JPH05312778A - Ion concentration sensor - Google Patents

Abstract

PURPOSE:To miniaturize an ion concentration sensor and to make detection sensitivity more stable, which uses an ion sensitive transistor having a sensitive film at its gate. CONSTITUTION:A p-channel transistor 21 having sensitive films 7 on each surface of n- and p-type substrates 2 and 3, respectively, an n-channel transistor 22 and a gate electrode 30 common to the both are formed on a semiconductor chips 10, and, the sensitive film 7 and the gate electrode 30 are made to contact to a detection - object 40, such as electrolyte. In addition, a gate voltage Vg whose polarity is switchable between positive and negative is applied to the gate electrode 30. Then, under that condition, the differential signal between an ion detecting signals S1 and S2 through transistors 21 and 22 is taken out as an ion concentration signal So.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for detecting ion concentration such as hydrogen ion concentration pH, and more particularly to an ion concentration sensor suitable for miniaturization.

[0002]

2. Description of the Related Art Conventionally, as is well known, the ion concentration, particularly the hydrogen ion concentration, is often measured using a reference electrode or a glass electrode, but it has the advantage of being accurate, but it is difficult to miniaturize it. There is a problem in managing the electrolyte in the reference electrode so that the measured value does not drift,
For example, there is a demand for a new ion concentration sensor that is simple in structure and easy to miniaturize so that it can be used for living organisms.

In order to meet such a demand, an ISFET (Ion Sensitive Field-Effect Transistor) in which a transistor has a sensor function by using a film having ion sensitivity as a gate insulating film of an insulated gate field effect transistor
An ion concentration sensor called istor) is known, and various innovations have been made in the reference electrode to be combined therewith. A conventional example in which a field effect transistor is also used in this reference electrode is described below with reference to FIG. Briefly explained.

FIG. 4A shows a state in which the ISFET 71 and the reference gate transistor 72 are immersed in a detection object 40 such as an electrolytic solution for detecting the ion concentration in the container 41 and its related circuit. ISFET 71 has a sensitive film 73 and a reference gate transistor 72.
Each has a reference electrode 74 at its gate. Since the sensitive film 73 is an insulating film, in order to apply a gate voltage to the ISFET 71, for example, the gate electrode 75, which is a platinum pseudo reference electrode, is immersed in the detection target 40. A direct current voltage is applied to both the transistors 71 and 72 from the power supply 80 to keep constant currents flowing through the constant current circuits 81 and 82, and the voltage of the power supply 80 is applied to the gate electrode 75. Since the source-drain resistance of the ISFET 71 having the sensitive film 73 changes according to the ion concentration of the detection target 40, when the ion concentration is a reference concentration, for example, the hydrogen ion concentration pH is 7, the sources of both transistors 71 and 72 are If the drain-to-drain voltages are balanced in opposite directions, the voltage difference between the two can be taken out as the detection voltage Vo representing the ion concentration of the detection target 40 and applied to the measurement circuit 60 as shown in the figure.

FIG. 4B is a sectional view showing a structural example of the reference electrode 74 of the reference gate transistor 72. The source layer 72 of the transistor 72 is formed on the surface of the p-type substrate 11 of the semiconductor chip 10, for example.
a and the drain layer 72b are diffused in the n-type, the electrode film 13 disposed on the insulating film 12 is connected to them, and the electrode film 13 is covered with the protective film 14 so as not to contact the detection target 40. .. The reference electrode 74 is obtained by filling a small hole in an insulating layer 74a provided on the surface of the substrate 11 which is the substrate of the transistor 72 with an agar gel or the like which holds a buffer electrolyte solution 74b such as potassium chloride and the like. Is covered with a porous film 74c of, for example, a metal so that the electrolytic solution 74b is in a liquid junction with the detection target 40.

