CN1069134C - Gas sensor for detecting nitrogen dioxide and its prodn technology - Google Patents

Abstract

The present invention relates to a hanging grid field effect type gas sensor for detecting the LB film decoration of NO2, and a manufacture method thereof. The gas sensor has the structure that a hanging grid field effect transistor comprises an LB film of cobalt porphyrin quaternary ammonium or phthalocyanine copper sulfuret or derivatives thereof, which is drawn on the whole surface of a gate region. The present invention comprises the following manufacture processes: a chip of the hanging grid field effect transistor is drawn in liquid floated with the LB film, so that the LB film is attached to the surface of the chip and is dried in vacuum; the film is repeatedly drawn and dried, so that the LB film reaches appropriate thickness and is cut into individual chips to lead AL electrodes. The gas sensor of the present invention has the advantages of high sensitivity, good selectivity, rapid response, good consistency and simple technology, and is suitable for bulk production.

Description

Gas sensor for detecting nitrogen dioxide and its manufacturing process

The invention belongs to a gas sensor, in particular to a suspended grid field-effect gas sensor modified by an organic monomolecular film and a manufacturing process thereof.

Suspended gate field effect (SGFET) type gas sensor is a kind of gas sensor modified on the gate area of field effect transistor with gas sensitive material, and the surface potential of the semiconductor in the gate area is changed by the change of charge distribution formed after the gas sensitive material absorbs a certain gas. A gas sensor that causes a change in the gate voltage of the field effect transistor, resulting in a change in the detected leakage current.

The prior art closest to the present invention is disclosed by a document titled "Recognition of the _Hydrogen and _Ammonia by Modified Gate Metallijation of the Suspended-gate FET", "Sensors and Actuators" 1990, (B1): 21. The gas sensor is completed on the traditional n-channel enhancement type field effect transistor, except that there is no metal layer and gate electrode in the gate area, other structures and field effect Transistor is consistent, then make gas-sensitive material SnO ₂ only be deposited in the gate area of field effect transistor. We claim that the gate area position does not vapor-deposit metal layer, and the field effect transistor that does not draw gate electrode is suspended gate field effect transistor. The gas sensor structure of the suspended gate field effect type is shown in Figure 1. A certain concentration of n-type carriers is diffused in two places on the P-type Si substrate to form a drain-source region, and the source electrode s and the drain electrode are respectively drawn out in the n-region D, i.e. the AL electrode, constitutes a suspended gate field-effect transistor. _SnO2 layers are deposited on the surface of the gate region between the two n regions. The process of making this gas sensor is roughly, at first utilizing the conventional method of making a semiconductor device Make the chip of enhanced suspended gate field effect transistor tube core, demarcate cutting.Described demarcation cutting is exactly to demarcate by tube core position on the chip, divide tube core one by one, but do not make tube core separate again, each tube core The cores are still connected together, and the chip is still a whole, so that gas-sensitive materials can be deposited. Then, a gas-sensitive transition metal or ceramic material (such as SnO ₂ ) is vapor-deposited on the surface of each die gate area at the same time using a special process technology. Finally, the chip is divided into individual dies, each of which leads to a source electrode S and a drain electrode D.

The gas sensor with the SnO ₂ -SGFET structure has low power consumption because no gate bias is applied, the change signal is easy to detect, the device is easy to realize integration, and the consistency is good. However, because gas-sensitive transition metals or ceramic materials generally need to have a strong ability to adsorb gases at high temperatures, the sensitivity of the sensor is low at room temperature; Both are sensitive, so poor selectivity leads to inaccurate detection; due to the use of enhanced field effect transistors, it is generally necessary to cause significant leakage current changes at higher gas concentrations, which is not conducive to the detection of low-concentration gases. However, in the manufacturing process, only the gas-sensitive material is deposited (evaporated) on the surface of the gate area, so the manufacturing is cumbersome and difficult.

The invention designs a suspended grid field-effect gas sensor modified with organic monomolecular film. The organic monomolecular film is modified on the whole tube core, so that the gas sensor can work at normal temperature, has high sensitivity, good selectivity, and can detect more Low concentration of gas, and the purpose of simpler process.

The field-effect gas sensor modified by the organic monomolecular film of the present invention, the organic monomolecular film (also known as LB film) adopts cobalt porphyrin quaternary ammonium salt and its derivatives or sulfurized copper phthalocyanine and its derivatives as raw materials. Monomolecular film, these two kinds of raw materials LB film modified in field effect transistor fabricated gas sensor for detecting _NO2 gas. The gas sensor can be called a gas sensor with LB-SGFET structure.

The specific structure of the present invention is referred to Fig. 2. On the surface of the gate area of the field effect transistor without metal layer and gate electrode (that is, the suspended gate field effect transistor), an organic monomolecular film of cobalt porphyrin quaternary ammonium salt or sulfurized copper phthalocyanine or their derivatives is drawn. In Figure 2, the suspended gate field effect transistor is the n-region with two n-type carriers diffused on the P-type Si substrate, and an organic monomolecular film is attached to the entire surface including the gate region. The two n-regions are respectively The source electrode S and the drain electrode D are drawn out.

The raw material for preparing organic monomolecular film is preferably four-p-dimethylaminocobalt porphyrin quaternary ammonium salt or its derivatives or tetra-4-(2,4-di-t-pentylphenylthio) copper phthalocyanine or its derivatives things. They are more sensitive to _NO2 gas and thus can improve the sensitivity and selectivity of gas sensors.

