 k Ω m=1.643 IS=16.86 nA
10-7
Real device
10-8 10-9 0
(b)
0.2
0.4
0.6
0.8
Voltage [V]
Fig. 7. (a) Measured I–V curve of a manufactured sensor; (b) forward bias region plotted using a logarithmic scale along y-axis.
desired and a more realistic approximation for a structure with such large dimensions would be given by using a distributed model. The second important feature which resulted from the measurements is the very low ‘breakdown’ voltage and the general non-abrupt behaviour in reverse bias. This again can be explained by the shallow nature of the n–p junction. The extremely thin n region becomes readily depleted, hence the reduced ‘breakdown’ voltage. Furthermore, once depleted, the entire surface region behaves as a leaking resistor through which the current would increase (although non-linearly) with the applied reverse bias. Consequently, in such a case one cannot speak of any abrupt reverse bias characteristic with a clearly defined breakdown threshold, as is the case with an ideal n–p junction. Finally, a quick and simple test was done to measure the device sensitivity to light, as this is the most important factor reflecting the device’s capability to operate as a photodiode. Note, however, that the
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22 Voltage [V] -3
-2.5
-2 -2
without light
R
~ 4 nA -1.5
-1
with light
-0.5
~ 0.635 µA
White light G B n+
21
AB
p+
-1
-3
-4
Current [µA]
AG -2
Depleted regions
reference measurement could not be carried out in complete darkness and the surrounding environment may have contributed in altering the real value of this parameter. Fig. 8 shows the obtained currents in the vicinity of 0 V bias for two situations, without and with the microscope light source switched on at maximum, respectively. The device does respond as a photodiode; however, the rather small current difference between the two situations is mainly due to the relatively low power of the microscope light source. Furthermore, the optical characteristic of the AR coating was not measured and could be very different from that initially desired.
5. Conclusions A colour sensor that incorporates both the filter and the detectors in a vertically stacked structure can be realized. Such a sensor is based on the wavelength dependence of light absorption in silicon and the complete solid-state structure is easily processed using CMOS-compatible technologies. Moreover, such a sensor allows a large degree of flexibility in usage. Although the positioning of the separate junctions cannot be altered by the user, the signals provided by them can be manipulated as required using separate amplifiers AR ; AG and AB ; respectively, each of them with variable amplification factors, and connected as shown in Fig. 9. By programming or varying the amplification factor of each channel, one could obtain any analogue combination between the three separate spectral characteristics of each of the detecting junctions, thus resulting in a programmable overall spectral response. Only a simplified version of the detector (for the violet–blue range) was fabricated until now. The first tests and measurements clearly demonstrated that the
Output
AR
-5
Fig. 8. Measured I–V characteristics in reverse bias, showing the simplified sensor’s optical response when microscope light is turned off or on, respectively.
∑
n epi
p substrate
Fig. 9. Schematic representation of the signal processing circuits structure of a fully integrated smart colour sensor chip.
simplified version of the device could be used as a photodiode. Further work is necessary to improve the existing device (e.g. to improve the junction’s doping gradient and reduce the serial parasitic resistance, to include a second detector as well and to perfect the AR coating, to perform detailed spectral characterizations) but the results obtained are extremely encouraging.
Acknowledgements We extend our gratitude and thanks to our students, Mr. Lim Kok Theam and Ms. Yeo Ai Chin, who performed the processing and characterization. We are also grateful for all the staff in the Microfabrication Laboratory who assisted in the completion of the activity done until now. This work was carried out as part of the research project RG 11/00.
References + [1] Gopel W. Ultimate limits in the miniaturization of chemical sensors. Sensors Actuators A 1996;56A:83–102. [2] Cefai JJ, Barrow DA, Woias P, Muller E. Integrated chemical analysis micro-systems in space life science research. J Micromech Microeng 1994;4:172–85. [3] Wolffenbuttel RF. On-chip microsystems on silicon: opportunities and limitations. J Micromech Microeng 1996;6: 138–43. [4] Neaves PI, Hatfield JV. A new generation of integrated electronic noses. Sensors Actuators B 1995;26B–27B: 223–31. [5] Weimar U, Gopel W. Chemical imaging: ii. Trends in practical multiparameter sensor systems. Sensors Actuators B 1998;52B:143–61.