[0006]

As shown in FIG. 4, a source / drain of a reference gate transistor 72 including an ISFET 71, which corresponds to a normal glass electrode, and a reference gate transistor 72, which corresponds to a normal reference electrode, and which includes the above-described reference electrode 74. ISFET with sensitive film 73, while resistance between them is constant
This ion concentration can be detected by utilizing the fact that the resistance between the source and drain of 71 changes according to the ion concentration of the detection target 40 with the gate electrode 75 applied with a predetermined gate voltage, and the ISFET 71 and the reference gate transistor can be used. Although the ion concentration sensor can be downsized to about several mm square by incorporating 72 in the same semiconductor chip, the structure of the reference electrode 74 is shown in FIG.
In reality, it is very difficult to reduce the size even further because it is still quite complicated as in (b).

Furthermore, since the buffer electrolyte solution 74b of the reference electrode 74 is in a liquid junction with or flows through the object 40 to be detected, the ion concentration thereof gradually changes after long-term use, causing an error or drift in the ion concentration detection value. There is. of course,
Similar to a normal ion densitometer, if the electrolyte in the reference electrode is replaced, the detection accuracy will be restored to its original value. However, as shown in Fig. 4 (b), the electrolyte will be replaced when the reference electrode 74 is 1 mm or less in size. Since it is very troublesome, the ion concentration sensor is actually disposable, and especially in applications where a small sensor is inserted or embedded in a living body, even if the sensor itself is inexpensive, it cannot be easily replaced. In view of the current situation, an object of the present invention is to obtain an ion concentration sensor that can be easily miniaturized and has high stability of detection accuracy.

[0008]

According to the ion concentration sensor of the present invention, a pair of field-effect transistors in which opposite conductivity types are given to a substrate having an ion-sensitive film on the surface thereof and a pair of field-effect transistors are provided. A common gate electrode is provided, and the difference signal between the ion detection signals from both transistors is detected by contacting the sensitive film and the gate electrode to the detection target and applying a gate voltage that sequentially changes positive and negative to the gate electrode. It is achieved by taking out as an ion concentration signal representing

It is preferable to use a thin film of silicon nitride, alumina, tantalum oxide or the like formed by the CVD method as the sensitive film having the above structure, and a pair of transistors having such sensitive film at the gate are provided together with the gate electrode. Fabrication in a single semiconductor chip is advantageous for miniaturization of the sensor. The ion concentration sensor of the present invention is used, of course, in a state where a current is applied to the pair of transistors.
It is desirable to adjust the ratio of the currents flowing through both transistors so as to enhance the detection sensitivity. Also, the gate voltage applied between both transistors and the gate electrode should be at least several
The positive and negative polarities should be alternated at a frequency of Hz, preferably several tens of Hz or higher.

Further, the ion concentration sensor of the present invention is No. 1
It is used with the sensitive film and gate electrode of the paired transistor always in contact with the detection target such as electrolyte solution, but at this time, a slight voltage drop occurs at the interface between the sensitive film and the gate electrode of the detection target. Therefore, setting the gate voltage higher than the sum of these voltage drops is very advantageous in ensuring the detection operation. Further, as described above, two pairs of transistors whose substrates are of opposite conductivity type are provided, the transistors of each pair are connected in series between a pair of power points, and the transistors of different sets of substrates are of opposite conductivity type. Are connected so as to correspond to each other, and the ion concentration signal is taken out from the interconnection point of the transistor pairs of each set, which is advantageous in enhancing the linearity of the detection sensitivity and the detection characteristic.

[0011]

[Function] The conventional ion concentration sensor described in FIG.
Although it was a so-called fixed operation type in which T and the reference gate transistor play their respective unique roles, the present invention uses a pair of transistors or ISFETs having a sensitive film to make them both of opposite conductivity types, and By applying a gate voltage with positive and negative alternating to the common gate electrode in order to make both transistors alternately function as the conventional ISFET and the reference gate transistor, it is an alternating operation type.
The reference electrode of the reference gate transistor, which is the main cause of the conventional problems, is omitted to facilitate the miniaturization of the sensor and to eliminate the drift of the ion concentration value.

That is, in the ion concentration sensor of the present invention, one of the pair of transistors of mutually opposite conductivity type operates as an original ISFET which flows a current corresponding to the ion concentration according to the positive or negative polarity of the gate voltage. The other operates, as it were, as a conventional reference gate transistor in a state in which it is substantially non-conductive or a small current flows. Thus, in the present invention, since the pair of transistors operate in a so-called differential manner, the difference signal between the ion detection signals of both transistors can be extracted as the ion concentration signal.