The thickness of the organic monomolecular film on the prepared gas sensor should be appropriate. For the sensor of NO ₂ gas, the thickness of LB film can be 6-60nm, preferably 15-45nm. When preparing the LB film with the aforementioned cobalt porphyrin quaternary ammonium salt, sulfurized copper phthalocyanine or their derivatives, the number of layers on the suspended gate field effect transistor is controlled to be about 4 to 40 layers, and the total thickness of the LB film can be 6 ~60nm. Too few layers are not conducive to obtaining higher sensitivity, and too many layers are not conducive to fast response and stability.

The LB film-modified field-effect gas sensor can use either an enhancement-type suspended-gate field-effect transistor or a depletion-type suspended-gate field-effect transistor. Their detection ranges are different. The depletion type can detect lower concentration of NO ₂ gas, that is, it has higher sensitivity; the enhanced type has better stability. These two gas sensors can be used according to different environments in actual detection.

The suspended grid field-effect type gas sensor modified by LB film of the present invention is made like this:

First, make the tube core chip of the suspended gate field effect transistor according to the conventional production method, use a dicing machine to demarcate and cut the single tube core, and carry out hydrophilic treatment to the chip; prepare cobalt porphyrin quaternary ammonium salt or Its derivatives or organic monomolecular film of sulfurized copper phthalocyanine or its derivatives, after that,

A． Place the chip in a liquid with an organic monomolecular film floating on it, and pull the organic monomolecular film onto the surface of the gate area of the chip under the condition of (20±2)°C;

B． Dry the chip under a vacuum of 10 ^-2 Torr, then put it into the liquid with organic monomolecular film floating, and pull the second layer of organic monomolecular film;

C． The process of A and B above is repeated until the thickness of the multi-layer organic monomolecular film on the surface of the gate region on the chip reaches 6-60 nm, and then dried.

Finally, the chip is divided into individual dies, and an AL lead is formed in the drain-source region (two n-regions) of the suspended gate field effect transistor by means of static pressure welding.

The aforementioned preparation of organic monomolecular films by conventional methods is the existing method for preparing LB films. Generally, the micropowder of the raw material (such as cobalt porphyrin quaternary ammonium salt) is dissolved in an organic solvent such as chlorine, and dripped on the liquid surface, such as dripping on the water surface, and then pushed to form a film.

Due to the orderliness of the LB-SGFET structure gas sensor of the present invention, after it absorbs gas molecules and exchanges electrons with them, a dipole layer is formed in the film, thereby generating an additional electric field in the gate region, causing changes in leakage current. Due to the order of the LB film, the gas sensor has high sensitivity; especially when the depletion type field effect transistor is used, the very small gate voltage change caused by the lower gas concentration will also be reflected from the leakage current change out, so that the detected gas concentration reaches the PPb level. Since the special material for preparing the LB membrane of the present invention absorbs _NO2 gas molecules and exchanges electrons with them, it has extremely high selectivity. Due to the ultra-thin characteristics of the LB film, the response and recovery to the gas are rapid, so the gas sensor of the LB film has the characteristics of fast response and fast recovery. Since the LB film is directly drawn on the chip of the suspended gate field effect transistor, it does not need to be selectively deposited on the gate area, which simplifies the process, and the fabrication of the device can also be integrated with good consistency, so it is convenient for large-scale Mass production.

Description of drawings:

Fig. 1 is a schematic diagram of the structure of a suspended gate field effect transistor for recognizing hydrogen and ammonia.

Fig. 2 is a structural schematic diagram of the gas sensor for detecting nitrogen dioxide of the present invention.

Claims (3)

1. A gas sensor for detecting _NO , its structure includes a suspended gate field-effect transistor with two n-region n-type carriers diffused on the P-type Si substrate, and the two n-regions lead out respectively The active electrode (S) and the drain electrode (D) are characterized in that an organic monomolecular film (LB) is attached to the surface of the suspended gate field effect transistor, and the organic monomolecular film (LB) is made of cobalt porphyrin quaternary ammonium Salt and its derivatives or sulfide copper phthalocyanine and its derivatives are raw materials; the number of layers of organic monomolecular film (LB) is 4 to 40 layers, and the thickness is 6 to 60nm; the so-called suspended gate field effect transistor is enhanced type suspended gate field effect transistor or depletion type suspended gate field effect transistor. 2. The gas sensor for detecting NO according to claim ₁ , characterized in that said organic monomolecular film (LB) is made of four-p-dimethylaminocobalt porphyrin quaternary ammonium salt or derivatives thereof , or tetra-4-(2,4-di-pentylphenylthio) copper phthalocyanine or its derivatives as raw materials; its thickness is 15-45nm. 3, a kind of claimed in claim ₁ is used to detect NO The manufacturing process of the gas sensor, at first make the tube core chip of suspension gate field effect transistor by conventional method, carry out demarcation cutting by single tube core with dicing machine , carry out hydrophilic treatment on the chip; adopt the conventional method to prepare the organic monomolecular film, that is, dissolve the micropowder of the film-making raw material in an organic solvent, and then drop it on the surface of the liquid to form a film; finally divide the chip into individual Die, use electrostatic pressure welding method to form drain electrode and source electrode leads in the drain-source region of the suspended gate field effect transistor, it is characterized in that, after carrying out hydrophilic treatment to the chip, draw on the surface of the gate region of the suspended gate field effect transistor Preparation of organic monomolecular film, the process is divided into; A. Place the chip in a liquid with an organic monomolecular film floating on it, and draw the organic monomolecular film on the surface of the gate area of the chip under the condition of (20±2)°C; B. Dry the chip under a vacuum of 10 ^-2 Torr, and then pull the second layer of organic monomolecular film; C. Repeat the process of A and B to make the thickness of the multi-layer organic monomolecular film on the surface of the gate area on the chip reach 6-60nm. After drying, divide it into individual dies and install AL leads.

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