22
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
[6] Terry SC, Jerman JH, Angell JB. A gas chromatographic air analyzer fabricated on a silicon wafer. IEEE Trans Electron Devices 1979;186:405–18. [7] Yee GM, Maluf NI, Hing PA, Albin M, Kovacs GTA. Miniature spectrometers for biochemical analysis. Sensors Actuators A 1997;58A:61–6.
[8] Kwa TA, Wolffenbuttel RF. Integrated grating/detector array fabricated in silicon using micromachining techniques. Sensors Actuators A 1992;31A:259–66. [9] Properties of silicon, EMIS Datareviews Series No.4, INSPEC. London, New York: The Institution of Electrical Engineers, 1988 [Chapter 28].
Colour sensor for (bio)chemical/biological discrimination and detection Daniel Puiu Poenar*, Tse Man Siu, Tan Ooi Kiang Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Block S2, Nanyang Avenue, Singapore 639798, Singapore
Abstract Life science is a field of dynamic development and can benefit from the usage of microelectronics in numerous applications. Various devices for separation of different (bio)chemical components from a mixture could be miniaturized in silicon, but they need detectors at their output to identify and characterize the separated elements. The colour sensor is such a detector, and it was preferred because other classical approaches typically used in chemistry or biology employ IR or UV-based analysis, for which it is more difficult to design, optimize and fabricate a silicon-based sensor. Unlike classical detection (using three different filters placed on separate detectors) the proposed device is based on an entirely different approach, using vertically stacked detectors within a single structure that can be fabricated using CMOS-compatible processing. The main requirements for the design of such a vertically stacked multi-junction structure are presented, together with details regarding the most critical processing steps and process parameter values obtained after simulation which were used in the manufacturing of the first version of the device, including some optical design aspects. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Colour sensor; Active optical filtering
1. Introduction Miniaturization is an important driving force for the modernization of many devices into integrated microanalysis systems for various applications [1–2], particularly for those which can be implemented in silicon [3]. The characterization of the elements synthesized or separated in these microsystems require suitable detectors at their output. The principles they use can range from either simple resistance/capacitance measurement or frequency changes in surface acoustic waves for electronic noses [4,5], to thermal detection in chromatographs [6] and spectrometric identification in bio/
*Corresponding author. Tel.: +65-790-5237; fax: +65-7920415. E-mail address: [email protected] (D.P. Poenar).
chemical analysis [7,8]. In these latter cases, the particular optical sensors employed were simple photodiodes and CCD imagers, respectively. However, instead of using this more complex approach, which requires first splitting the light into spectral components and then detecting them, it may be more convenient to identify directly the light’s spectral characteristics using a colorimetric methodology. Typically, this is done by using 3 different filters (usually made of dyed polymers), one for each fundamental colour, red (R), green (G) and blue (B), deposited on independent and separate detectors horizontally placed in a mosaic-like arrangement. However, this approach may have two disadvantages: (i) One complete colorimetric sensing unit (‘pixel’) needs an area three times larger than that of a single detector, and
1369-8001/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 5 1 - 3
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
18
(ii) In some cases the polymer’s properties degrade due to aging, leading to an uncontrollable and unreproducible variation in time of the detector’s spectral selectivity. A completely distinct device can be based on the different penetration of various wavelengths into silicon at different depths. Thus, more sensors (e.g. simple pn junctions) can be incorporated in a single solid-state structure, with each of them ‘tuned’ properly to extract the carriers photo-generated at the depths corresponding to different colour ranges. Because the device is based only on silicon’s inherent absorbing properties, its spectral tuning and reliability can be better controlled. Furthermore, both the ‘filters’ and the detectors are integrated together and fabricated using CMOS-compatible processing, thus being potentially attractive for any industrial manufacturer. Moreover, this also provides flexibility in controlling the overall spectral responsivity of the detector.