[0013]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a sectional view of a semiconductor chip incorporating the ion concentration sensor of the present invention and its related circuit diagram, FIG.
3 is a waveform diagram of main signals showing an operation example of the embodiment of FIG.
FIG. 3 is an ion concentration sensor and its related circuit diagram showing another embodiment of the present invention. In these embodiments, a pair of transistors and a gate electrode are formed in a single semiconductor chip for downsizing the ion concentration sensor, and the gate voltage applied to the gate electrode has a positive and negative polarity at a frequency of about several tens Hz. Shall be replaced.

In FIG. 1 (a), a semiconductor chip 10 is obtained by growing an epitaxial layer 2 of n type on a p type semiconductor substrate 1 as in the case of an ordinary CMOS integrated circuit device. P-channel transistor 21 of a pair of transistors 21 and 22 used for the ion concentration sensor of
Is an n-type substrate 2 whose epitaxial layer is
The channel transistors 22 are each formed with the well diffused in the epitaxial layer as the p-type substrate 3. In the transistor 21, the source layer 21a and the drain layer 21b are diffused in p-type from the surface of the n-type substrate 2, and the substrate connection layer 21c is diffused in n-type in the illustrated example. Similarly, in the transistor 22, the source layer 22a and the drain layer 22b are diffused in the n-type from the surface of the p-type substrate 3 and the substrate connecting layer 22 is formed.
c is diffused in p-type. These diffusion layers may be formed in the same manner as a normal field effect transistor, and the substrate connection layers 21c and 22c are not always necessary.

Wirings for both transistors 21 and 22 may be provided in the same manner as in a normal integrated circuit device. First, the surface of the semiconductor chip 10 is covered with an insulating film 4, and then a window is opened at a required place. An aluminum wiring film 5 connected to the source / drain layers of both transistors is provided in the window on the upper side thereof as shown in the figure. It should be noted that an opening is also provided at a position where the sensitive film 7 should be provided later when the window of the insulating film 4 is opened. In the example shown in the figure, when the wiring film 5 is provided, the substrate connection layer 21c of the transistors 21 and 22 is formed. 22c is short-circuited with the source layers 21a and 22a, respectively. Further, a dense protective film 7 made of silicon nitride or the like is formed on the upper side of the wiring film 5 so that the wiring film 5 does not come into contact with the electrolytic solution or the like as the detection target 40, and the sensitive film 7 is formed on the protective film 7.
An opening for the gate and a window for the gate electrode 30 are provided.

The sensitive film 7 is preferably a thin film of silicon nitride, alumina, tantalum oxide, or the like. In this embodiment, the silicon nitride film is contacted with the surface of the substrate 2 or 3 in the above-mentioned opening by, for example, plasma CVD. By the method, a photo-etching process is performed to form a pattern in which the peripheral edge overlaps with the protective film 6 as shown in the figure. This silicon nitride sensitive film 7 is particularly suitable for detecting the hydrogen ion concentration. As the metal for the gate electrode 30, platinum is preferably used as in the case of FIG. 4 (a). By depositing this on the protective film 2 by a sputtering method or a vapor deposition method and performing photoetching, As shown in the figure, a pattern common to both transistors 21 and 22 is formed on the protective film 6 between them, and is connected to the lower wiring film 6 through the above-mentioned window formed in the protective film 6. ..

The ion concentration sensor shown in FIG. 1 (a) is shown in FIG.
It is incorporated in the detection circuit 50 shown in (b) and used while the sensitive film 7 and the gate electrode 30 are in contact with the detection target 40. In FIG. 1B, the gate electrode 30 is shown in a position facing the sensitive film 7. The transistors 21 and 22 are connected between the pair of power supply potential points V and E in series with the adjusting resistors 51 and 52, respectively, and the gate voltage Vg at which the polarity generated by the pulse generating circuit 55 is sequentially changed between positive and negative is the gate electrode. Granted to 30. Ion detection signals S1 and S2 are derived from the interconnection points of the transistors 21 and 22 and the resistors 51 and 52, respectively, and the difference signal between these signals is given to the measurement circuit 60 as the ion concentration signal So.

