WO2016066007A1

WO2016066007A1 - Mems methane sensor, and application and manufacturing method thereof

Info

Publication number: WO2016066007A1
Application number: PCT/CN2015/091746
Authority: WO
Inventors: 丁恩杰; 马洪宇; 刘晓文; 赵小虎; 赵端
Original assignee: 中国矿业大学
Priority date: 2014-10-31
Filing date: 2015-10-12
Publication date: 2016-05-06
Also published as: CN104316578B; CN104316578A

Abstract

An MEMS methane sensor, and application and manufacturing method thereof, being suitable to be used in a coal mine detection; the sensor comprises a P-type silicon substrate (01), an N-type silicon (02) provided on the P-type silicon substrate (01), and a silicon heating assembly (101) machined and prepared by the N-type silicon (02) on the P-type silicon substrate (01); and the silicon heating assembly (101) comprises two fixed ends (102), a silicon heater (1011) and two silicon cantilevers (1012). The sensor utilizes a common monocrystal silicon as a material of the heating assembly serving as a sensitive assembly, and does not need a catalyst carrier or a catalyst material. The machining process is compatible with a CMOS process, and the releasing of the heating assembly employs a wet silicon etching process capable of simultaneously etching in two directions. The sensor has a low cost, a high sensitivity and a low power consumption, and the measurement is free of the effect of oxygen concentration, carbon deposition and poisoning.

Description

MEMS methane sensor and application and preparation method thereof

Technical field

The invention relates to a methane sensor, an application thereof and a preparation method thereof, and is particularly suitable for a MEMS methane sensor used in coal mine safety detection, and an application and a preparation method thereof.

Background technique

With the development of the Internet of Things, current methane sensors are unable to meet the needs of individual equipment for low-power, long-life, low-cost methane sensors that detect low-concentration methane. At present, the low-concentration methane used in coal mines is still a catalytic combustion-type methane sensor based on traditional platinum wire heating. The power consumption is large, and the use of the catalyst leads to unstable methane detection performance, short calibration time, and carbon deposition. Poisoning, activation and other shortcomings caused by the use of catalysts; and the high price of infrared methane sensors, the sensing components are seriously affected by dust and water vapor; these two methane sensors can not meet the low power consumption of the Internet of Things Application requirements for methane sensors. Other methane sensors are also unable to adapt to the complex environment of high humidity in coal mines.

Summary of the invention

SUMMARY OF THE INVENTION The object of the present invention is to provide a MEMS methane sensor which has a simple structure and can detect a low concentration of methane (0 to 4%) without using a catalyst, and an application and a preparation method thereof.

In order to achieve the above technical problem, the MEMS methane sensor of the present invention uses P-type silicon as a substrate, and a P-type silicon substrate is provided with N-type silicon; and a silicon heating element is prepared by processing N-type silicon on the P-type silicon substrate. The silicon heating element comprises two fixed ends, a silicon heater, two silicon cantilevers; the single silicon cantilever is at least 300um in length; one end of the single silicon cantilever is connected to the silicon heater, and the other end is fixed Connected to the end to provide electrical connection for the silicon heater; the two silicon cantilevers are arranged side by side in parallel, form a U-shaped cantilever structure integrally with the silicon heater, suspend the silicon heater in the air; the silicon heater of the silicon heating element And the outer surface of the silicon cantilever is provided with a passivation protective layer; the fixed end is disposed on the P-type silicon substrate, and includes a silicon oxide layer on the N-type silicon, the N-type silicon, and a pad metal used as the electric extraction pad, The electrical lead pad pad metal is disposed on the silicon oxide layer above the N-type silicon, and the electrical lead-out pad Pad metal forms an ohmic contact by directly contacting the underlying N-type silicon through the window of the silicon oxide layer, and electrically extracting the pad Pad metal. There is no silicon oxide in contact with the N-type silicon layer underneath ;

Provided around the silicon heating element and its fixed end with an isolation trench for removing N-type silicon, the isolation trench for the silicon heating element and its fixed end of the N-type silicon and the rest on the P-type silicon substrate The N-type silicon is in a high-resistance state, and in particular, there is no circuit path between the two fixed ends of the silicon heating element provided on the P-type silicon substrate except for the electrical path formed by the silicon cantilever and the silicon heater.

A methane detection application using the sensor of claim 1 using two methane sensors, one in contact with ambient air and the other in a hermetic package that remains sealed from ambient air; the two used are based on wet A bidirectionally etched silicon MEMS methane sensor forms a Wheatstone bridge detection bridge arm; a voltage is applied across two fixed ends of a wet bidirectionally etched silicon MEMS methane sensor or current is applied to place the operating point of the silicon heating element In the current-resistance characteristic curve, the working point region on the left side of the turning point and the silicon heater of the heating element generate heat, and the electric heating temperature is above 500 degrees Celsius; the turning point is the maximum resistance of the resistor as the current or voltage increases. Point, when the current or voltage continues to increase, the resistance does not continue to increase but decreases; when methane gas appears, with ambient air The temperature of the contacted silicon heater is lowered, causing a significant change in the electrical resistance of the silicon heating element including the silicon heater, and the Wheatstone bridge formed by the wet bidirectionally etched silicon-based MEMS methane sensor achieves a low concentration Detection of methane gas.

