JP2001296270A - Measuring method and apparatus for semiconductor depletion layer capacity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple semiconductor depletion layer capacity measuring method having a low noise and a low background, and to provide its device. SOLUTION: In this semiconductor depletion layer capacity measuring method, a sensor, having a structure comprising first and second electrolytes 3,5-insulator layer 2-bulk semiconductor 1 is set, and a bias voltage is applied to a depletion layer 1A of the semiconductor bulk 1, and the first and second electrolytes 3 and 5 are irradiated with two optical pulses 9 and 10 having shifted phases corresponding respectively to the back of the semiconductor bulk 1. The outputs from electrodes 4 and 6 connected to the first and second electrolytes 3 and 5 are led out, and photoelectric current detection having a low noise and a low background is executed by differential detection method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chemical sensor, and more particularly, to a LAPS which is an apparatus for measuring the capacitance of a semiconductor depletion layer.
(Light Addressable Poten
tiometricSensor).

[0002]

2. Description of the Related Art Here, a depletion layer (depletion layer) is used.
The term “layer” refers to a thin layer in which conduction electrons and holes hardly exist in a pn junction surface of a semiconductor or inside a semiconductor having a MOS structure. Since the thickness of the depletion layer changes sensitively with the applied voltage, it governs the characteristics of the diode and the transistor.

Here, the measurement of the semiconductor depletion layer capacitance is actually a measurement of chemical species such as pH corresponding to the depletion layer capacitance,
It refers to the measurement of the surface potential of the surface of a living body or the like, and the detection and measurement of semiconductor defects.

In recent years, LA has been used in various fields of chemistry and biology.
Biosensor based on PS [T. Yoshino
bu, H .; Iwasaki, M .; Nakao, S .; No
mura, T .; Nakanishi, S .; Takama
tsu and K. Tomita, Bioimage
s, “Visualization of pH ch
angle of E. Coli with a nov
el pH imaging microscopic
e ", 5 (1997), pp 143-147, GG
ehring, D.E. L. Patternson, and
S. Tu, “Useof right address
able potentiometric sensor
for the detection of Esc
herichia coli O157: H7 ″, An
al. Biochem. , 258 (2) (1998),
pp 293-298. J. J. Valdes, M .;
T. Goode, R .; R. Brubaker, V .; L.
Motin, W.C. H. Boyleston, and
J. P. Chambers, "Detectionof.
Bacterial DNA sequences
via oligonucleotide-based
biosensors ", Proc. Int. Sym
p. Int. Soc. Environ. Biotech
nol. , 3rd (1997), pp 341-345].
The application of is becoming popular.

[0005] In these applications, it is common to detect changes in pH and ions in the environment. LAPS
According to the conventional measurement method according to the present invention, several operations must be performed on the light response characteristics of LAPS,
It took time to quantitatively measure. In the case of an interface between a living body and a semiconductor that detects and measures the electrical activity of a neuron, the nerve action potential is modulated by a very strong photocurrent (background) signal. It was difficult to pick up the decay signal of the extracellular potential. While improving LAPS measurement techniques is of great importance, only a few reports so far have been reported [Luc J. et al. B
ouse S. Mostarshed and D.M.
Hafeman, "A combined measu
rement of surface potenti
al and zeta potential of
an electoryte / insulator
interface of LAPS ", Sensors
and Actuators B: 10 (199
2), pp 67-71; Adami M. Sar
tore, E. Baldini, A .; Rossi an
d C.I. Nicolini ,, “New measure
ing principal for LAPS ", S
sensorand Actuators B: 9 (1
992), pp 25-31; Sasaki, Y .;
Kanai, H .; Uchida, and T.W. Kats
ube, “Highly Sensitive Tas
te sensors with a new dif
ferential LAPS method ", Se
nsors and Actuators B: 24-
25 (1995), pp 819-822].

[0006] Bousse et al. (1992) studied using two sensors and two LEDs to measure the output potential difference between LED1 and LED2 over time, and to measure the combined surface potential and zeta potential. did. A
(1992) also reported that two sensors and two L
The differential measurement by LAPS was examined using an ED and a comparator.