2. Basic principle The detector’s structure is designed starting from just one fundamental principle: the variation of silicon’s optical properties with the wavelength l: As it is wellknown, each material can be characterized by its optical admittance N* ¼ n ik; where n is the refractive index and k the extinction coefficient. Both n and k are wavelength dependent, and in this particular case we are interested in the dependence k ¼ kðlÞ for monocrystalline silicon, which is shown in Fig. 1 [9]. It can be seen that k decreases about 2 orders of magnitude throughout the entire visible range, causing the violet (the short wavelength range) to be strongly absorbed in a very thin
layer at the very surface of the wafer, whereas red (the long wavelength range) penetrates to a considerable depth and requires a much broader region in which it is fully absorbed. Using these data it is possible to calculate the position and width of each specific detector as required for the detection of a certain hue, i.e. the depth at which a certain pn junction is placed and the necessary width for its depleted region required in order to place the selectivity peak at the desired wavelength. The exact results are presented in Fig. 2, which shows both the values for the depths where the depletion regions should be situated and their corresponding widths, as well as the resulting spectral selectivity curves for these values. Each curve i corresponds to the absorption taking place in a depleted region of width wi situated at a depth Di ; respectively. The complete structure of such a detector is shown in Fig. 3. The top-most n2þ –p junction must be extremely shallow and with the smallest width of its depleted region (with the values of D1 and w1 ; respectively, as indicated in Fig. 2) in order to detect the violet–blue range, because the shortest wavelengths will be absorbed very rapidly in silicon. As the value of the extinction coefficient k dramatically decreases for longer wavelengths, the next p2n junction should be situated deeper and with a wider depleted region (having the values of D2 and w2 ; respectively, in Fig. 2) in order to detect the green–yellow range. Similarly, the deepest junction with the broadest depletion region (junction depth and depleted region widths values of D3 and w3 ; respectively, in Fig. 2) will respond only to the long wavelengths of the visible spectrum, i.e. the orange–red range. Consequently, this vertically stacked multi-junction structure ensures that both the filter and the detector are
70 60
1
Absorption [%]
Extinction coefficient k
10
0
10
50
Colour sensor selectivity for the component junctions the depths Di & depletion widths wi [nm]: D1=50; w1=200; D2=650, w2=2000; D3=3000; w3=12000 1
2 3
40 30 20
-1
10
10 0 350 400 450
-2
10
-3
10
350
400
450
500
550
600
650
700
750
800
Wavelength [nm]
Fig. 1. Variation of monocrystalline silicon’s extinction coeffi* with the cient (imaginary part of the optical admittance N) wavelength.
500 550 600 650 Wavelength [nm]
700
750 800
Fig. 2. Simulated spectral selectivity curves for the three separate detectors that are supposed to be stacked vertically in a colour sensor. Curve 1 corresponds to the top-most junction (for the detection of the violet–blue spectral region), curve 2 represents the absorption in the middle junction (for the detection of the green–yellow range), and curve 3 shows the spectral selectivity for the third and deepest junction (for the detection of the orange–red range), respectively.
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
AR coating
n
p
GND or -V3
+V2
GND
n++
p+
n+
Depleted region (G detection)
n epi Depleted region (R detection)
~ ~
~ ~
p+ substrate
+V1
‘Active’ sensing region
Fig. 3. Schematic representation of the cross-section through the ideal structure of a complete colour sensor. The values of the junction depths and depletion region widths for each junction are those shown in Fig. 2.
Active sensing areas
AR coating Silicon Oxide (FOX)
Contacts
n+ shallow n (~15 nm deep)
p+ p substrate
Fig. 4. The simplified first version of the sensor, sensitive in the violet–blue range of the visible spectrum.
integrated together, in a single solid-state device which can deliver three R–G–B signals. Due to various practical constraints we were forced to split the realization of such a device into smaller subsequent stages. The first stage dealt with the design and implementation of the first (top-most) and most difficult junction, resulting in a modified and simplified version of the sensor, as shown in Fig. 4. The next section details the realization of this element of the colour sensor.
