JP2007192628A - Gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a humidity sensor enhanced in reliability by preventing an electrode part such as a sensing electrode part or a connection electrode for connecting a substrate from being exposed under a measuring environment and increasing the opening degree of a humidity-sensitive part to enhance the responsiveness of the sensor. <P>SOLUTION: The whole surface of a sensor chip is exposed to the measuring environment on its one surface side 3 and its one surface side 5 has sensing electrodes 7 and 8 and a signal processing circuit part 9 for processing the signals from the sensing electrodes after a semiconductor film 6 is formed to the sensor chip. The sensing electrodes are passed through a terminal substrate 1 by a wiring member 11 to be electrically connected to the outside. The sensing electrodes 7 and 8 and the signal processing circuit part 9 are covered with and protected by a protective film 10 consisting of a humidity resistant. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a gas sensor in which a signal processing circuit and a gas sensor unit are formed on the same semiconductor substrate, and in particular, the gas sensor unit includes a porous film that functions as a moisture-sensitive film whose capacitance value or resistance value changes according to humidity, The present invention relates to an electrode configured to detect a change in capacitance value or resistance value of a porous membrane.

(1) Structure of moisture-sensitive part The moisture-sensitive part generally uses a moisture-sensitive film whose capacitance value or resistance value changes according to humidity, and is roughly classified into two types. The first is a structure with a sensing electrode on one side of a sensor chip made of semiconductor, ceramic, organic material, etc., to detect the change in capacitance or resistance of the moisture sensitive film. The second is moisture sensitive. It has a structure with sensing electrodes on both sides of the membrane.

  The sensor chip is provided with a connection electrode routed from the sensing electrode, and the sensing electrode is electrically connected to a wiring member for external connection through the connection electrode.

-Long-term stability of moisture-sensitive film Although organic materials may be used as the moisture-sensitive film, there are problems in durability in a high-temperature and high-humidity atmosphere, and long-term reliability may be deteriorated. A long-term stability is achieved by forming a porous region on a silicon substrate to form a gas sensor, and the sensor is made of an inorganic material and the surface of the porous silicon is covered with thermally and scientifically stable silicon oxide. Excellent.

・ Single-chip signal processing circuit Furthermore, if the signal processing circuit is formed on the non-porous part of the same substrate, the sensor part and the signal processing circuit are made into one chip, and there are advantages such as downsizing and cost reduction. There is such a problem.

Sensors using porous silicon as a moisture-sensitive part are disclosed in, for example, Sensors and Actuators B 95 188-193, 2003 and JP 6-23709.
Japanese Patent Publication No. 6-23709 Sensors and Actuators B 95 188-193, 2003

-Corrosion of electrodes However, in the humidity sensor as described above, there is a problem that the electrode portion provided on the sensor chip, that is, the sensing electrode and the connection electrode are easily corroded when exposed to a high humidity environment. . For this reason, it is conceivable to use a corrosion-resistant electrode material, such as a noble metal, which does not easily corrode as the electrode part, but the electrode material is limited. In particular, when the sensor part is placed on the same substrate as the signal processing circuit and the electrode of the sensor part is to be formed by the electrode / wiring formation technology in the semiconductor process when forming the signal processing circuit, it is common in the semiconductor process. It is desirable to use typical materials.

・ Advantage of electrode placement When forming a moisture sensitive part using an organic material on a semiconductor substrate on which a signal processing circuit is formed, the electrode that senses the moisture sensitive part is formed in advance at the position where the moisture sensitive material is placed. In general, a moisture sensitive material is formed thereon by a method such as coating. However, when porous silicon is used as a moisture sensitive material, it is not easy to dispose an electrode under the porous layer because a part of the semiconductor substrate is made porous by a method such as anodization.

  On the other hand, in the case of a structure in which the electrode is disposed on the upper surface of the moisture sensitive portion, the electrode on the moisture sensitive film becomes an obstacle to moisture permeation, and the moisture absorption characteristics of the moisture sensitive film are limited. In addition, the electrode member is also restricted in terms of corrosion and cost.

