CN102169126B - Hot air speed and air direction sensor based on thinning process and manufacturing method thereof - Google Patents

Abstract

The invention provides a wafer-level packaged thermal wind speed and direction sensor and a preparation method thereof, the sensor chip adopts a standard CMOS process to manufacture a heating element and a thermal sensing temperature measuring element; The dry etching process prepares a 50 micron deep thermal isolation groove between the heating element and the thermal sensing and temperature measuring element to reduce the lateral heat conduction effect between them; the silicon substrate of the sensor chip is thinned by thinning process Thin until the thickness is in the range of 80 microns to 100 microns, reduce the heat conduction and heat capacity of the chip substrate; use the ceramic substrate to stick to the back of the thinned silicon chip to protect the silicon chip and sense the change of wind speed and direction in the external environment. The wind speed and direction sensor proposed by the present invention greatly reduces the heat conduction loss on the silicon substrate and the heat capacity of the sensor chip while realizing wafer-level packaging, and can obtain a larger output signal and a faster Response time.

Description

Thermal wind speed and direction sensor based on thinning process and its preparation method

technical field

The invention relates to a wafer-level packaged thermal wind speed and direction sensor realized by silicon substrate thinning process technology. The sensor chip is prepared by standard CMOS integrated circuit technology, and especially relates to a low-power integrated wind speed sensor based on ceramic packaging. Wind direction sensor and preparation method thereof.

Background technique

In the design of CMOS integrated wind speed and direction sensors, packaging has always been a technical bottleneck hindering its development. On the one hand, the encapsulation material is required to have good thermal conductivity and to protect the sensor, and the impact of the encapsulation material on the sensitivity, reliability and price of the sensor needs to be considered in the design, which limits the sensor’s own encapsulation. Design freedom. On the other hand, the thermal flow sensor requires the sensitive part of the sensor to be exposed to the measurement environment, and at the same time requires the processing circuit to be isolated from the environment, so as not to affect the performance of the processing circuit, which creates a contradiction in the packaging requirements.

Most of the silicon wind speed and direction sensors reported in the past directly expose the sensitive surface of the silicon wafer to the natural environment, so as to be able to sense the change of the external wind speed. As a result, silicon wafers are easily subject to various contaminations, resulting in unstable performance and even damage. If a ceramic substrate with high thermal conductivity is used, and the silicon chip of the sensor is packaged by flip-chip packaging or thermally conductive adhesive, the above contradictions can be better avoided, but the heat generated by the sensor after packaging is extremely large. Part of it is dissipated from the silicon-based substrate in the form of heat conduction, and only a small part is heat-exchanged with the outside air through the ceramic, which greatly reduces the amplitude of the output sensitive signal, and the sensitive signal can be improved by increasing the power consumption of the sensor. Amplitude, but it causes a large power consumption of the whole sensor system.

Contents of the invention

The object of the present invention is to provide a thermal wind speed and direction sensor based on thinning technology and its preparation method based on wafer-level packaging realized by silicon substrate thinning technology. The designed sensor structure and packaging form are conducive to ensuring a relatively The sensor system has low power consumption while having a large sensitive signal amplitude.

The present invention adopts following technical scheme:

A thermal wind speed and direction sensor based on a thinning process, including a thinned silicon chip, the back of the thinned silicon chip is connected to a ceramic substrate through a thermally conductive glue, an N well is arranged on the front of the thinned silicon chip, and the N well There is an oxide layer on the top of the N well, and there are 4 diffused resistance heating elements and 4 heat sensing temperature measuring elements in the middle of the N well. The 4 heat sensing temperature measuring elements are thermocouple temperature measuring elements and are distributed in 4 diffusion resistance Around the heating element, there are electrical lead-out pads on the edge of the oxide layer. The electric lead-out pads of the 4 diffusion resistance heating elements and the 4 electric lead-out pads of the heat sensing and temperature-measuring elements are respectively connected to the electric lead-out pads by metal leads. The pads are connected, and a heat isolation groove is arranged between the 4 diffusion resistance heating elements and the 4 heat sensing temperature measuring elements, and the heat isolation groove is deep and thinned in the silicon chip substrate.