Therefore, a schematic configuration of a conventional chemical sensor LAPS is as shown in FIG.

FIG. 10 is a schematic configuration diagram of a conventional chemical sensor LAPS, FIG. 11 is a waveform diagram of each part of the chemical sensor LAPS, FIG.
FIG. 11B is a waveform diagram of a photocurrent output from the comparator.

As shown in these figures, an insulator layer 102 is formed on a bulk semiconductor (Si) 101, and a single electrolyte 103 as a measurement sample is provided thereon. A voltage V of an AC voltage source (variable voltage value) 105 is applied to the bulk semiconductor 101, one end of the AC voltage source 105 is input to a first input terminal of an amplifier (amplifier) 106, and the
3 is brought into contact with an electrode 104, the output of which is
6 to the second input terminal.

On the other hand, from the back surface of the bulk semiconductor (Si) 101, an irradiation light pulse 107 as shown in FIG.
Is irradiated.

With this configuration, the depletion layer 101A
When a bias voltage is applied to the LAP / pH sensor, the thickness of the depletion layer 101A is correlated with the surface potential of the depletion layer 101A. Depletion local value of the capacitance generated when irradiated with light sources amplitude modulation bulk semiconductor (Si) 101, as shown in FIG. 11 (b), can be read by the AC photocurrent I _out.

[0012]

However, in the configuration of the conventional chemical sensor LAPS described above, FIG.
As shown in (b), there is a problem that a high background is obtained, noise is increased, and an accurate photocurrent waveform cannot be obtained.

The present invention has been made in view of the above circumstances, and has as its object to provide a method and an apparatus for simply measuring the capacitance of a semiconductor depletion layer with low noise and low background.

[0014]

According to the present invention, there is provided a method for measuring a capacitance of a semiconductor depletion layer, comprising: a sensor having a first and a second electrolyte-insulator layer-semiconductor bulk structure; And applying a bias voltage to a depletion layer of the semiconductor bulk, and irradiating a back surface of the semiconductor bulk with light pulses of two phases shifted respectively corresponding to the first and second electrolytes, And output from an electrode connected to the second electrolyte is derived, and photocurrent detection with low noise and low background is performed by a differential detection method.

[2] The method for measuring the capacity of a semiconductor depletion layer according to the above [1], wherein the pH of the electrolyte is measured.

[3] In the semiconductor depletion layer capacitance measuring device, a sensor having a first and second electrolyte-insulator layer-semiconductor bulk structure, means for applying a bias voltage to the depletion layer of the semiconductor bulk, Means for irradiating the back surface of the semiconductor bulk with two out-of-phase light pulses respectively corresponding to the first and second electrolytes, and differential detection of the output from the electrodes connected to the first and second electrolytes And a means for deriving the photocurrent with low noise and low background by a simple method.

[4] In the semiconductor depletion layer capacitance measuring device according to the above [3], the insulator layer is made of Si ₃ N ₄ / SiO
_{2. The} semiconductor bulk is a Si bulk.

[5] In the semiconductor depletion layer capacitance measuring apparatus according to the above [3], the light pulse is 10 kHz to 30 kHz.
Hz laser light.

[6] In the semiconductor depletion layer capacitance measuring device according to the above [3], the first and second electrolytes are a measuring cell and a control cell.

[0020]

Embodiments of the present invention will be described below in detail.

FIG. 1 is a schematic configuration diagram of a semiconductor depletion layer capacitance measuring system showing an embodiment of the present invention, and FIG. 2 is a diagram showing waveforms at various parts of the semiconductor depletion layer capacitance measuring system.
2A is a waveform diagram of the first irradiation light pulse, FIG. 2B is a waveform diagram of the second irradiation light pulse, FIG. 2C is a waveform (cancelled) input to the amplifier, FIG. 2D is a waveform diagram of a photocurrent output from the amplifier.