Method for preparing MEMS methane sensor, which comprises two preparation methods;

The steps of the preparation method (1) are as follows:

In the first step, N-type silicon is prepared by doping or diffusion on the front side of the (100) crystal orientation P-type silicon substrate, and the thickness of the N-type silicon is 3 to 30 um;

The second step, thermal oxidation to form a silicon oxide layer, thickness of 0.5 to 1 um;

In the third step, a photoresist is prepared on the front side of the P-type silicon substrate, and after lithography, a silicon heating element, an isolation trench disposed around the fixed end of the silicon heating element, and a pattern of the front etching window are formed, and the reactive ion engraving is performed. The etching method dry etching the exposed silicon oxide layer and the underlying N-type silicon, the etching depth is greater than the sum of the thickness of the N-type silicon and the silicon oxide layer formed in the second step, and removing the photoresist;

In the fourth step, the silicon oxide layer formed in the second step is photolithographically formed on the front side of the P-type silicon substrate to form a metal contact hole;

In the fifth step, a metal layer is formed by sputtering or depositing or evaporating on the front side of the P-type silicon substrate. The material of the metal layer may be gold or aluminum and annealed to expose the metal layer and the exposed N on the P-type silicon substrate. Type silicon forms an ohmic contact;

In the sixth step, the electrical lead-out pad Pad metal, the metal connecting line and the total metal connecting end are formed after etching the metal layer as needed, and the electrical lead-out pad Pad metal and metal connecting line of each silicon heating element are formed. The metal layers are in communication, and the metal connection line is connected to the total metal connection end through the metal layer; the total metal connection end is disposed at the edge of the P-type silicon substrate, and when a potential is applied at the total metal connection end, the entire silicon wafer is All of the silicon heating elements of the N-type silicon form a good electrical connection and have the same potential as the total metal connection, the metal connection line is disposed in the scribe groove;

In the seventh step, a photoresist is prepared on the front side of the P-type silicon substrate, a front etching window pattern is formed on the P-type silicon substrate, and a front etching window formed by dry etching by reactive ion etching is formed. The P-type silicon exposed by the pattern has an etching depth greater than 20 um, forming a front etching window of the wet silicon etching to remove the photoresist;

In the eighth step, lithography aligned with the front etch window is performed on the back surface of the P-type silicon substrate to form a pattern of the back etch window; the formed back etch window pattern and the front etch window pattern may be smaller than the single The front side etching window pattern required for the front side wet etching etching through the silicon wafer is performed, and the formed back etching window pattern overlaps with the center of the front etching window pattern, and the sides are the same direction, and the formed back etching window pattern is formed. Less than the front etch window pattern, the size of the etch window should ensure that the silicon wafer is formed through the via hole. The size of the via hole is much larger than the outer dimension of the silicon heater, and the silicon heater of the silicon heating element is located on the front etched window and the back side. The center position of the etch window; the silicon oxide layer in the exposed back etch window pattern and the underlying silicon are dry etched by reactive ion etching, and the etching depth is 10 to 30 um to form the back surface required for wet etching. Etching window

In the ninth step, an etch protection layer is separately formed on the front side and the back side of the P-type silicon substrate and patterned to expose the front side wet etching window prepared in the eighth step, and the front side wet etching window formed in the seventh step. And a total metal connection connecting the metal of the fixed end of the silicon heating element;

In the tenth step, the prepared silicon wafer is placed in a tetramethylammonium hydroxide solution or a potassium hydroxide solution, and the P-type silicon is simultaneously subjected to silicon wet etching on the front and back sides of the silicon wafer by a PN junction self-stop method. Applying a positive voltage to the N-type silicon on the P-type silicon substrate through etching at the total metal connection on the silicon wafer, the positive voltage being higher than the passivation potential of the PN junction from the stop etching to make the P-type silicon The PN junction formed by the substrate and the N-type silicon is in a reverse bias state; under the action of stopping the etching of the PN junction, the N-type silicon of the silicon heating element is not etched; after the etching is completed, not only the silicon heating element is released, Front and back engraving a through hole is also formed in the etch window, and a center projection of the silicon heater of the silicon heating element coincides with a projection of a center of the through hole;

In the eleventh step, the front and back etching protective layers prepared in the ninth step are removed, and dried, and after drying, the silicon oxide on the surface of the silicon heating element generated in the second step is removed by using a hydrofluoric acid solution or a hydrofluoric acid gas mist;

In the twelfth step, the silicon exposed on the outer surface of the silicon heating element is oxidized to form a thin layer of silicon oxide having a uniform thickness, the thickness of which is from ten to 100 nm, as a passivation protective layer;

The thirteenth step, dicing along the scribe groove, in particular, cutting the metal connection line in the partial scribe groove and the connection of the metal of the electrical extraction pad Pad, and obtaining a plurality of MEMS methane sensors based on wet bidirectionally etched silicon after the cleavage;

Or the steps of the preparation method (2) are:

In the first step, N-type silicon is prepared by doping or diffusion on the front side of the crystal orientation P-type silicon substrate, and the thickness of the N-type silicon is 3 to 30 um;

In the fourth step, a metal layer is formed on the front side of the P-type silicon substrate by sputtering, deposition or evaporation. The material of the metal layer is aluminum, the thickness is 1 to 2 um, and the metal layer is annealed to form N-type silicon. Ohmic contact between; forming a metal layer by sputtering or depositing or evaporating on the back side of the P-type silicon substrate, the material of the metal layer being aluminum;