A method for preparing a thermal wind speed and direction sensor based on a thinning process is as follows:

The first step, the preparation of the silicon chip

Step 1, thermally growing a first thermal oxide layer on the surface of the silicon chip;

Step 2, chemical vapor deposition of a silicon nitride layer on the first thermal oxide layer;

Step 3, using RIE technology to etch the silicon chip to define the active area;

Step 4, chemical vapor deposition of the second oxide layer;

Step 5, using CMP technology to polish the silicon chip;

Step 6, removing the silicon nitride layer by wet etching, and preparing the field oxide layer;

Step 7, implanting phosphorus ions to prepare an N well;

Step 8, thermally growing the gate oxide layer;

Step 9, boron ion implantation, preparing one end of the heating element and the heat sensing temperature measuring element;

Step 10, chemical vapor deposition of a third oxide layer, wherein the first thermal oxide layer, the second oxide layer and the third oxide layer are combined into one oxide layer;

Step 11, using a dry etching process to prepare a through hole on the heat sensing temperature measuring element and a through hole on the heating element;

Step 12, using a sputtering process to prepare the aluminum pad for electrical lead-out, the other end of the heat sensing temperature-measuring element, and the electrical lead-out pad of the heating element;

Step 13, using a dry etching process to prepare a thermal isolation groove between the heating element and the heat sensing temperature measuring element;

The second step, thinning and encapsulation

Step 1, hanging a paraffin layer on the front side of the silicon chip;

Step 2, adhering a glass slide on the front side of the silicon chip through a paraffin layer at an ambient temperature of 80°C;

Step 3, using a thinning process to thin the substrate of the silicon chip until the thickness of the substrate is in the range of 80 microns to 100 microns to obtain a thinned silicon chip;

Step 4, hang-coat the thermally conductive adhesive on the back of the thinned silicon chip, attach the ceramic substrate, and cure the thermally conductive adhesive at an ambient temperature of 100°C;

Step 5, removing the glass slide and the paraffin layer at an ambient temperature of 80°C;

The third step is dicing to complete the preparation of the wind speed and direction sensor.

In the present invention, a heating element and a thermal sensing temperature measuring element are prepared on a silicon base using a CMOS process, and a 50-micron deep thermal isolation groove is prepared between the heating element and a thermal sensing temperature measuring element by using a DRIE dry etching process for use in Reduce the lateral heat conduction effect between the heating element and the heat-sensing temperature-measuring element; use the thinning process of silicon substrate mechanical grinding and polishing to thin the silicon chip substrate that already contains the heating element and the heat-sensing temperature-measuring element , remove most of the silicon substrate until the thickness of the silicon substrate is in the range of 80 microns to 100 microns, which can greatly reduce the heat capacity of the chip, and can reduce the response time of the sensor while improving the sensitivity of the chip; A ceramic substrate with a certain thermal conductivity is pasted and sealed to the back of the thinned silicon chip through a thermally conductive adhesive. On the one hand, the ceramic substrate protects the thinned silicon chip from the pollution of the external environment and provides mechanical support. On the other hand, it is used as an intermediate heat transfer medium to realize Heat exchange between the thinned silicon chip and the external environment. In the process of thinning the silicon chip substrate, the silicon chip is first attached to the glass slide with paraffin wax, and the glass slide is used to provide the necessary mechanical support for the thinned silicon chip, and then the still attached glass slide is used The thinned silicon chip of the thin film is pasted and sealed on the ceramic substrate, and finally the paraffin is melted to remove the glass slide to complete the preparation of the sensor. Such preparation steps can prepare a sensor chip with a substrate thickness of 80 microns to 100 microns, and Provides the necessary mechanical support for fragile chip structures throughout the post-processing of sensor packaging and dicing.

In the present invention, the heat generated by the heating element in the thinned silicon chip is conducted to the ceramic substrate through the substrate of the thinned silicon chip and the heat conduction effect of the heat-conducting glue, and a temperature field is established in the ceramic substrate, and the upper surface of the ceramic substrate is exposed. In the external environment, changes in the wind in the external environment will affect the temperature field distribution in the ceramic substrate. The changed temperature field distribution in the ceramic substrate can be transmitted back to the thinned silicon chip substrate through the transfer of thermal conductive adhesive, and the thinned silicon chip The heat sensing temperature measuring element in the device measures the change of the temperature distribution of the temperature field through the thinned silicon substrate. Under the condition of no wind outside, the distribution of temperature field presents a completely symmetrical state. When the outside wind blows over the upper surface of the ceramic substrate, the wind will take away part of the heat from the upper surface of the ceramic substrate in the form of heat convection, and establish a temperature gradient distribution field along the direction of the wind in the ceramic substrate, heat transfer sensing The temperature element measures the change of the temperature field distribution through the heat conduction of the thinned silicon substrate and the heat-conducting glue, and then the wind speed and wind direction can be calculated.