In these figures, 1 is a bulk semiconductor (Si), 2 is an insulator layer, 3 is a first electrolyte (measurement cell) as a measurement sample formed on the insulator layer 2, and 4 is its first electrolyte. Reference numeral 1 denotes an electrode of the electrolyte 3, 5 denotes a second electrolyte (control cell) as a control sample formed on the insulator layer 2, 6 denotes an electrode of the second electrolyte 5, and 7 denotes a DC voltage source (voltage value). Variable) and 8 are amplifiers connected to the first electrolyte 3 and the second electrolyte 5 and to the DC voltage source 7. Reference numeral 9 denotes a first irradiation light pulse, and reference numeral 10 denotes a second irradiation light pulse. These pulses have two phases shifted (out-of-phase).

With such a configuration, LAPS is EI
S [first and second electrolytes _{3, 5} -insulator (Si ₃ N ₄ /
SiO ₂ ) layer 2—bulk semiconductor (Si) 1] structure. When a bias current from the DC voltage source 7 is applied to the depletion layer 1A having this structure, the thickness of the depletion layer 1A correlates with the surface potential of that portion, and in a LAP / pH sensor, this surface potential depends on the pH value of the electrolyte. Dependent. The local value of the depletion capacitance can be read by the AC photocurrent generated when the amplitude-modulated irradiation light pulse is irradiated on the bulk semiconductor (Si) 1.

According to the present invention, in LAPS measurement, two phases are shifted (out) instead of one irradiation light pulse.
t-of-phase) first and second irradiation light pulses 9,
10, a low noise / low background result can be obtained by a kind of differential detection method.

More specifically, the heterostructure (Si ₃
The Si layer (N ₄ / SiO ₂ / Si) is irradiated at two different locations by first and second irradiation light pulses 9, 10 (laser 9 and laser 10). The measurement cell 3 and the control cell 4 are placed on the Si ₃ N ₄ surface immediately above the irradiation point. As shown in FIGS. 2A and 2B, the intensities of the first and second irradiation light pulses 9 and 10 are modulated such that the phases are shifted (out-of-phase) at the same frequency. Irradiation light pulse intensity, luminous flux diameter, cell area,
If the electrolyte volume and other parameters are the same, the photocurrents generated in the two cells are the same and opposite in phase.

Hereinafter, a specific example will be described.

FIG. 3 is a configuration diagram of a semiconductor depletion layer capacitance measuring system showing a specific example of the present invention.

In this figure, reference numeral 11 denotes a bulk semiconductor (S
i) and an insulator (Si ₃ N ₄ / SiO) formed thereon
₂ ) A sensor composed of layers, 12 is a measuring cell, 13 is an electrode made of an Ag / AgCl rod connected to the measuring cell, 14 is a bias voltage source (variable voltage application) applied to this electrode 13, and 15 is Earth, 16 is a control cell, 1
Reference numeral 7 denotes an electrode made of an Ag / AgCl rod connected to the control cell 16, reference numeral 18 denotes a control bias voltage source (variable voltage application), reference numeral 19 denotes ground, and reference numeral 20 denotes a metal contact that contacts the back surface of the sensor 11. Metal contact 20
Is connected to a first input terminal of an amplifier (pre-amplifier) 23. The second input terminal of the amplifier 23 is connected to the ground 2
4 is connected.

On the other hand, for example, 1
A first laser beam 21 of 0 kHz to 30 kHz and a second laser beam 22 whose phase is shifted by the same frequency by the phase shift circuit 25 are irradiated.

FIG. 4 shows L of the semiconductor depletion layer capacitance measuring device.
4A is a time sequence of an APS signal, and FIG. 4A is a diagram showing a photocurrent characteristic by a first laser beam, and FIG.
FIG. 4 shows a photocurrent characteristic by the second laser beam,
(C) is a photocurrent characteristic diagram due to the combination of the first laser light and the second laser light.