In the fifth step, the metal layer is patterned to expose the silicon oxide layer in the front etching window pattern formed in the third step and the N-type silicon underneath, and the exposed silicon is dry-etched by reactive ion etching. Etching depth of 30 um, forming a front etch window required for front side wet etching;

In the sixth step, lithography aligned with the front etch window is performed on the back surface of the P-type silicon substrate to form a pattern of the back etch window; the formed back etch window pattern and the front etch window pattern are smaller than the single The front side wet etching engraves the front etching window pattern required to pass through the silicon wafer, and the formed back etching window pattern overlaps with the center of the front etching window pattern, and the sides are the same direction, and the formed back etching window pattern is smaller than The front side etches the window pattern. The size of the etch window should be such that the silicon wafer can be cut through to form a through hole. The size of the through hole is much larger than the outer shape of the silicon heater, and the silicon heater of the silicon heating element is located at the center of the two etching windows. Position; dry etching the exposed silicon oxide layer in the back etched window pattern and the underlying N-type silicon by reactive ion etching, etching depth of 10 to 30 um, forming the backside etching required for wet etching window;

In the seventh step, the prepared silicon wafer is placed in a tetramethylammonium hydroxide solution, and the P-type silicon is simultaneously subjected to a silicon wet etching on the front and back sides of the silicon wafer by a PN junction self-stop method to release the silicon heating element. When etching, a positive voltage higher than the pn junction self-stopping passivation potential is applied to the N-type silicon through the front metal, so that the PN junction formed by the P-type silicon substrate and the N-type silicon layer is in a reverse bias state. The N-type silicon of the silicon heating element is not corroded under the protection of the PN junction from the stop protection. After the etching is completed, not only the silicon heating element is released, but the back etching window and the front etching window form a through hole, and the silicon of the silicon heating element The projection of the center of the heater coincides with the projection of the center of the through hole;

In the eighth step, a photoresist is prepared on the fixed end of the silicon heating element, and the photoresist is dried to remove residual metal except the fixed end metal layer;

In the ninth step, the silicon oxide on the surface of the silicon heating element is removed by using a hydrofluoric acid solution or a hydrofluoric acid gas mist; and the photoresist formed in the eighth step is removed;

In the tenth step, the exposed silicon is oxidized to form a thin layer of silicon oxide having a uniform thickness of from ten to 100 nm as a passivation protective layer;

In the eleventh step, dicing, dicing, and obtaining the MEMS methane sensor based on wet bidirectional etched silicon.

Advantageous Effects: The wet MEMS methane sensor of the present invention uses silicon as a heating material without using metal; the silicon heater of the present invention is processed using a common silicon wafer and is away from the silicon substrate; A technique for simultaneously etching silicon in both directions. Due to the adoption of the above scheme, the following effective effects are obtained:

1. The MEMS methane sensor of the present invention can realize high sensitivity detection of low concentration methane gas by using a silicon heating element as a heating element and a detecting element without using a catalyst; the silicon heater of the MEMS methane sensor of the invention adopts more The parallel structure of silicon heating strips has a high temperature surface area in contact with air, which contributes to the improvement of sensitivity, and the sensitivity can reach 10mV/CH ₄ %. Such sensitivity can directly push the instrument and meet the requirements of national standards;

2. Unlike catalytic combustion type methane sensors, which require oxygen to participate in the catalytic combustion reaction, the detection of methane by the methane sensor of the present invention is not affected by oxygen in the air;

3. The MEMS methane sensor of the present invention does not use a catalyst and a catalytic carrier. Therefore, the performance of the sensor is not affected by the catalyst, and there is no problem of sensitivity reduction, poisoning, activation, etc. caused by a decrease in catalyst activity;

4. The silicon heater of the MEMS methane sensor of the present invention is suspended in the air and away from the silicon substrate, and the distance is more than 300 um, and the silicon heater can be heated to a high temperature of 500 ° C or higher with a low power, which is well lowered. The heat lost through the silicon wafer, so has the advantage of low power consumption, the power consumption of a single silicon heating element is about 80 ~ 90mW;

5. The silicon heating element of the MEMS methane sensor of the invention uses silicon as a heating material, and the raw material cost is greatly reduced; the processing technology is simple, compatible with the CMOS process, and easy to mass-produce; the invention adopts a wet silicon etching process and is inexpensive to use. The chemical solution can complete the release of the device of the present invention, and the processing cost is lower without using expensive dry etching equipment and processing cost compared with the dry etching; and the silicon heating element of the present invention adopts the wet method The silicon etching releases the silicon from the front and back sides by simultaneously etching the silicon, which can save about half of the etching time and improve the etching processing efficiency by about 1 time. The above comprehensive solution greatly saves the cost and has good performance. benefit;

6. The silicon heating element of the MEMS methane sensor of the present invention is processed by using stable single crystal silicon, which makes the methane sensor of the invention have good stability and long life under high temperature working condition; There is no disadvantage that the metal heating material such as platinum or tungsten is easily volatilized and migrated at a high temperature of 500 degrees Celsius or higher, and there is no disadvantage that the polycrystalline silicon resistor is easily changed at a high temperature and cannot be controlled; and at the same time, the silicon heating element of the present invention The passivation layer provided on the outer surface also reduces the influence of the external environment on the above components, thereby further improving the stability of the performance of the methane sensor of the present invention;

7. The methane sensor of the invention is processed by a MEMS process, and has a small size, which not only makes the sensor consume low power, but also has a fast response speed of up to 40 ms;

8. The methane sensor of the present invention can be mass-produced by a CMOS process, and has good consistency, so that it can be batch-calibrated, thereby further improving sensor performance and reducing the cost of the sensor calibration link;

Advantages: The method for preparing a MEMS methane sensor based on wet bidirectionally etched silicon can be compatible with a CMOS process; easy mass production and calibration; low cost and good consistency; and the size of the methane sensor of the present invention Small, fast response, low sensor power consumption, high sensitivity, good linearity of output signal; sensor performance is not affected by catalyst, comprehensive optimization and compensation of sensor performance without considering the complex effects of catalyst, simple and easy, low Concentration methane has a high sensitivity detection capability.