In the sensor structure, the ceramic substrate sealed to the back of the thinned silicon chip with thermally conductive adhesive is used to protect the underlying thinned silicon chip and provide necessary mechanical support, and on the other hand, it is used as a sensitive element that senses changes in the external wind . Only the upper surface of the ceramic substrate of the whole sensor is in contact with the wind environment, and other components are isolated from the external environment through the ceramic substrate, so that it can avoid being polluted by the external environment. The structure of the sensor of the invention is suitable for preparing a two-dimensional wind speed and direction sensor.

In this sensor design scheme, in the first step of silicon chip preparation, the standard CMOS process is used to prepare the heating element and the thermal sensing temperature measuring element; in the second step of the preparation of the thermal isolation groove, the DRIE dry etching process is used; the third step In the thinning of the silicon chip substrate, the thinning process of mechanical grinding and polishing of the silicon substrate is used; in the fourth step of the ceramic substrate mount package, the thermal conductive adhesive is applied to the back of the ceramic substrate using the suspension coating technology, and then the thinning process is performed. The back of the silicon chip is pasted and sealed; the fifth step is dicing. The whole sensor preparation process is compatible with the standard CMOS process, and can realize the wafer-level packaging of the sensor chip.

The present invention obtains following effect:

1. The present invention utilizes the DRIE dry etching process to prepare a 50 micron deep thermal isolation groove between the heating element on the front surface of the silicon chip and the thermal sensing temperature measuring element, which can effectively reduce the heating element and the thermal sensing temperature measuring element. The lateral heat conduction effect between elements; the thinning process of silicon substrate mechanical grinding and polishing is used to thin the silicon chip substrate that already contains heating elements and thermal sensing temperature measuring elements, and remove most of the silicon substrate until the silicon The thickness of the substrate is in the range of 80 microns to 100 microns, which can greatly reduce the heat capacity of the chip, reduce the thermal response time of the sensor and improve the sensitivity of the sensor.

2. The chip structure with thermal isolation grooves on the front surface of the thinned silicon chip can make most of the heat generated by the heating element conduct vertically to the back of the thinned silicon chip through the thinned silicon substrate, which can be largely isolated The lateral heat conduction between the heating element and the heat sensing temperature measuring element can greatly reduce the unnecessary power consumption caused by the heat conduction effect of the silicon substrate.

3. In the process of thinning the silicon chip substrate, first use paraffin to attach the front side of the silicon chip to the glass slide, use the glass slide to provide the necessary mechanical support for the thinned silicon chip after thinning, and then use the thermal conductive adhesive to attach the silicon chip to the glass slide. The backside of the thinned silicon chip with the glass slide attached is pasted and sealed on the ceramic substrate, and finally the paraffin is melted in an environment of 80°C-100°C to remove the slide glass and complete the preparation of the sensor. Thinned silicon chips with base thicknesses ranging from 80 microns to 100 microns provide essential mechanical support for fragile chip structures throughout packaging and dicing of sensors

4. The present invention uses a ceramic substrate with a certain thermal conductivity to be pasted and sealed to the back of the thin-film silicon chip through a thermally conductive adhesive, which can provide the necessary mechanical support for the fragile chip structure and protect the sensor from the pollution of the external environment. A thermal path is provided between the silicon chip and the external environment.

The traditional CMOS integrated wind speed and direction sensor, in reducing the heat conduction of the silicon-based substrate, one method is to use a wet etching process to prepare a heat-insulating cavity on the back of the silicon chip and the area corresponding to the heating element. The disadvantage is that the prepared heat-sensitive film Too fragile, thermal stress has a great influence on signal detection, and the packaging of the sensor cannot be realized. Another method is to prepare a porous silicon insulation layer under the heating element. In this way, the preparation process is not compatible with the standard CMOS process, and the preparation process of porous silicon is poorly consistent, which increases the difficulty of signal conditioning for back-end sensors. The sensor structure proposed by the present invention is prepared by a standard CMOS process, and the structural design of preparing thermal isolation grooves and thinning the silicon chip substrate can effectively reduce the heat loss caused by the heat conduction effect of the sensor heating element, making most of the The heat is exchanged with the air through the ceramic substrate to sense the change of wind speed and direction in the external environment; the preparation of a thin silicon chip substrate can greatly improve the sensitivity of the sensor, so it can obtain a larger output with lower power consumption Signal. Compared with the traditional single-chip packaged wind speed and direction sensor, this form of wafer-level packaging greatly reduces the packaging cost of MEMS devices on the one hand, and on the other hand ensures the consistency of the deviation caused by the sensor package to a large extent. , reducing the cost of sensor back-end signal conditioning.