FIG. 2 shows the above-mentioned ion detection signal S1, when the gate voltage Vg having the waveform shown in FIG.
The waveforms of S2 and the ion concentration signal So are shown. Figures (b) to (d) show the cases (e) to (d) when the cation concentration of the target 40 is high.
(g) corresponds to the case where the cation concentration is low or the anion concentration is higher than the cation concentration, respectively. First,
The case where the cation concentration is high will be described. The p-channel transistor 21 is a transistor when the gate voltage Vg is positive.
21 becomes conductive and a current according to the ion concentration flows, but when the gate voltage Vg is negative, almost no current flows. Therefore, the ion detection signal from the transistor 21 in FIG.
S1 has a waveform in which the voltage Vg takes a low value within a positive time as shown in the figure.

On the other hand, an n-channel transistor 22
In the case of high cation concentration, there is almost no conduction,
As described above, since it operates as a reference gate transistor, the ion detection signal S2 by the transistor 21 of FIG. 2 (c) has a small change and has a waveform in which the gate voltage Vg drops slightly within a negative time. The difference signal between these ion detection signals S1 and S2, the ion concentration signal So in FIG. 2 (d), has a positive pulse and a voltage Vg takes a slightly negative value within a negative time as shown in the figure. The measuring circuit 60 that receives the ion concentration signal So is preferably configured to convert the positive and negative peak values of the pulse waveform into a digital value and calculate the ion concentration value from the average value.

When the anion concentration is high, the ion detection signal S1 shown in FIG. 2 (e) is almost flat and the gate voltage Vg has a waveform that slightly drops within a negative time.
S2 has a waveform in which the voltage Vg takes a low value within a negative time, and the difference signal between them, the ion concentration signal So in Fig. 2 (g), is a negative pulse and has a slightly positive value during a positive voltage Vg. The waveform becomes In this case, the transistor 21 operates as a reference gate transistor, so that the ion concentration signal So has a negative pulse-like waveform, which indicates that the anions in the detection target 40 are more dominant than the cations. .. In addition,
The adjusting resistors 51 and 52 in FIG. 1 (b) are preferably adjusted and placed so that the ion concentration value obtained from the ion concentration signal So becomes 0 when the hydrogen ion concentration pH is 7, for example.

FIG. 3 is a circuit diagram corresponding to FIG. 1 (b) showing a different embodiment of the present invention. In this embodiment, two pairs of transistors or ISFETs of opposite conductivity type each having a sensitive film 7 are used, and a series circuit of the transistors 21 and 22 of the first pair and the adjusting resistor 53 and the transistor 23 of the second pair are used. A series circuit of 24 and adjusting resistor 54 is connected between power supply potential points V and E, and p-channel transistors 21 and 24 are arranged so as to correspond to n-channel transistors 23 and 22, respectively, as shown. Further, in the example of the drawing, the gate electrodes 31 and 32 provided for each pair are commonly provided with a gate voltage Vg that is sequentially switched between positive and negative by the pulse generation circuit 55, and the transistors 21 and 22 of the first pair are connected. Interconnection point and second pair of transistors 2
Ion detection signals S3 and S4 from 3 and 24 interconnection points, respectively
And the difference signal between the two is given to the measurement circuit 50 as the ion concentration signal So.

As can be easily understood, in this embodiment, the four transistors 21 to 24 form a kind of modified bridge circuit, so that the currents flowing through the first and second transistor pairs by the adjusting resistors 53 and 54. By balancing the above, the ion concentration detection sensitivity can be increased as compared with the previous embodiment, and the nonlinearity of the ion concentration detection characteristics of each transistor pair can be compensated for, and the linearity can be improved. In addition,
In both the embodiment of FIG. 1 and the embodiment of FIG. 3, it is advantageous to set the gate voltage Vg higher than the sum of the voltage drops that can occur near the interface between the sensitive film to be detected and the gate electrode. In this embodiment, it is desirable to set the value slightly higher than that in the embodiment of FIG.