DRAWINGS

1 is a top plan view of a MEMS methane sensor of the present invention.

2 is a top plan view of a MEMS methane sensor of the present invention on a silicon wafer after completion of preparation of an etch window pattern.

Figure 3 is a cross-sectional view taken along line A-A of Figure 1 of the present invention.

4 is a schematic view showing the structure of a silicon heater of a silicon heating element of the MEMS methane sensor of the present invention.

FIG. 5 is a schematic view showing the pad metal, the metal connecting line and the partial dicing groove on the silicon wafer when the wet bidirectionally etched silicon based MEMS methane sensor of the present invention is prepared.

6 is a current-resistance characteristic curve of a silicon heating element of a wet bi-directional silicon-etched MEMS methane sensor of the present invention.

In the figure: 01-P type silicon substrate, 02-N type silicon, 101-silicon heating element, 1011-silicon heater, 1012-silicon cantilever, 1013-silicon heating strip, 102-fixed end, 103-isolation trench , 20-silicon oxide layer, 21-electric lead pad pad metal, 22-passivation protective layer, 31-metal connection line, 32-metal connection end, 40-scriber groove.

detailed description

An embodiment of the present invention will be further described below with reference to the accompanying drawings:

As shown in FIG. 1, FIG. 2, and FIG. 3, the MEMS methane sensor of the present invention includes a P-type silicon substrate 01, and a P-type silicon substrate 01 is provided with N-type silicon 02; on the P-type silicon substrate 01. N-type silicon 02 processing to prepare a silicon heating element 101; the silicon heating element 101 includes two fixed ends 102, a silicon heater 1011, two silicon cantilevers 1012; the single silicon cantilever 1012 is at least 300 um in length; One end of the silicon cantilever 1012 is connected to the silicon heater 1011, and the other end is connected to a fixed end 102 to provide electrical connection for the silicon heater 1011; the two silicon cantilevers 1012 are arranged side by side in parallel with the silicon heater 1011. The cantilever structure suspends the silicon heater 1011 in the air; the silicon heater 1011 of the silicon heating element 101 and the outer surface of the silicon cantilever 1012 are provided with a passivation protective layer 22; the fixed end 102 is disposed on the P-type silicon liner On the bottom 01, a silicon oxide layer 20 on the N-type silicon 02, the N-type silicon 02, and an oxide metal pad 21 on the N-type silicon 02 are used, and the electric extraction pad Pad metal 21 is disposed on the N-type silicon 02. On the silicon layer 20, and electrically drawing the pad Pad metal 21 directly through the window of the silicon oxide layer 20 to the underlying N-type silicon 02 Constituting the ohmic contact, the electrical lead 21 and its pad Pad metal contact portions 02 in the N-type silicon layer 20 without the silicon oxide layer.

An isolation trench 103 from which N-type silicon is removed is disposed around the silicon heating element 101 and its fixed end 102, and the isolation trench 103 makes the silicon heating element 101 and its fixed end 102 N-type silicon and P The remaining N-type silicon on the type silicon substrate 01 is in a high resistance state, in particular, between the two fixed ends 102 of the silicon heating element 101 disposed on the P-type silicon substrate 01, except for being heated by the silicon cantilever 1012 and silicon. There is no other circuit path beyond the electrical path formed by the device 1011.

4 is a schematic structural view of a silicon heater 1011 of a silicon heating element of a wet bi-directional silicon-etched MEMS methane sensor according to the present invention; the silicon heater 1011 shown in FIG. 4 has a structure of a plurality of silicon heating strips 1013. Parallel to increase the high temperature surface area in contact with air.

Figure 6 is a graph showing the current-resistance characteristics of a silicon heating element of a wet bi-etched silicon based MEMS methane sensor of the present invention.

A methane detection application using the sensor of claim 1 using two methane detection sensors, one in contact with ambient air and the other in a hermetic package that remains sealed from ambient air; the two used are based A wet bi-directionally etched silicon MEMS methane sensor forms a Wheatstone bridge detection bridge arm; a voltage is applied across the two fixed ends 102 of the wet bi-etched silicon based MEMS methane sensor or an electric current is applied to the silicon heating element 101 The working point is located in the working point region on the left side of the turning point in the current-resistance characteristic curve, and the silicon heater 1011 of the heating element 101 generates heat, characterized in that the electric heating temperature is above 500 degrees Celsius; the turning point is that the resistance increases with current or voltage. The maximum resistance occurs, when the current or voltage continues to increase, the resistance does not continue to increase but decreases; when methane gas occurs, the temperature of the silicon heater 1011 in contact with the ambient air decreases, including The resistance of the silicon heating element 101 of the silicon heater 1011 is significantly changed by the MEMS methane transmission based on the wet bidirectional etching of silicon. Wheatstone bridge constituted enable detection of low concentrations of methane gas. The power consumption of the two silicon heating elements is around 180mW, and the output signal is around 10mv/CH ₄ %.