Description of drawings

FIG. 1 is a schematic diagram of steps 1 to 4 of the fabrication process of a silicon chip.

FIG. 2 is a schematic diagram of steps 5 to 9 of the silicon chip preparation process.

FIG. 3 is a schematic diagram of steps 10 to 13 of the silicon chip fabrication process.

Figure 4 is a top view of a silicon chip.

FIG. 5 is a schematic diagram of step 1 to step 3 of thinning the silicon chip substrate and sealing the ceramic package.

FIG. 6 is a schematic diagram of step 4 to step 5 of thinning the silicon chip substrate and sealing the ceramic package.

Figure 7 is the final diced monolithic sensor chip.

Detailed ways

Example 1

A preparation method of a thermal wind speed and direction sensor based on a thinning process is as follows:

The first step, the preparation of the silicon chip

Step 1, thermally growing a first thermal oxide layer 2 on the surface of the silicon chip 1;

Step 2, chemical vapor deposition of a silicon nitride layer 3 on the first thermal oxide layer 2;

Step 3, using RIE technology to etch the silicon chip 1 to define the active region 4;

Step 4, chemical vapor deposition of the second oxide layer 5;

Step 5, using CMP technology to polish the silicon chip 1;

Step 6, removing the silicon nitride layer 3 by wet etching, and preparing the field oxide layer 6;

Step 7, phosphorus ion implantation, preparing N well 7;

Step 8, thermally growing the gate oxide layer 8;

Step 9, boron ion implantation, preparing the heating element 9 and one end 10 of the heat sensing temperature measuring element 15;

Step 10, chemical vapor deposition of a third oxide layer, wherein the first thermal oxide layer 2, the second oxide layer 5 and the third oxide layer are combined into an oxide layer 11;

Step 11, using a dry etching process to prepare the through hole 12 on the heat sensing temperature measuring element 15 and the through hole 17 on the heating element 9;

Step 12, using a sputtering process to prepare the aluminum pad 14 for electrical extraction, the other end 13 of the heat sensing temperature measuring element 15, and the electrical extraction pad 18 of the heating element 9;

Step 13, using a dry etching process to prepare a thermal isolation groove 16 between the heating element 9 and the heat sensing temperature measuring element 15;

The second step, thinning and encapsulation

Step 1, hanging a paraffin layer 19 on the front side of the silicon chip 1;

Step 2, adhering the glass slide 20 on the front side of the silicon chip 1 through the paraffin layer 19 at an ambient temperature of 80° C.;

Step 3, using a thinning process to thin the substrate of the silicon chip 1 until the thickness of the substrate is in the range of 80 microns to 100 microns to obtain a thinned silicon chip 21;

Step 4, hang coating the thermally conductive adhesive 22 on the back of the thinned silicon chip 21, and paste the ceramic substrate 23, and cure the thermally conductive adhesive at an ambient temperature of 100°C;

Step 5, removing the glass slide 20 and the paraffin layer 19 at an ambient temperature of 80°C;

The third step is dicing to complete the preparation of the wind speed and direction sensor.