  In other words, when using a moisture-sensitive part of an inorganic material that is excellent in long-term stability, especially porous silicon, porous silicon oxide, etc., electrode placement to overcome electrode corrosion and electrode moisture-sensitive surface coating There was a problem that the advantages and disadvantages of the deterioration of moisture sensitivity due to a decrease in the aperture ratio contradict each other.

  In view of the above problems, the present invention solves the problem of deterioration of the sensor in a high humidity atmosphere by using porous silicon or porous silicon oxide for the sensor part, and also forms a substrate on which porous silicon is formed. When the signal processing circuit is formed on the same substrate and the sensor chip is miniaturized by utilizing the fact that it is silicon, the electrodes such as sensing electrodes and electrodes for connecting to the substrate are prevented from being exposed to the measurement environment. In addition, an object of the present invention is to provide a highly reliable humidity sensor that is excellent in responsiveness by increasing the aperture ratio of the moisture sensitive portion.

  In order to achieve the above object, in the present invention, sensing electrodes 7 and 8 for detecting a change in capacitance value or a change in resistance value, a signal processing circuit unit 9 for electric signal processing, Wiring 11 for electrical connection and a protective film 10 made of a moisture-resistant material for protecting them are provided, and one surface 3 side is characterized by a structure in which the entire surface is exposed to the measurement environment.

  According to the present invention, since the sensing electrodes 7, 8 and the signal processing circuit unit 9 are covered with the protective film 10 and can be configured on the one surface 5 side of the sensor chip 2, it is prevented from being exposed to the measurement environment. be able to.

  As a result, a material having no corrosion resistance (for example, a material such as Al or an Al alloy) can be used as a material for the electrode portion, so that the sensor chip can be manufactured by a normal semiconductor process, and miniaturization and mass production are possible. It becomes possible.

  The invention according to claim 1 is characterized in that the one surface 3 side of the sensor chip is used as the moisture sensitive portion. According to this, water molecules trying to enter the moisture sensitive film are not hindered by the electrode. In addition, the same effect is obtained when water molecules are desorbed, which is preferable because the responsiveness of the sensor can be increased.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments shown in the drawings will be described below. FIG. 1 is a diagram showing a schematic cross-sectional configuration of a capacitive humidity sensor according to an embodiment of the present invention.

・ Description of sensor structure (Fig. 1)
〔The entire〕
In FIG. 1, reference numeral 1 denotes a mounting substrate for flip-chip mounting a sensor chip, and the electrodes of the sensor chip are connected by a wiring member 11 such as a solder ball. Reference numeral 2 denotes a sensor chip in which one surface 5 side of the sensor chip on which the signal processing circuit is formed is installed to face the mounting substrate. The sensor chip is provided with a signal processing circuit unit 9 for signal processing on a semiconductor substrate and a moisture sensitive film 4 formed by making the semiconductor substrate porous. The one surface 3 side of the sensor chip 2 is arranged so that the moisture sensitive film 4 whose capacitance value changes according to the humidity is exposed to the measurement environment. A semiconductor film 6 and two sensing electrodes 7 and 8 are formed on the one surface 5 side of the sensor chip 2. The sensing electrodes 7 and 8 are for detecting the capacitance change of the moisture-sensitive porous membrane 4, and the shape is not particularly limited.

  In the present invention, the surface 3 side of the sensor chip is a moisture sensitive part, and the capacitance between the two sensing electrodes 7 and 8 changes according to the surrounding humidity change, so that the humidity can be detected based on this capacitance change. Yes. As in this example, the moisture sensitive portion does not necessarily have to be formed on the entire surface of the second main surface, and may be a part of the moisture sensitive portion.

  The sensing electrodes 7 and 8 are electrically connected to the signal processing circuit unit 9 that constitutes a switched capacitor circuit, for example, and changes the capacitance between the sensing electrodes 7 and 8 that change according to the surrounding humidity change. Humidity can be detected by converting to voltage and outputting. Further, the humidity detection does not necessarily have to be a capacitance detection type as in this example, and for example, a resistance detection type that detects a change in resistance value between the sensing electrodes 7 and 8 may be used as another embodiment. Good. In that case, the signal processing circuit 9 provided for signal processing needs to constitute a circuit that converts a resistance change into a voltage and outputs the voltage.