[0023]

As described above, in the ion concentration sensor of the present invention, a pair of field effect transistors in which conductivity types opposite to each other are imparted to a substrate having a sensitive film for ions on its surface and a gate common to them. An electrode is provided, the sensitive film and the gate electrode are brought into contact with a detection target such as an electrolyte solution whose ion concentration is to be detected, and a gate voltage applied to the gate electrode that sequentially alternates between positive and negative is applied to the gate electrode. By taking out as the ion concentration signal representing the ion concentration of the detection target, the following effects can be achieved.

(A) A pair of ion-sensitive transistors having different conductivity types alternately play the role of a reference gate transistor according to the positive / negative of the gate voltage, so that it is not necessary to provide a reference electrode as in the conventional case, and the transistor pair is not required. The ion concentration sensor can be miniaturized by forming the gate electrode in a semiconductor chip of 2 to 3 mm square or less. (b) Since the electrolyte that is liquid-junctioned to the detection target that was conventionally held on the reference electrode is no longer necessary, there is no cause for drift in the ion concentration detection value, and the detection sensitivity of the ion concentration sensor is maintained stable for a long period of time. can do.

(C) Since a pair of ion-sensitive transistors are operated differentially to extract the ion concentration signal, the non-linearity of the detection characteristic of each transistor is automatically compensated, and the ion concentration is accurately measured with a good linear characteristic. Can be detected, and the ion concentration detection range can be expanded as compared with the conventional case. (d) Since the transistor pair including the sensitive film and the gate electrode can all be formed in the semiconductor chip by utilizing the semiconductor process technology, it is possible to provide an ion concentration sensor with good performance at low cost.

[Brief description of drawings]

FIG. 1 shows an embodiment of an ion concentration sensor of the present invention,
Figure (a) is a cross-sectional view of a semiconductor chip with an ion concentration sensor built in, and Figure (b) is a related circuit diagram.

2 shows the main signals associated with the embodiment of FIG.
(a) is the gate voltage, (b) is the ion detection signal from a p-channel transistor when the cation concentration is high,
The figure (c) is the ion detection signal by the n-channel transistor in the same case, the figure (d) is the ion concentration signal in the same case, and the figure (e) is the p-channel transistor when the anion concentration is high. 2F is a waveform diagram of the ion concentration signal by the n-channel transistor in the same case, and FIG. 6G is a waveform diagram of the ion concentration signal in the same case.

FIG. 3 is a related circuit diagram showing another embodiment of the ion concentration sensor of the present invention.

4A and 4B show a conventional ion concentration sensor, FIG. 4A is a related circuit diagram thereof, and FIG. 4B is a sectional view of a reference electrode of a reference gate transistor.

[Explanation of symbols]

2 Epitaxial layer as substrate 3 Well as substrate 7 Sensitive film 10 Semiconductor chip with ion concentration sensor 21 p-channel type transistor or ISFET 22 n-channel type transistor or ISFET 23 n-channel type transistor or ISFET 22 p-channel transistor or ISFET 30 gate electrode 31 gate electrode 32 gate electrode 40 detection target or electrolyte S1 ion detection signal by p-channel transistor S2 ion detection signal by n-channel transistor S0 ion concentration signal Vg gate Voltage

Claims (3)

[Claims] 1. A pair of field effect transistors having opposite conductivity types applied to a substrate having a sensitive film for ions on the surface thereof, and a gate electrode common to the transistors, and a sensitive film and a gate electrode to be detected. Ion characterized in that the difference signal of the ion detection signals by both transistors is taken out as an ion concentration signal representing the ion concentration of the detection target while the gate voltage is applied to the gate electrode which makes contact with Concentration sensor. 2. The sensor according to claim 1, wherein the gate voltage is set to be larger than the sum of the voltage drops respectively generated near the interface between the sensitive film and the gate electrode in the object to be detected. Ion concentration sensor. 3. The sensor according to claim 1, wherein the substrate is provided with two pairs of transistors of opposite conductivity type, and each pair of transistors is connected in series between a pair of power supply points and substrates of different pairs are provided. An ion concentration sensor characterized in that transistors of opposite conductivity type are connected so as to correspond to each other, and an ion concentration signal is taken out from an interconnection point of each pair of transistors.