The invention is a scheme for realizing the preparation of a CMOS integrated wind speed and direction sensor and the wafer-level packaging. The side where the sensor chip is in contact with the wind in the external environment is the upper surface of the ceramic substrate 23, and a thermal connection is established between the thermally conductive adhesive 22 and the thinned silicon chip 21. Since the ceramic material has a certain thermal conductivity, the thinned silicon chip The heat generated by the heating element 9 in 21 is conducted to the ceramic substrate 23 through the thinned silicon substrate and the thermally conductive glue 22 , and a certain temperature field distribution is established on the upper surface of the ceramic substrate 23 . Under the condition of no wind, the temperature field is symmetrically distributed around the center of the ceramic substrate 23 on the ceramic substrate 23; under the condition that there is a certain wind speed in the external environment, the symmetrical distribution is broken, and a temperature gradient field is generated. The direction remains the same. Four heat sensing temperature measuring elements 15 are distributed in a symmetrical layout around the heating element 9 on the thinned silicon chip 21, and a thermal isolation groove 16 is arranged between the heating element 9 and the heat sensing temperature measuring element 15 for Reduce the lateral heat conduction effect between them and increase the useful signal of the sensor. The temperature field on the upper surface of the ceramic substrate 23 can be transmitted to the thinned silicon chip 21 by utilizing the heat conduction characteristics of the thermally conductive adhesive 22, and then conducted to the heat-sensing and temperature-measuring element 15 through the silicon substrate, and then the temperature field on the upper surface of the ceramic substrate 23 can be measured Changes. The information of wind speed and wind direction in the external environment can be obtained by processing the output signals of the four heat sensing temperature measuring elements 15 .

The traditional CMOS integrated wind speed and direction sensor is generally packaged directly with the ceramic substrate by flip-chip flip-chip or heat-conducting glue. Since the thermal conductivity of silicon is much greater than that of ceramics, most of the heat generated by the heating element on the silicon after packaging is dissipated from the silicon substrate through heat conduction, and only a small amount of heat passes through the ceramic substrate and the air. Heat convection heat exchange is generated, which on the one hand greatly reduces the output signal of the sensor, on the other hand increases the working power of the sensor and reduces the efficiency of the sensor. Based on this problem, the predecessors proposed to make a cavity on the back of the silicon substrate or make a layer of porous silicon under the heating element to reduce the heat conduction of the silicon substrate, so that the sensor packaging or process consistency and CMOS process compatibility challenged.

In the present invention, the CMOS process is used to prepare the sensor chip; the DRIE dry etching process is used to prepare a thermal isolation groove on the front of the sensor chip, which is used to increase the useful signal of the sensor and reduce the interference caused by the heat conduction effect to the useful signal; The process-compatible thinning process thins the substrate of the sensor chip, which can greatly reduce the useless power consumption caused by heat conduction and increase the sensitivity of the sensor; the ceramic substrate is sealed to the back of the thinned sensor chip with thermally conductive adhesive Package the sensor chip.

Example 2

A thermal wind speed and direction sensor based on a thinning process is characterized in that it includes a thinned silicon chip 21, the back side of the thinned silicon chip 21 is connected with a ceramic substrate 23 through a thermally conductive glue 22, and the thinned silicon chip 21 is connected to a ceramic substrate 23. An N well 7 is provided on the front side, an oxide layer 11 is provided on the N well 7, and 4 diffused resistance heating elements 9 and 4 heat sensing temperature measuring elements 15 are arranged in the middle of the N well 7. The temperature element 15 is a thermocouple temperature measuring element and is distributed around the four diffused resistance heating elements 9, and an electric lead-out pad 14 is provided on the edge area of the oxide layer 11, and the electric lead-out pad 18 of the four diffused resistance heating elements 9 and the electric lead-out pads 13 of the four heat-sensing temperature-measuring elements 15 are respectively connected to the electric-lead-out pads 14 by metal leads, and between the four diffused resistance heating elements 9 and the four heat-sensing temperature-measuring elements 15, a The thermal isolation groove 16 is deep and thinned in the substrate of the silicon chip 21 .

Claims (1)

1. A thermal wind speed and direction sensor based on a thinning process, characterized in that it includes a thinned silicon chip (21), and the back of the thinned silicon chip (21) is connected to a ceramic substrate ( 23), an N well (7) is provided on the front side of the thinned silicon chip (21), an oxide layer (11) is provided on the N well (7), and 4 diffusion resistors are provided in the middle of the N well (7) The heating element (9) and 4 heat sensing temperature measuring elements (15), the 4 heat sensing temperature measuring elements (15) are thermocouple temperature measuring elements and distributed around the 4 diffusion resistance heating elements (9), In the edge area of the oxide layer (11), there are electric lead-out pads (14), electric lead-out pads (18) of the four diffused resistance heating elements (9) and electric lead-out pads (18) of the four heat-sensing and temperature-measuring elements (15). The lead-out pads (13) are respectively connected to the electric lead-out pads (14) through metal leads, and a thermal isolation groove ( 16), the thermal isolation groove (16) is deep and thinned in the silicon chip (21) substrate.

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