The preparation method of the MEMS methane sensor includes two preparation methods;

The steps of the first method of preparation are:

In the first step, N-type silicon 02 is prepared by doping or diffusion on the front side of a 100-crystal P-type silicon substrate 01, and the thickness of the N-type silicon 02 is 3 to 30 um;

The second step, thermal oxidation to form a silicon oxide layer 20, a thickness of 0.5 to 1 um;

In the third step, a photoresist is prepared on the front surface of the P-type silicon substrate 01, and after lithography, a pattern of the silicon heating element 101, the isolation trench 103 disposed around the fixed end of the silicon heating element, and the front etching window 104 is formed. The exposed silicon oxide layer 20 and the underlying N-type silicon 02 are dry-etched by a reactive ion etching method, and the etching depth is greater than the sum of the thicknesses of the N-type silicon 02 and the silicon oxide layer 20 formed in the second step, and the light is removed. Engraved

In the fourth step, the silicon oxide layer 20 formed in the second step is photolithographically formed on the front side of the P-type silicon substrate 01 to form a metal contact hole;

In the fifth step, a metal layer is formed by sputtering or deposition or evaporation on the front surface of the P-type silicon substrate 01. The material of the metal layer may be gold or aluminum, and annealed to expose the metal layer and the P-type silicon substrate 01. N-type silicon 02 forms an ohmic contact;

In the sixth step, the metal layer is lithographically etched as needed to form an electrical extraction pad Pad metal 21, a metal connection line 31 and a total metal connection end 32, and the electrical extraction of each silicon heating element 101 is formed. The pad Pad metal 21 and the metal connection line 31 are both connected by a metal layer, and the metal connection line 31 is connected to the total metal connection end 32 through a metal layer; the total metal connection end 32 is disposed at the edge of the P-type silicon substrate. When an electrical potential is applied at the total metal connection 32, the N-type silicon of all of the silicon heating elements 101 on the entire silicon wafer forms a good electrical connection and has the same electrical potential as the total metal connection 32, which is provided Inside the slot 40;

As shown in FIG. 5, when preparing the MEMS methane sensor, a schematic diagram of the pad pad metal, the metal connecting line and the partial dicing groove on the silicon wafer is electrically extracted. The metal connection line 31 on the silicon wafer, the partial dicing groove 40, the total metal connection end 32 and the electrical extraction pad Pad metal 21 described in the sixth step;

In the seventh step, a photoresist is prepared on the front surface of the P-type silicon substrate 01, and a front etching window 104 is formed by lithography on the P-type silicon substrate 01, and the front surface formed by dry etching is performed by reactive ion etching. The P-type silicon exposed by the etch window 104 is etched to a depth greater than 20 um to form a front etch window 104 of the wet silicon etch to remove the photoresist;

In the eighth step, lithography aligned with the front etch window 104 is performed on the back side of the P-type silicon substrate 01 to form a pattern of the back etch window 105; the formed back etch window 105 pattern and front etch window 104 are formed. The pattern may be smaller than the front side etching 104 window pattern required for the front side wet etching to pass through the silicon wafer, and the formed back etching window 105 pattern overlaps the center of the front etching window 104 pattern, and the directions of the sides are the same. The formed back etch window 105 pattern is smaller than the front etch window 104 pattern, and the etch window is sized to ensure that the silicon wafer is formed through the via hole, the through hole size is much larger than the outer dimension of the silicon heater 1011, and the silicon heating element 101 Silicon heater 1011 is located in the front etch window 104 and the center position of the back etching window 105; the silicon oxide layer in the exposed back etching window 105 and the underlying silicon are dry-etched by a reactive ion etching method, and the etching depth is 10 to 30 μm to form a wet method. Etching the desired backside etch window 105;

In the ninth step, an etch protection layer is separately formed on the front side and the back side of the P-type silicon substrate 01 and patterned to expose the front side wet etching window 105 prepared in the eighth step, and the front side wet etching formed by the seventh step is formed. Eclipse window 104 and a total metal connection end 32 connecting all of the silicon heating element 101 fixed end 102 metal;

In the tenth step, the prepared silicon wafer is placed in a tetramethylammonium hydroxide solution or a potassium hydroxide solution, and the P-type silicon is simultaneously subjected to silicon wet etching on the front and back sides of the silicon wafer by a PN junction self-stop method. Applying a positive voltage to the N-type silicon 02 on the P-type silicon substrate 01 through the total metal connection 32 on the silicon wafer during etching, the positive voltage being higher than the passivation potential of the PN junction from the stop etching, so that The PN junction formed by the P-type silicon substrate 01 and the N-type silicon 02 is in a reverse bias state; under the action of stopping the etching of the PN junction, the N-type silicon 02 of the silicon heating element 101 is not etched; after the etching is completed Not only the silicon heating element 101 is released, but also a through hole is formed in the etching window of the front and back surfaces, and the projection of the silicon heater 1011 of the silicon heating element 101 coincides with the projection of the center of the through hole;

In the eleventh step, the front and back etching protective layers prepared in the ninth step are removed, and dried, and after drying, the silicon oxide on the surface of the silicon heating element 101 formed in the second step is removed by using a hydrofluoric acid solution or a hydrofluoric acid gas mist. ;

The twelfth step, the silicon exposed on the outer surface of the silicon heating element 101 is oxidized to form a thin layer of thin silicon oxide having a thickness of ten to 100 nm as a passivation protective layer 22;

In the thirteenth step, the scribe is diced along the scribe groove, and in particular, the connection between the metal connection line 31 in the partial scribe groove 40 and the electrical extraction pad Pad metal 21 as shown in FIG. 5 is cut, and a large number of copies are obtained after the cleavage. The invention relates to a wet bi-etched silicon based MEMS methane sensor.

