Membrane type flow sensor chip and preparation method thereof

Abstract

The application relates to a MEMS hot-film flow sensor chip and a preparation method thereof, and the method comprises the following steps: arranging a silicon nitride layer on the front and back surfaces of a substrate respectively; forming a silicon support layer on the silicon nitride layer on the front surface of the substrate; forming a groove array on the designated area on the front surface of the silicon support layer; filling the groove array by using a thermal oxidation method to make the designated area all be thermally oxidized to form a silicon oxide layer; sputtering a metal layer on the silicon oxide layer to form an upstream and downstream bridge resistance and a heating resistance; sputtering the metal layer on the non-designated area on the front surface of the silicon support layer to form an ambient resistance, and the non-designated area is located on both sides of the designated area; depositing a passivation protective layer on the metal layer; performing photoetching and etching on the middle area of the silicon nitride layer on the back surface of the substrate, and etching the substrate from the back surface to stop at the bottom of the silicon nitride layer on the front surface of the substrate, so as to form a suspended membrane structure. The mechanical strength is improved by optimizing the thickness of the membrane area, and the deformation or rupture of the membrane area is prevented.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of temperature sensing, and in particular to a MEMS hot-film flow sensor chip and a preparation method thereof. BACKGROUND

[0002] The MEMS hot-film flow sensor chip is mainly divided into a heat distribution type and a heat loss type. The measurement principle of the heat distribution type is that when a fluid flows, the temperature field distribution near the heating element is no longer symmetrical, the temperature sensed by the upstream temperature sensing element decreases, the temperature sensed by the downstream temperature sensing element increases, and the fluid flow size is obtained by detecting the temperature difference between the upstream and downstream temperature sensing elements. The measurement principle of the heat loss type is that the heat transfer effect between the fluid and the surface of the heating element (Heater), and the temperature change caused by the heat loss of the heating element is measured by the temperature sensing element to indirectly measure the change of the flow. The influencing factors of the measurement accuracy of the MEMS hot-film flow sensor include the heating element, the temperature sensing element and the heat insulation treatment. Among them, the heating element and the temperature sensing element can affect the response speed of the sensor, and good heat insulation treatment can improve the sensitivity of the sensor. In order to strengthen the heat insulation, the MEMS hot-film flow sensor chip is usually provided as a suspended structure, that is, the sensor is provided on a membrane suspended in the middle and supported at both ends. After packaging, a back cavity is formed under the sensor, which does not contact the packaging base but contacts air or vacuum, so as to achieve the purpose of reducing the interference of the external environment temperature.

[0003] Due to the structural characteristics of the membrane suspended in the middle and supported at both ends, uneven stress distribution is easily generated in the whole membrane during work, which may cause deformation or rupture of the membrane area. The mechanical stress can be resisted by optimizing the thickness of the membrane. At present, the membrane is usually prepared by using a silicon processing technology, and common technical means include a low pressure chemical vapor deposition (LPCVD) process combined with a front and back (back cavity) etching process. Since the rate of the LPCVD process is slow and the processing period is long, the processing demand for optimizing the thickness of the membrane cannot be met. The etching process has low control accuracy for the thickness of the membrane due to the influence of factors such as the purity, concentration and temperature of the etching solution used. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application provides a MEMS hot-film flow sensor chip and a preparation method thereof, which aims to improve the thickness of the chip by optimizing and controlling the thickness of the membrane, prevent deformation or rupture of the membrane area, and thus improve the service life and reliability of the chip.

[0005] The technical scheme adopted by the present application is as follows:

[0006] The present application provides a preparation method of a MEMS hot-film flow sensor chip, comprising:

[0007] A layer of silicon nitride is arranged on the front and back surfaces of the substrate, respectively;

[0008] forming a silicon support layer on the silicon nitride layer on the front surface of the substrate by using a silicon epitaxial growth method;

[0009] forming a groove array in a designated area on the front surface of the silicon support layer by using photolithography and etching, and the bottom surface of the groove being connected to the silicon nitride layer;

[0010] filling the groove array by oxidation by using a thermal oxidation method, and forming a silicon oxide layer by thermal oxidation in the designated area;

[0011] sputtering a metal layer on the silicon oxide layer, forming a pattern by using photolithography and etching, and forming an upstream bridge resistor, a downstream bridge resistor and a heating resistor by alloying the metal pattern; sputtering a metal layer on the non-designated area on the front surface of the silicon support layer, forming a pattern by using photolithography and etching, and forming an ambient resistor by alloying the metal pattern, the non-designated area being located on both sides of the designated area;

[0012] depositing a passivation protective layer on the metal layer, and forming a PAD window by photolithography and etching;

[0013] performing photolithography and etching on the middle area of the silicon nitride layer on the back surface of the substrate, exposing the substrate, and etching the substrate from the back surface to form a back cavity, and the etching being self-stopped at the bottom of the silicon nitride layer on the front surface of the substrate, thereby forming a suspended membrane structure.

[0014] Further technical solutions are as follows:

[0015] forming a silicon support layer on the silicon nitride layer on the front surface of the substrate by using a silicon epitaxial growth method, comprising:

[0016] cleaning the surface of the silicon support layer to be grown to remove impurities;

[0017] high-temperature annealing in a hydrogen environment to remove oxides on the surface of the silicon support layer to be grown;

[0018] preheating the surface of the silicon support layer to be grown in an epitaxial furnace, and the preheating temperature is 1000°C;

[0019] introducing a certain flow of reaction gas and carrier gas into the furnace, and performing silicon epitaxial growth under a certain pressure;

[0020] gradually reducing the temperature of the epitaxial furnace to room temperature, and then taking out and cooling.