3. Design and implementation of the simplified sensor The most delicate element of the sensor is the very shallow n-type region. This was realized using an indirect phosphorus doping by diffusion through a thin oxide layer, followed by a short annealing and the growth of another thin oxide film. Such an approach was preferred instead of a more straightforward implantation through an oxide at very low energies
because of the damage which this latter method could cause to the silicon. The subsequent annealing has to be carried out at relatively low temperatures in order to achieve the desired shallow junction, which means that probably only a partial crystalline network recovery and possible incomplete activation of all the dopant atoms would take place. Both these factors would result in increased generation-recombination, diminished carrier lifetimes and other undesired effects that would adversely affect the photodiode’s performance. Simulations carried out using the TSUPREME4 software helped in determining accurately the values of the critical processing parameters, especially the dopant’s drive-in time and temperature, as well as those for the subsequent thermal oxide growth step. The optimal outcome was given by a short initial oxidation (8501C for 25 min) followed by the phosphorus drive-in step (9501C for 45 min), which resulted in a junction depth of about 0.12 mm. Fig. 5 shows the layout of the manufactured device. It can be noticed that a single large area device was tested, with multiple interdigitated teeth, in order to ease the detection of the generated photocurrents that otherwise would have been much reduced in magnitude if a small area device (as those used in high resolution arrays) had been employed. Additionally, an antireflectant (AR) multiplayer coating has to be deposited on top of any optical sensor in order to maximize its detection efficiency. Obviously, the simplified sensor shown in Fig. 4 would be sensitive only in the violet–blue part of the visible range. Therefore, the AR coating necessary to maximize the light absorption efficiency within the detector should be designed to have a transmissivity peak into silicon for this shortwavelength region. A simple two-layer (nitride on oxide) design was adopted for this initial simplified sensor, and Fig. 6 presents the simulated characteristic curves as well as the values for the physical parameters of each layer.
NWELL P+CONT PBASE NSURF N+CONT
~1.1 mm
Depleted region (B detection)
19
~1.1 mm
Fig. 5. The layout of the manufactured device, with a blow-up showing a detail of a quarter of the chip, highlighting the structure with interdigitated teeth.
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
20 λ 0=425 nm 100
T
R
95
19
90
14
85
9
80
125 105 85
4
Wavelength [nm]
65
0
75 400 Violet
450 Blue
500
550
Green
600 Orange
650
700
Current [[µ µA]
145 Reflectance [%]
Transmittance [%]
165
24
750
45
800
Red
25
Yellow l
Fig. 6. Simulated transmittance and reflectance for the AR coating designed for the fabrication of the simplified detector ( SiO2 and 210 A ( Si3N4). (100 A
-3
-1.4
-0.6
55 -5
-0.2
1
Voltage [V]
(a)
10-4
4. Measurements and results
10-5 Current [A]
Fig. 7a shows the current–voltage characteristic measured on one of the chips, and which was repeatable across the wafer. Although the general shape of the curve clearly demonstrates that the device is indeed behaving like a diode, two important features can be noticed from the measurement. Firstly, if the forward bias region of the I–V curve is plotted using a logarithmic scale for the y axis as shown in Fig. 7b, one can notice the deviation from ideality of the diode. A simple model of a resistor connected in series with an ideal diode was considered in order to account for this behaviour, which we believed, would be mainly due to the high resistance typically associated with the sensor’s thin n-type regions. A simulation was performed to find the best fit to the experimental data, from which the characteristic parameter values were extracted. The simulation ultimately provided the values that are also shown in Fig. 7b: for the diode, the nonideality factor m ¼ 1:643 and IS ¼ 16:86 nA using IF ¼ IS expðVF =mVT Þ as the description of the diode’s static characteristic (with VT the thermal voltage VT ¼ kT=q ¼ 25:875 mV at room temperature, i.e. T ¼ 300 K), whereas the series resistor has the value RD3:1 kO. All these values reflect the intrinsic attributes of the fabricated device, as detailed below. The very shallow junction was obtained after diffusion through an oxide, and this explains the slightly larger value of the non-ideality factor. Remembering that an ideal abrupt junction has m ¼ 1 and that a linearly graded junction is characterized by mD3; it becomes clear that the slope of the doping gradient finally obtained in silicon is intermediary between these two cases. This high resistance value is also to be expected for such a shallow junction realized with long fingers in a large area device. Of course, this simple model can be improved upon if a better understanding of the device’s functioning is
-2.2
D
10-6 R = 3.1 k Ω m=1.643 IS=16.86 nA
10-7
Real device
10-8 10-9 0
(b)
0.2
0.4
0.6
0.8
Voltage [V]
Fig. 7. (a) Measured I–V curve of a manufactured sensor; (b) forward bias region plotted using a logarithmic scale along y-axis.