The steps of the second preparation method are as follows:

In the fourth step, a metal layer is formed on the front surface of the P-type silicon substrate 01 by sputtering, deposition or evaporation. The material of the metal layer is aluminum, the thickness is 1 to 2 um, and the metal layer is annealed to form 21 and N. An ohmic contact between the type of silicon 02; sputtering or depositing or evaporating a metal layer on the back side of the P-type silicon substrate, the material of the metal layer being aluminum;

In the fifth step, the metal layer is patterned to expose the silicon oxide layer 20 in the pattern of the front etching window 104 formed in the third step and the N-type silicon oxide 02 underneath, and the dry etching is performed by reactive ion etching. Silicon, etching depth of 30um, forming a front etching window 104 required for front side wet etching;

In the sixth step, lithography aligned with the front etch window 104 is performed on the back side of the P-type silicon substrate 01 to form a pattern of the back etch window 105; the formed back etch window 105 pattern and front etch window 104 are formed. The pattern is smaller than the front etching window 104 pattern required for the front side wet etching to pass through the silicon wafer, and the formed back etching window 105 pattern overlaps the center of the front etching window 104 pattern, and the directions of the sides are the same. The formed back etching window 105 pattern is smaller than the front etching window 104 pattern, and the etching window size should ensure that the silicon wafer can be penetrated to form a through hole, and the through hole outer shape Far greater than the outer dimensions of the silicon heater 1011, and the silicon heater 1011 of the silicon heating element 101 is located at the center of the two etch windows; dry etching the exposed backside etch window 105 pattern by reactive ion etching The silicon oxide layer 20 and the underlying N-type silicon 02, etching depth of 10 to 30 um, forming a back etching window 105 required for wet etching;

In the seventh step, the prepared silicon wafer is placed in a tetramethylammonium hydroxide solution, and the P-type silicon is simultaneously subjected to a silicon wet etching on the front and back sides of the silicon wafer by a PN junction self-stop method to release the silicon heating element. When etching, a positive voltage higher than the pn junction self-stopping passivation potential is applied to the N-type silicon 02 through the front metal, so that the PN junction formed by the P-type silicon substrate and the N-type silicon layer is in a reverse bias state. The N-type silicon 02 of the silicon heating element 101 is not corroded under the PN junction self-stop protection. After the etching is completed, not only the silicon heating element 101 is released, but also the back etching window 105 and the front etching window 104 form a through hole. The projection of the center of the silicon heater 1011 of the silicon heating element 101 coincides with the projection of the center of the via;

In the eighth step, a photoresist is prepared on the fixed end 102 of the silicon heating element 101, and the photoresist is dried to remove residual metal except the metal layer of the fixed end 102;

In the ninth step, the silicon oxide 23 on the surface of the silicon heating element 101 is removed by using a hydrofluoric acid solution or a hydrofluoric acid gas mist; and the photoresist formed in the eighth step is removed;

The tenth step, oxidizing the exposed silicon to form a thin layer of thin silicon oxide having a thickness of ten to 100 nm as a passivation protective layer 22;

Claims

A MEMS methane sensor characterized in that it comprises a P-type silicon substrate (01), a P-type silicon substrate (01) is provided with N-type silicon (02); and the P-type silicon substrate (01) The upper N-type silicon (02) is processed to prepare a silicon heating element (101); the silicon heating element (101) comprises two fixed ends (102), a silicon heater (1011), and two silicon cantilevers (1012); The single silicon cantilever (1012) is at least 300 um in length; the single silicon cantilever (1012) has one end connected to the silicon heater (1011) and the other end connected to a fixed end (102) as a silicon heater (1011) Providing an electrical connection; the two silicon cantilevers (1012) are arranged side by side in parallel, form a U-shaped cantilever structure integrally with the silicon heater (1011), and suspend the silicon heater (1011) in the air; the silicon heating element (101) The silicon heater (1011) and the outer surface of the silicon cantilever (1012) are provided with a passivation protective layer (22); the fixed end (102) is disposed on the P-type silicon substrate (01), including N-type silicon ( 02), a silicon oxide layer (20) on the N-type silicon (02) and used as an electrical extraction pad Pad metal (21), the electrical extraction pad Pad metal (21) is provided in the N-type silicon (02) On the upper silicon oxide layer (20), and the electric extraction pad Pad metal (21) is oxidized The window of the silicon layer (20) is in direct contact with the underlying N-type silicon (02) to form an ohmic contact, and the contact portion of the electrically conductive pad Pad metal (21) and the underlying N-type silicon layer (02) has no silicon oxide layer (20). ;