(Electrode part)
Since the moisture sensitive part has a porous structure, the protective film 10 is formed so that water vapor penetrates the porous layer and reaches the sensing electrodes 7 and 8 and the signal processing circuit part 9 and does not corrode. .

  Examples of the preferred protective film arrangement of the present invention include the following.

(Example 1)
A protective film is disposed so as to cover a portion exposed to the first main surface side of the moisture sensitive portion in the gap between the sensing electrodes 7 and 8. The protective film is preferably an insulating film. Alternatively, a protective film may be substantially formed by filling a gap between the resin material such as silicon and the polymer material 12 with the mounting substrate.

(Example 2)
If the moisture sensitive part is a capacitance change type, a protective film made of an insulating material is formed on the entire surface of the moisture sensitive part on the first surface 5 side of the sensor chip prior to the formation of the sensing electrode, and the electrode is disposed thereon. To do. By arranging in this way, corrosion on the side of the electrode facing the moisture sensitive part can also be prevented.

(Example 3)
A semiconductor film is formed on the entire surface 5 side of the sensor chip of the moisture sensitive part, part of which is used as two sensing electrodes, and the gap is electrically isolated by pn junction separation or oxidation by oxidizing the semiconductor film. Alternatively, the separation method may be used.

  In any method, it is preferable to fill the resin substrate 12 with no gap between the mounting substrate and the sensor chip to prevent intrusion of moisture or the like.

(Material of protective film)
The protective film may be, for example, a silicon nitride film, a silicon oxide film, a silicon film (single crystal, polycrystal, amorphous), or a resin, but is a material having no moisture permeability. This is a component requirement. Further, the conductivity of the protective film may be appropriately selected depending on the target sensor characteristics. For example, in the case of capacitance change, any of conductivity, insulation, and semiconductor can be selected, but in the case of resistance change, insulation is not preferable.

  Other members will be described below.

  FIG. 2 shows a manufacturing process of the sensor chip.

(Semiconductor substrate)
As the semiconductor substrate 20 for forming the porous layer, not only a single crystal silicon wafer manufactured by the CZ method, the MCZ method or the FZ method, but also a wafer whose substrate surface has been subjected to hydrogen annealing treatment, an epitaxial silicon wafer, or the like is used. be able to. Of course, not only silicon but also a compound semiconductor substrate such as a GaAs substrate or an InP substrate can be used as the compound semiconductor substrate.

(Moisture sensitive film)
A porous Si layer functioning as a moisture sensitive film is formed from one surface of the substrate 20 by anodization. The fact that the porous membrane functions as a moisture-sensitive membrane is described, for example, in the literature A. Kaan Kalkan et al .; IEEE Vol. 25, NO. 8, pp. 256-528 (2004). The thickness of the porous Si layer can be used from several hundred μm to about 0.1 μm. More preferably, it is in the range of 0.1 μm to 10 μm. The porosity can be freely set, but it is necessary that the pores communicate from the surface of the porous layer to the innermost part so that the gas penetrates into the pores of the porous layer. The porous layer may be composed of a single porous layer or a plurality of layers having different porosities. In particular, when a porous layer is used also in the separation step described later, a two-layer structure of a low porosity layer that functions as a moisture-sensitive film and a high porosity layer that functions as a separation layer can be used. The porosity of the high porosity layer is 10% to 90%, and the porosity of the low porosity layer is 70% or less. Formation of a plurality of layers having different porosities can be realized by changing the current density during anodization or changing the type or concentration of the conversion solution.

  In the case of a capacitance detection type sensor, it is preferable that the porous Si layer is oxidized. By forming the oxidized porous Si layer, an insulating film is formed and the water molecule adsorption rate is increased.