As described above, when voltages having two phases shifted from each other are applied to the common back contact of the sensor 11, FIG.
As shown in FIG. 4C, since the mutual noise and the basis element are canceled, the noise and the background are low as plotted in FIG. When the pH value of the electrolyte or the surface potential of one cell changes, the photocurrent of that cell also changes, so that a difference signal proportional to the change is output.

This measuring method was used for pH measurement. Therefore, the pH can be measured directly from the difference signal if only the calibration value of the sensor is obtained.

Therefore, in this case, since there is no need to perform an operation of smoothing the detected data or finding the difference twice, the measurement is quicker than the conventional method.

Using LAPS, in the usual way, LAPS
The pH of the electrolyte was measured from the amount of the bias shift of the photocurrent response. The shift amount of the photocurrent response in the voltage direction is measured p
It is determined as the difference between the inflection point potentials of the H buffer and the control pH buffer (regardless of the choice of control). LA
Regarding PS, the inflection point potential is the bias voltage when the second derivative of the response photocurrent crosses zero.

FIG. 5 is a diagram showing the response photocurrent / V characteristic with respect to the bias / V of LAPS, and FIG. 6 is a diagram showing how to find the inflection point potential.

Since the work of obtaining the inflection point P potential of the measurement data is affected by noise of the measurement data, the curve must be previously smoothed by filtering. In addition, the steeper the curve, the higher the calculation accuracy. This is because if the gradient is steep, the point crossing the zero value is more accurately determined, and the accuracy of the pH measurement is improved [M. Sartore, M .; Adami, and
C. Nicolini, "Computer simu
ration and optimization of
a light-addressable pote
ntiometric sensor ”, Biosen
sors Bioelectronics. , 7 (19
92) 57-64].

[0037] In this method, first, first to adjust both the bias, for example when the bias voltage of the measurement cell was V _mi, control solution in the two cells (in this case pH-
Determine the "minimum difference" signal (v _r) of 4.55).

Next, by changing the pH buffer in the "measurement cell", a difference signal ( _vmpH ) corresponding to a new pH value is _obtained . Then, the bias voltage of the "measurement cell" is changed, and a "minimum difference" signal for the changed bias voltage _VmpH is obtained. The change in the bias voltage of the measurement cell here becomes the pH sensitivity of the Si ₃ N ₄ layer of the LAPS used. One or both of the difference signal and the bias change are used to calculate a quantitative value of pH. If a personal computer is used for bias scanning, pH measurement can be performed automatically and quickly.

Hereinafter, an experimental example will be described with reference to FIG.

The sensor 11 has a hetero structure composed of an Au / Si / SiO ₂ / Si ₃ N ₄ layer of n-type Si (1 to 10 Ω · cm, thickness 180 μm) of <100> orientation. The two infrared lasers (λ = 830 nm) are a first laser 21 and a second laser 22,
Irradiation is performed from the back side of No. 1. The two lasers have a frequency of 10k
Hz to 30 kHz, and the phases are shifted by 180 ° from each other by the phase shift circuit 25. In order to obtain the photocurrent signal with the minimum difference, it is sometimes necessary to finely adjust the phase by the phase shift circuit 25.

The two cells 12, 16 are separated by 25 mm.
It is installed on the surface of Si ₃ N ₄ just above the irradiation point. Each cell is as small as 8 mm in diameter, uses a silicone rubber seal, and is in contact with the electrolyte. One cell is used as a control buffer and the other as a measurement buffer. Ag / AgCl rod electrodes 13 and 17 are used for the two cells 12 and 16, respectively, and serve as electrodes for applying a specified control voltage to the electrolyte 16 by the DC power supply 18. Ag / AgC
The electrodes 13 and 17 of the 1 rod are connected to the Ag rod in the laboratory
It was prepared by thick electrodeposition of gCl. 10MΩ resistance and 1
A parallel connection of a 00 μF capacitor was connected in series to the power supply to prevent leakage of DC current. The difference signal of the photocurrent is
The signal was received by the metal contact 20 on the rear surface, amplified by a factor of 10 ⁶ , and converted into a voltage by a preamplifier (comparator) 23. An oscilloscope (not shown) was used for reading.