An isolation trench (103) with N-type silicon removed is disposed around the silicon heating element (101) and its fixed end (102), and the isolation trench (103) causes the silicon heating element (101) and The N-type silicon of the fixed end (102) is in a high resistance state with the remaining N-type silicon on the P-type silicon substrate (01), especially the silicon heating element provided on the P-type silicon substrate (01) ( There is no circuit path between the two fixed ends (102) of 101) except for the electrical path formed by the silicon cantilever (1012) and the silicon heater (1011).
A methane detection application using the sensor of claim 1 wherein two of said two-way wet etching silicon based MEMS methane sensors are used, one of which is in contact with ambient air and the other of which is hermetically sealed, The air inside the package is kept sealed from the ambient air; the two MEMS methane sensors based on wet bidirectionally etched silicon form the Wheatstone bridge detection bridge arm; in the wet-process bidirectionally etched silicon-based MEMS methane sensor A voltage is applied to the two fixed ends (102) or a current is applied to cause the operating point of the silicon heating element (101) to be located in the operating point region on the left side of the turning point in the current-resistance characteristic curve, and the silicon heater of the heating element (101) ( 1011) Heating, characterized in that the electric heating temperature is above 500 degrees Celsius; the turning point is the maximum point of resistance that occurs when the resistance increases with current or voltage, and when the current or voltage continues to increase, the resistance does not continue to increase but decreases. Small; when methane gas is present, the temperature of the silicon heater (1011) in contact with the ambient air is lowered, so that the silicon heating element (101) including the silicon heater (1011) is charged The resistance changes significantly, and the low-concentration methane gas is detected by the Wheatstone bridge composed of the wet bi-directional silicon-etched MEMS methane sensor.
The method for preparing a wet bi-etched silicon-based MEMS methane sensor according to claim 1, comprising: two preparation methods;

The steps of the preparation method (1) are as follows:

In the first step, N-type silicon (02) is prepared by doping or diffusing the front side of the (100) crystal orientation P-type silicon substrate (01), and the N-type silicon (02) has a thickness of 3 to 30 um;

The second step, thermal oxidation to form a silicon oxide layer (20), thickness of 0.5 to 1 um;

In the third step, a photoresist is prepared on the front side of the P-type silicon substrate (01), and a silicon heating element (101) is formed after photolithography, an isolation trench (103) disposed around the fixed end of the silicon heating element, and a front side etching are formed. a pattern of the window (104), and dry etching the exposed silicon oxide layer (20) and the underlying N-type silicon (02) by reactive ion etching, the etching depth is greater than that of the N-type silicon (02) and the The sum of the thicknesses of the silicon oxide layers (20) formed in two steps, removing the photoresist;

In the fourth step, the silicon oxide layer (20) formed in the second step is photolithographically formed on the front side of the P-type silicon substrate (01) to form a metal contact. hole;

In the fifth step, a metal layer is formed by sputtering or deposition or evaporation on the front surface of the P-type silicon substrate (01). The material of the metal layer may be gold or aluminum, and annealed to make the metal layer and the P-type silicon substrate (01). The exposed N-type silicon (02) on the ohmic contact;

In the sixth step, lithography is performed on the metal layer as needed, and the metal layer is etched to form an electrical extraction pad Pad metal (21), a metal connection line (31), and a total metal connection end (32). The electrical extraction pad Pad metal (21) of the heating element (101) and the metal connection line (31) are both connected by a metal layer, and the metal connection line (31) is connected to the total metal connection end (32) through the metal layer; The total metal connection end (32) is provided at the edge of the P-type silicon substrate, and when a potential is applied at the total metal connection end (32), the N-type silicon of all the silicon heating elements (101) on the entire silicon wafer is well formed. Connected and have the same potential as the total metal connection end (32), the metal connection line (31) is disposed in the scribe groove (40);

In the seventh step, a photoresist is prepared on the front side of the P-type silicon substrate (01), and a front etching window (104) pattern is formed on the P-type silicon substrate (01), and a reactive ion etching method is used. Etching the formed front etch window (104) pattern exposed P-type silicon, etching depth greater than 20um, forming a front etch window (104) of wet silicon etching, removing the photoresist;

In the eighth step, lithography aligned with the front etch window (104) is performed on the back side of the P-type silicon substrate (01) to form a pattern of the back etch window (105); the formed back etch window (105) Both the pattern and the front etch window (104) pattern may be smaller than the front side etch (104) window pattern required for the front side wet etching of the silicon wafer, and the formed back etch window (105) pattern and front side engraved The center of the etch window (104) overlaps, and the directions of the sides are the same. The formed back etch window (105) pattern is smaller than the front etch window (104) pattern, and the size of the etch window should ensure that the silicon wafer is formed through. The hole and the through hole are much larger than the outer dimensions of the silicon heater (1011), and the silicon heater (1011) of the silicon heating element (101) is located at the center of the front etching window (104) and the back etching window (105). The silicon oxide layer in the exposed back etching window (105) and the underlying silicon are dry etched by a reactive ion etching method, and the etching depth is 10 to 30 um to form a back etching required for wet etching. Window (105);

In the ninth step, an etch protection layer is separately formed on the front side and the back side of the P-type silicon substrate (01) and patterned to expose the back surface wet etching window (105) prepared in the eighth step, and the seventh step is formed. a front wet etching window (104) and a total metal connection end (32) connecting all of the silicon heating element (101) fixed end (102) metal;