  If water molecules are adsorbed on the porous layer, the water molecules have a large dielectric constant, so that the dielectric constant of the porous film changes greatly according to the amount of invaded water, and as a result, the capacitance value between both sensing electrodes also changes. It has become.

(Semiconductor film)
As the semiconductor film 22 forming the signal processing circuit, a compound semiconductor film such as GaAs, InP, or GaAs can be used in addition to the non-porous single crystal silicon thin film. When the semiconductor film is single crystal silicon, SiH ₂ Cl ₂ , SiHCl ₃ , SiCl ₄ , or SiH ₄ may be used as a source gas, and HCl gas may be added as necessary. The formation method is not limited to the CVD method, and an MBE method, a sputtering method, or the like is also possible. The semiconductor film is preferably a single crystal, but may be polycrystalline or amorphous.

  When a porous layer is formed by anodization, a protective film in which a protective film such as a nitride film or an oxide film is provided on the inner wall of the porous hole prior to forming the semiconductor film 22 on the porous layer It is preferable to perform a forming step or a heat treatment step in an atmosphere containing hydrogen. Of course, it is also preferable to perform the heat treatment step after the protective film forming step.

(Separation layer)
In addition, when forming a porous layer using anodization, the said porous layer can also be comprised with several layers from which porosity differs. For example, a two-layer structure of a low porosity layer and a high porosity layer can be formed from the epi film region 21 side. By adjusting the porosity to form a two-layer structure, it is selectively separated from the second layer, which is suitable for separation.

(Manufacturing method)
First, a silicon substrate is anodized to form a two-layered porous layer having a porous film 23 functioning as a moisture-sensitive film and a porous layer 24 functioning as a separation layer on the surface. For example, specific conditions in this example are shown.

  A P-type single crystal Si substrate having a specific resistance of 0.01 Ω · cm was prepared, and the substrate surface was anodized in an HF solution to form two porous wet layers. Hereinafter, the layer 23 functioning as a moisture sensitive film is referred to as a first layer, and the layer 24 functioning as a separation layer is referred to as a second layer. The anodizing conditions were as follows.

First layer Current density: 10 (mA · cm-2)
Anodizing solution: HF: H2O: C2H5OH = 1.5: 1: 7.5
Time: 2 (minutes)
Thickness of porous Si layer: 3 (μm)
Second layer Current density: 33 (mA · cm-2)
Anodizing solution: HF: H2O: C2H5OH = 1.5: 1: 7.5
Time: 8 (minutes)
Thickness of porous Si layer: 12 (μm)
The first layer functioning as a moisture-sensitive layer was adjusted to have a porosity so that a high-quality epitaxial Si layer could be formed on the porous Si layer and used as a moisture-sensitive film. Specifically, it was 60%. The porosity of the second layer was adjusted to function as a separation layer.

  Thereafter, the surface of the porous Si layer was immersed in hydrofluoric acid, and only the oxide film on the surface of the porous Si layer was removed leaving the oxide film on the inner wall of the hole. Next, a single crystal Si layer 22 was epitaxially grown by 3 μm on the porous oxide film by CVD. The growth conditions are as follows.

Source gas: SiH2C12 / H2
Gas flow rate: 0.5 / 180 1 / min
Gas pressure: 80 Torr
Temperature: 950 ° C
Growth rate: 0.3μm / min
Thus, the member 20 was formed.

Further, the thickness of the epi film can be adjusted according to the growth conditions, and can be arbitrarily formed according to the processing circuit on the epi film.
Thereafter, two sensing electrodes 30 and 31 are formed on the epi film formed on the porous layer. The sensing electrode area and the distance between sensing electrodes may be designed according to the ability of the signal detection processing circuit on the epi film, but the electrode area is large and the capacitance between the electrodes is increased by taking the distance between the electrodes small. be able to. The shape of the sensing electrode is not limited, but in this example, each sensing electrode has a comb-tooth shape, and the comb-tooth portions engage with each other and face each other. In particular, in the case of a capacitive sensor, by adopting such a comb-shaped electrode, the capacitance can be increased, so that the arrangement area of the electrode can be reduced. In this example, Pt—Pd is used as the electrode material, but there is no limitation. For example, materials made of Al or Al alloy as the main component can be used, and other materials that can be used in ordinary semiconductor production lines such as Pt, Ti, Au, Al, Cu, Poly-Si, etc. Can do.