Various buffers in the range of pH-3.55 to pH-5.65 were used for the measurement by the semiconductor depletion layer capacitance measuring device. Buffers are prepared in the lab,
Each pH value was always measured immediately before the experiment. First, the measurement cell 12 and the control cell 16 were filled with a buffer of pH -4.55 as a control solution. The response photocurrent was measured by irradiating only the first laser 21, irradiating only the second laser 22, and simultaneously irradiating the first laser 21 and the second laser 22.

Next, as shown in the characteristic diagram of the converted photocurrent / V with respect to the bias / V in FIG. 7, a point where the photocurrent does not reach saturation even at the highest pH value to be measured is carefully selected. Was used as the operating point for pH measurement. When strongly inverted, the photocurrents of both cells do not become the same value. This is probably due to non-uniformities in the sensing surfaces of both cells, mismatches in laser intensity and beam diameter.

[0044] In order to obtain a minimum difference signal (v _r), DC bias of both cells was adjusted to close to the operating point.
In this experiment, 1.390 V (V _bi ) and 1.445 V (V _mi ) were measured for the measurement cell 12 and the control cell 16, respectively.
Was adjusted to a minimum difference signal (v _r) is, 13.3MVp-
p. The base photocurrent signals of the measurement cell 21 and the control cell 16 are 211 mVp-p and 210 mVp, respectively.
-P. The phase shift between the two lasers was adjusted around 180 ° to obtain the minimum difference signal. here,
As an example, the buffer of the control cell 16 is adjusted to pH-4.5
5 and the buffer of the measuring cell 12 is adjusted to pH-3.
Change it to 55. Due to the change in the pH value, the measurement cell 1
2 and the amplitude of the photocurrent change, and the difference signal (v
_mpH ) was 25.3 mVp-p. change in photocurrent by pH changes, the voltage difference between the two difference signals, that is, calculated as (v _mpH -v _r).

FIG. 8 shows the difference signals for the various pH buffers. As the pH value increases, the voltage difference
It was assumed to be positive for the H buffer and negative when the pH value decreased.

Conceptually, the change in photocurrent is due to a change in the insulator surface potential that is sensitive to the pH of the buffer. Here, when the DC bias of the measurement cell 12 is increased, the photocurrent of the cell increases, v _mpH (difference signal) decreases, and a new bias of 1504.5 mV is applied.
(V _mpH ) to v _r (minimum difference signal). The difference between the initial bias (V _mi ) and the new bias (V _mpH ) in the measurement cell is the pH sensitivity (mV / pH) of the sensor 11 itself. Measurement buffer pH
Value is at higher than the pH value of the control buffer, decrease the bias of the measuring cell 12, to obtain a minimum difference signal (v _r).

As shown in FIG. 9, plotting the difference between the bias of the control pH buffer and the bias for various pH values gives the pH sensitivity. The average value of pH sensitivity in the tested pH range was 57.6 mV / pH,
This was very close to the Nernst value. This measurement system has some limitations on the pH measurement range.

As can be seen from FIG. 7, the sensor switches from accumulation to inversion with only a 350 mV change in surface potential. When the sensitivity is around 58 mV / pH, as obtained in this experiment, the pH range that can be safely measured is 6 pH (eg, pH-4 to p-p).
H-10). The entire process of pH measurement can be automated by using a personal computer for data collection and bias scanning in the measurement buffer.

As described above, a general measurement method for LAPS has been described. In principle, the measurement system described here provides a means for low noise and low background when applying LAPS to biosensors. As one of the applications of the new measurement system, pH measurement was studied. In the pH range from 3.55 to pH 5.65, the pH sensitivity was 57.9 mV / pH, which was very close to the Nernst value. A measuring device capable of directly and quickly measuring pH with high sensitivity without using an expensive device such as a potentiostat or a lock-in amplifier could be provided.

It should be noted that the present invention is not limited to the above-described embodiment, and various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0051]

As described above, according to the present invention, the following effects can be obtained.