desired and a more realistic approximation for a structure with such large dimensions would be given by using a distributed model. The second important feature which resulted from the measurements is the very low ‘breakdown’ voltage and the general non-abrupt behaviour in reverse bias. This again can be explained by the shallow nature of the n–p junction. The extremely thin n region becomes readily depleted, hence the reduced ‘breakdown’ voltage. Furthermore, once depleted, the entire surface region behaves as a leaking resistor through which the current would increase (although non-linearly) with the applied reverse bias. Consequently, in such a case one cannot speak of any abrupt reverse bias characteristic with a clearly defined breakdown threshold, as is the case with an ideal n–p junction. Finally, a quick and simple test was done to measure the device sensitivity to light, as this is the most important factor reflecting the device’s capability to operate as a photodiode. Note, however, that the
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22 Voltage [V] -3
-2.5
-2 -2
without light
R
~ 4 nA -1.5
-1
with light
-0.5
~ 0.635 µA
White light G B n+
21
AB
p+
-1
-3
-4
Current [µA]
AG -2
Depleted regions
reference measurement could not be carried out in complete darkness and the surrounding environment may have contributed in altering the real value of this parameter. Fig. 8 shows the obtained currents in the vicinity of 0 V bias for two situations, without and with the microscope light source switched on at maximum, respectively. The device does respond as a photodiode; however, the rather small current difference between the two situations is mainly due to the relatively low power of the microscope light source. Furthermore, the optical characteristic of the AR coating was not measured and could be very different from that initially desired.
5. Conclusions A colour sensor that incorporates both the filter and the detectors in a vertically stacked structure can be realized. Such a sensor is based on the wavelength dependence of light absorption in silicon and the complete solid-state structure is easily processed using CMOS-compatible technologies. Moreover, such a sensor allows a large degree of flexibility in usage. Although the positioning of the separate junctions cannot be altered by the user, the signals provided by them can be manipulated as required using separate amplifiers AR ; AG and AB ; respectively, each of them with variable amplification factors, and connected as shown in Fig. 9. By programming or varying the amplification factor of each channel, one could obtain any analogue combination between the three separate spectral characteristics of each of the detecting junctions, thus resulting in a programmable overall spectral response. Only a simplified version of the detector (for the violet–blue range) was fabricated until now. The first tests and measurements clearly demonstrated that the
Output
AR
-5
Fig. 8. Measured I–V characteristics in reverse bias, showing the simplified sensor’s optical response when microscope light is turned off or on, respectively.
∑
n epi
p substrate
Fig. 9. Schematic representation of the signal processing circuits structure of a fully integrated smart colour sensor chip.
simplified version of the device could be used as a photodiode. Further work is necessary to improve the existing device (e.g. to improve the junction’s doping gradient and reduce the serial parasitic resistance, to include a second detector as well and to perfect the AR coating, to perform detailed spectral characterizations) but the results obtained are extremely encouraging.
Acknowledgements We extend our gratitude and thanks to our students, Mr. Lim Kok Theam and Ms. Yeo Ai Chin, who performed the processing and characterization. We are also grateful for all the staff in the Microfabrication Laboratory who assisted in the completion of the activity done until now. This work was carried out as part of the research project RG 11/00.
References + [1] Gopel W. Ultimate limits in the miniaturization of chemical sensors. Sensors Actuators A 1996;56A:83–102. [2] Cefai JJ, Barrow DA, Woias P, Muller E. Integrated chemical analysis micro-systems in space life science research. J Micromech Microeng 1994;4:172–85. [3] Wolffenbuttel RF. On-chip microsystems on silicon: opportunities and limitations. J Micromech Microeng 1996;6: 138–43. [4] Neaves PI, Hatfield JV. A new generation of integrated electronic noses. Sensors Actuators B 1995;26B–27B: 223–31. [5] Weimar U, Gopel W. Chemical imaging: ii. Trends in practical multiparameter sensor systems. Sensors Actuators B 1998;52B:143–61.
22
D.P. Poenar et al. / Materials Science in Semiconductor Processing 5 (2002) 17–22
[6] Terry SC, Jerman JH, Angell JB. A gas chromatographic air analyzer fabricated on a silicon wafer. IEEE Trans Electron Devices 1979;186:405–18. [7] Yee GM, Maluf NI, Hing PA, Albin M, Kovacs GTA. Miniature spectrometers for biochemical analysis. Sensors Actuators A 1997;58A:61–6.
[8] Kwa TA, Wolffenbuttel RF. Integrated grating/detector array fabricated in silicon using micromachining techniques. Sensors Actuators A 1992;31A:259–66. [9] Properties of silicon, EMIS Datareviews Series No.4, INSPEC. London, New York: The Institution of Electrical Engineers, 1988 [Chapter 28].