  A protective film 32 made of a moisture resistant material is formed on these two sensing electrodes. In this example, the protective film 32 covers both the sensing electrodes and between the two sensing electrodes (between the comb teeth). Next, the epi film is etched to electrically separate the two sensing electrodes. Furthermore, the signal processing circuit unit 33 for processing the capacitance value change (signal from the sensing electrode) between the two sensing electrodes according to the change in the surrounding humidity through a normal semiconductor process on the epi film. Form. Similar to the sensing electrode, the signal processing circuit section is covered with a protective film 34 made of a moisture resistant material.

  In addition, after forming a signal processing circuit, you may carry out in the order of forming an electrode.

(Separation)
Next, the entire surface of the wafer is separated into the substrate side 36 and the device side 37 by the separation layer 35 prepared in advance.

  The device side must be coated with an organic resin 38 in advance so that it can withstand the separation, and the strength of the thin film must be increased to prevent breakage that may occur during the separation. In addition, a method of bonding a flexible sheet, a glass substrate, a plastic substrate or the like and removing it after separation is also effective.

  Separation utilized fluid pressure. Specifically, separation can be performed by spraying a fluid jet on the side surface of the separation layer 35. As the fluid, water, an etching solution, alcohol, or the like can be used for a liquid, and air, nitrogen gas, argon gas, or the like can be used for a gas. Ultrasonic vibration may be used during the separation. A feature of the fluid is that it can flow into a very small gap to increase the internal pressure, and the external pressure can be distributed and applied. As in the present invention, this is an optimal means for separating the entire surface of a thin layer in which a semiconductor device has already been fabricated.

  There is also a method of taking out the sensor chip by etching the substrate side as shown in FIG. The sensor unit is exposed to the measurement environment by etching the substrate from the one surface 41 side of the member 40. The etching depth at this time is sufficient if at least one surface 42 side of the sensor unit is exposed to the measurement environment. In this case, the two porous layers formed as the separation layer are not necessarily required.

(Implementation)
Thereafter, the device layer side 37 is cut into a chip size by dicing, and each chip is packaged.

  For forming a chip from the separation layer, a commonly used dicing apparatus can be used, and etching, laser ablation, an ultrasonic cutter, a high-pressure jet (for example, a water jet), and the like can also be used.

  At this time, as shown in FIG. 1, the surface 5 side of the sensor chip 2 is brought into contact with the package base and sealed, thereby preventing the problem that the sensing electrodes and peripheral circuits are easily corroded by being exposed to a humidity environment. Although it is conceivable to use a corrosion-resistant electrode such as a noble metal which is not easily corroded as the electrode part, this is not necessary in the present invention and can be freely selected without being restricted by the electrode material.

  Further, by using the one surface 3 side of the sensor chip 2 as a moisture sensitive film, the problem that the electrode prevents moisture from passing through is solved.

  In the present embodiment, as described above, the sensor chip 2 in which the circuit element unit for signal processing is integrated can be manufactured by a normal semiconductor process, and the sensor unit is not covered with an electrode. High sensitivity, miniaturization, high integration, mass production, and a small humidity sensor can be provided.

  The second embodiment of the present invention shown in the drawings will be described below. FIG. 4 is a diagram showing a schematic cross-sectional configuration of a gas sensor according to Embodiment 2 of the present invention.

  As the member 50 for forming the porous layer, not only a single crystal silicon wafer produced by the CZ method, the MCZ method or the FZ method as in Example 1, but also a wafer whose substrate surface has been subjected to a hydrogen annealing treatment, or an epitaxial layer. A silicon wafer or the like can be used. Of course, not only silicon but also a compound semiconductor substrate such as a SiGe substrate, a GaAs substrate, or an InP substrate can be used.