In the tenth step, the prepared silicon wafer is placed in a tetramethylammonium hydroxide solution or a potassium hydroxide solution, and the P-type silicon is simultaneously subjected to silicon wet etching on the front and back sides of the silicon wafer by a PN junction self-stop method. Applying a positive voltage to the N-type silicon (02) on the P-type silicon substrate (01) through the total metal connection end (32) on the silicon wafer during etching, the positive voltage being higher than the PN junction self-stop etching Passivating the potential so that the PN junction formed by the P-type silicon substrate (01) and the N-type silicon (02) is in a reverse bias state; under the action of stopping the etching of the PN junction, the N of the silicon heating element (101) The silicon (02) is not etched; after the etching is completed, not only the silicon heating element (101) is released, but also a through hole is formed in the etching windows of the front and back surfaces, and the silicon heater of the silicon heating element (101) The center projection of 1011) coincides with the projection of the center of the through hole;

In the eleventh step, the front and back etching protective layers prepared in the ninth step are removed, dried, and dried to remove the surface of the silicon heating element (101) produced in the second step by using a hydrofluoric acid solution or a hydrofluoric acid gas mist. Silicon oxide

In the twelfth step, the silicon exposed on the outer surface of the silicon heating element (101) is oxidized to form a thin layer of thin silicon oxide having a thickness of ten to 100 nm as a passivation protective layer (22);

In the thirteenth step, the dicing is performed along the scribe groove, and in particular, the connection between the metal connection line (31) in the partial dicing groove (40) and the electrical extraction pad Pad metal (21) is cut, and after the cleavage, a plurality of wet-based methods are obtained. Bidirectionally etched silicon MEMS methane sensor;

Or the steps of the preparation method (2) are:

In the first step, N-type silicon (02), N-type silicon is prepared by doping or diffusion on the front side of a (100) crystal orientation P-type silicon substrate (01). (02) thickness is 3 to 30 um;

The second step, thermal oxidation to form a silicon oxide layer (20), thickness of 0.5 to 1 um;

In the third step, a photoresist is prepared on the front side of the P-type silicon substrate (01), and a silicon heating element (101) is formed after photolithography, an isolation trench (103) disposed around the fixed end of the silicon heating element, and a front side etching are formed. a pattern of the window (104), and dry etching the exposed silicon oxide layer (20) and the underlying N-type silicon (02) by reactive ion etching, the etching depth is greater than that of the N-type silicon (02) and the The sum of the thicknesses of the silicon oxide layers (20) formed in two steps, removing the photoresist;

In the fourth step, a metal layer is formed on the front surface of the P-type silicon substrate (01) by sputtering, deposition or evaporation, the material of the metal layer is aluminum, the thickness is 1 to 2 um, and the metal layer is annealed to form ( 21) an ohmic contact with the N-type silicon (02); sputtering or depositing or evaporating a metal layer on the back side of the P-type silicon substrate, the material of the metal layer being aluminum;

In the fifth step, the metal layer is patterned to expose the silicon oxide layer (20) and the underlying N-type silicon (02) in the front etching window (104) formed by the third step, and is dried by reactive ion etching. Etching the exposed silicon to an etching depth of 30 um to form a front etch window (104) required for front side wet etching;

In the sixth step, lithography aligned with the front etch window (104) is performed on the back side of the P-type silicon substrate (01) to form a pattern of the back etch window (105); the formed back etch window (105) The pattern and the front etch window (104) pattern are smaller than the front etch window (104) pattern required for the front side wet etch through the silicon wafer, and the back etch window (105) pattern and front etch are formed. The center of the window (104) overlaps, and the directions of the sides are the same. The formed back etching window (105) pattern is smaller than the front etching window (104) pattern, and the etching window size is required to ensure that the silicon wafer is formed through the through hole. The through hole is much larger than the outer dimension of the silicon heater (1011), and the silicon heater (1011) of the silicon heating element (101) is located at the center of the two etching windows; the dry etching is performed by reactive ion etching The silicon oxide layer (20) and the underlying N-type silicon (02) in the etched back etch window (105) pattern are etched to a depth of 10 to 30 um to form a backside etch window required for wet etching ( 105);

In the seventh step, the prepared silicon wafer is placed in a tetramethylammonium hydroxide solution, and the P-type silicon is simultaneously subjected to a silicon wet etching on the front and back sides of the silicon wafer by a PN junction self-stop method to release the silicon heating element. Applying a positive voltage higher than the PN junction self-stopping passivation potential to the N-type silicon (02) through the front metal during etching, so that the PN junction formed by the P-type silicon substrate and the N-type silicon layer is reversed In the off state, the N-type silicon (02) of the silicon heating element (101) is not corroded under the PN junction self-stop protection. After the etching is completed, not only the silicon heating element (101) but also the back etching window (105) and The front etching window (104) forms a through hole, and the projection of the center of the silicon heater (1011) of the silicon heating element (101) coincides with the projection of the center of the through hole;

In the eighth step, a photoresist is prepared on the fixed end (102) of the silicon heating element (101), and the photoresist is dried to remove residual metal except the metal layer of the fixed end (102);

In the ninth step, the silicon oxide (23) on the surface of the silicon heating element (101) is removed by using a hydrofluoric acid solution or a hydrofluoric acid gas mist; and the photoresist formed in the eighth step is removed;

The tenth step, oxidizing the exposed silicon to form a thin layer of thin silicon oxide having a thickness of ten to 100 nm as a passivation protective layer (22);

In the eleventh step, dicing, dicing, and obtaining the MEMS methane sensor based on wet bidirectional etched silicon.