(A) A simple and low-noise, low-background semiconductor depletion layer capacitance measurement can be performed.

(B) A new, rapid and highly sensitive direct measurement of pH can be performed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a semiconductor depletion layer capacitance measurement system showing an embodiment of the present invention.

FIG. 2 is a diagram showing waveforms at various parts of a semiconductor depletion layer capacitance measuring system according to an embodiment of the present invention.

FIG. 3 is a configuration diagram of a semiconductor depletion layer capacitance measurement system showing a specific example of the present invention.

FIG. 4 is a time sequence of a LAPS signal of a semiconductor depletion layer capacitance measuring device showing a specific example of the present invention.

FIG. 5 shows bias / V of LAPS showing an embodiment of the present invention.
FIG. 7 is a response photocurrent / V characteristic diagram for the response to the following.

FIG. 6 shows a bias / V of LAPS showing an embodiment of the present invention.
FIG. 9 is a diagram showing a method of obtaining an inflection point potential of a response photocurrent / V with respect to the following.

FIG. 7 shows LAPS bias / V showing an embodiment of the present invention.
FIG. 10 is a characteristic diagram of converted photocurrent / V with respect to FIG.

FIG. 8 shows difference signals for various pH buffers of LAPS illustrating an embodiment of the present invention.

FIG. 9 is a plot of the difference between the bias of the control pH buffer and the bias for various pH values of LAPS showing an embodiment of the present invention.

FIG. 10 is a schematic configuration diagram of a conventional chemical sensor LAPS.

FIG. 11 is a waveform diagram of each part of a conventional chemical sensor LAPS.

[Explanation of symbols]

 Reference Signs List 1 bulk semiconductor (Si) 1A depletion layer 2 insulator layer 3 first electrolyte (measurement cell) 4 electrode of first electrolyte 5 second electrolyte (control cell) 6 electrode of second electrolyte 7 DC voltage source ( 8, 23 Amplifier 9 First irradiation light pulse 10 Second irradiation light pulse 11 Sensor 12 Measurement cell 13, 17 Electrode (Ag / AgCl rod) 14, 18 Bias voltage source (Variable voltage application) 15 , 19,24 Earth 16 Control cell 20 Metal contact 21 First laser beam 22 Second laser beam 25 Phase shift circuit

Claims (6)

[Claims] In a semiconductor depletion layer capacitance measuring method,
(A) setting a sensor having a first and second electrolyte-insulator layer-semiconductor bulk structure; (b) applying a bias voltage to a depletion layer of the semiconductor bulk; and (c) applying a bias voltage to a back surface of the semiconductor bulk. Irradiating two phase-shifted light pulses respectively corresponding to the first and second electrolytes,
(D) deriving the output from the electrodes connected to the first and second electrolytes and using a differential detection method to reduce the noise
A method of measuring a semiconductor depletion layer capacitance, comprising detecting a low background photocurrent. 2. The semiconductor depletion layer capacitance measurement method according to claim 1, wherein the pH of the electrolyte is measured. 3. A semiconductor depletion layer capacitance measuring device,
(A) a sensor having a first and second electrolyte-insulator layer-semiconductor bulk structure; (b) means for applying a bias voltage to a depletion layer of the semiconductor bulk; and (c) a back surface of the semiconductor bulk. Means for irradiating two phase-shifted light pulses respectively corresponding to the first and second electrolytes, and (d) differentially detecting the output from the electrodes connected to the first and second electrolytes. Means for deriving a photocurrent with low noise and low background derived by a method. 4. The semiconductor depletion layer capacitance measuring device according to claim 3, wherein said insulator layer is Si ₃ N ₄ / SiO ₂ , and said semiconductor bulk is Si bulk. . 5. The semiconductor depletion layer capacitance measuring device according to claim 3, wherein the light pulse is a laser beam of 10 kHz to 30 kHz. 6. The semiconductor depletion layer capacitance measuring device according to claim 3, wherein the first and second electrolytes are a measurement cell and a reference cell.