  A porous portion 52 that functions as a moisture sensitive portion is formed on the one surface 51 side of the substrate 50. The depth of the porous layer can be used from several hundred μm to 0.1 μm.

  After the porous layer 52 is formed by partial chemical conversion in the region used as the sensor, the semiconductor film 53 is formed in the same manner as in the first embodiment. Thereafter, a signal processing circuit 54, electrodes 55 and 56, and a protective film are formed for the semiconductor film. Thereafter, both electrodes are electrically separated by etching.

  After the device portion is formed in this manner, the device layer is separated from the one surface 57 side of the member 50 by etching. The etching depth at this time should be at least that one surface 58 of the sensor portion is exposed to the measurement environment.

  Thereafter, as described in Example 1, the sensor module is formed after dicing into chips by dicing.

  In this embodiment, an inexpensive sensor can be provided without including an epitaxial film forming step.

Schematic sectional view of a sensor module according to an embodiment of the present invention Manufacturing process of sensor chip shown in Fig. 1 Device layer separation form Manufacturing process of sensor according to Example 2

Explanation of symbols

1 Mounting board
2 Sensor chip
3 One side of sensor chip
4 Porous membrane
5 Other side of sensor chip
6 Semiconductor film 7, 8 Sensing electrode
9 Signal processing circuit
10 Protective film
11 Solder balls
12 Protection member
20 One side of sensor chip
21 Other side of sensor chip
22 Semiconductor film
23 Porous membrane for moisture sensitivity
24 porous membrane for separation,
30, 31 Sensing electrode
32 Protective film
33 Signal processing circuit
34 Protective film
35 Separation layer
36 One side of member
37 Other side of member
38 Protection member
40 substrates
41 One side of member
42 One side of the sensor
50 substrates
51 One side of the board
52 porous layer
53 Semiconductor film
55,56 electrodes
57 One side of the board
58 One side of the sensor

Claims (7)

In a gas sensor in which a gas sensor unit and a signal processing circuit unit are formed on the same semiconductor substrate
(1) The first main surface side has the signal processing circuit unit and the gas sensor unit electrode,
(2) a gas sensor portion made of a porous structure gas adsorbing member is disposed through the substrate, has a gas detection surface on the second main surface, and
(3) A gas sensor, wherein a surface portion of the signal processing circuit section and the electrode are covered with a moisture resistant material.   2. The gas sensor according to claim 1, wherein the gas sensor unit is made of either porous silicon or oxidized porous silicon, or a mixture thereof.   The gas sensor according to claim 1, wherein the gas sensor unit is a humidity sensor whose capacitance changes with humidity.   The gas sensor according to claim 3, wherein the moisture-resistant material is an insulating material.   2. The gas sensor according to claim 1, wherein an electrode surface side of the gas sensor unit is covered with a non-porous insulating moisture-resistant material, and an electrode is disposed thereon. A method for manufacturing a sensor in which a signal processing circuit, an electrode, and a moisture-sensitive film are formed on the same substrate, including at least the following steps.
(1) forming a porous layer at least partially on a semiconductor substrate;
(2) forming a semiconductor film for forming a signal processing circuit on the porous layer;
(3) forming an electrode on top of the porous layer;
(4) forming a signal processing circuit;
(5) removing the semiconductor substrate and exposing a lower portion of the porous layer;
and,
(6) Step of coating the electrode and the signal processing circuit with a moisture-resistant material A method for manufacturing a sensor in which a signal processing circuit, an electrode, and a moisture-sensitive film are formed on the same substrate, including at least the following steps.
(1) forming a porous layer at least partially on a semiconductor substrate;
(2) forming a semiconductor film for forming a signal processing circuit on the porous layer;
(3) forming an electrode on top of the porous layer;
(4) forming a signal processing circuit;
(5) a step of separating the substrate from the semiconductor layer in which the porous layer and the signal processing circuit are formed, and the electrode by separating the substrate from the porous layer or at the interface between the porous layer and the substrate;
and,
(6) Step of coating the electrode and the signal processing circuit with a moisture-resistant material

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