[0021] the reaction gas is a silicon source gas SiH4, and the flow is 50 sccm; the carrier gas is H2, and the flow is 2000 sccm; during epitaxial growth, the temperature is 800°C, the pressure in the furnace is 50 Torr, and the epitaxial growth rate is controlled at 10 nm / min.

[0022] the thickness of the silicon support layer is 5-10 μm;

[0023] The substrate is a silicon substrate, and the silicon nitride layer is grown on the silicon substrate by low-pressure chemical vapor deposition.

[0024] In the groove array, the grooves are equidistantly distributed, each groove has a trapezoidal cross section with a top side and a bottom side, and the bottom angle of the trapezoid close to the top side is 86°-89°; the maximum distance between adjacent two grooves is less than or equal to 1 μm, the maximum width of each groove is not greater than 1 μm, and the depth of the groove is the thickness of the silicon support layer; the projection surface of the groove is a polygon, a circle or a special shape.

[0025] The groove array is filled by oxidation by using a thermal oxidation method, comprising:

[0026] Based on the silicon between adjacent grooves in the groove array, thermal oxidation is performed along the surface of the base, so that the silicon between adjacent grooves is continuously consumed and the grooves are continuously filled until all of them become an oxide silicon layer.

[0027] After the metal is patterned, an alloying treatment is performed on the metal at a temperature greater than 400 ℃ by using a furnace tube process.

[0028] The passivation protective layer has a single-layer or multi-layer structure.

[0029] The material of the passivation protective layer is one or a combination of several of silicon oxide, silicon nitride and silicon oxynitride, or a polyimide material.

[0030] Before sputtering a metal layer on the oxide silicon layer, the surface of the oxide silicon layer is also polished to be planarized by using a chemical mechanical polishing process.

[0031] The application also provides a MEMS hot-film type flow sensor chip, which is prepared according to the preparation method and comprises:

[0032] A substrate has a back cavity formed in the middle of the back surface by etching, and support ends are formed at both ends of the back cavity.

[0033] A silicon nitride layer is deposited on the front surface and the back surface of the substrate, the silicon nitride layer on the front surface is used as a self-stopping layer for the bottom of the back cavity formed by etching and as a thermal insulation layer for the metal layer, and the silicon nitride layer on the back surface is used as a protective layer for the opening of the back cavity formed by etching.

[0034] A silicon support layer is grown on the silicon nitride layer on the front surface by silicon epitaxial growth, and is used as a thermal insulation layer for the metal layer and for enhancing the structural strength of the suspended membrane.

[0035] An oxide silicon layer is formed in a specified area of the silicon support layer by a thermal oxidation method, and the specified area is located in the middle of the silicon support layer.

[0036] A metal layer is formed on the silicon nitride layer and the silicon support layer by sputtering, etching, patterning and alloying treatment to form a corresponding resistance.

[0037] A passivation protective layer is formed on the metal layer to protect the metal layer.

[0038] The present application has the following advantages:

[0039] The present application has the following advantages:

[0040] The present application has the following advantages:

[0041] The present application has the following advantages:

[0042] Other features and advantages of the present application will be illustrated in the following description, or will be understood by those skilled in the art through the implementation of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The present application has the following advantages:

[0044] Figure 2 The present application has the following advantages:

[0045] Figure 3 The present application has the following advantages:

[0046] In the figure: 1, substrate; 2, silicon nitride layer; 3, silicon support layer; 4, groove; 5, silicon oxide layer; 6, metal layer; 7, passivation protective layer; 8, back cavity; 10, gap; 20, silicon island. DETAILED DESCRIPTION

[0047] The specific embodiments of the present application will be described below with reference to the accompanying drawings.

[0048] Referring to Figure 1 The present application provides a preparation method of a MEMS thermal film type flow sensor chip, comprising:

[0049] A layer of silicon nitride 2 is arranged on the front and back of the substrate 1 respectively;

[0050] A silicon support layer 3 is formed on the layer of silicon nitride 2 on the front of the substrate 1 by using silicon epitaxial growth method;

[0051] A groove array is formed on the front of the silicon support layer 3 by using photoetching and etching in the designated area, and the bottom of the groove 4 is connected with the layer of silicon nitride 3;

[0052] The groove array is filled with oxidation by using thermal oxidation method, and the designated area is entirely thermally oxidized to form a layer of silicon oxide 5;

[0053] A metal layer 6 is sputtered on the layer of silicon oxide 5, and a pattern is formed by using photoetching and etching, and the metal pattern is alloyed to form upstream and downstream bridge resistors and a heating resistor; a metal layer 6 is sputtered on the non-designated area on the front of the silicon support layer 3, and a pattern is formed by using photoetching and etching, and the metal pattern is alloyed to form an ambient resistor, wherein the non-designated area is located on both sides of the designated area;

[0054] A passivation protective layer 7 is deposited on the metal layer 6, and a PAD window (i.e. a pin window) is formed by photoetching and etching;

[0055] The middle area of the layer of silicon nitride 2 on the back of the substrate 1 is photoetched and etched to expose the substrate 1, and the substrate 1 is etched from the back to form a back cavity 8, and the etching is stopped at the bottom of the layer of silicon nitride 2 on the front of the substrate 1, thereby forming a suspended membrane structure.

[0056] In this embodiment, the silicon support layer 3 is set by using silicon epitaxial growth, the thickness of the entire suspended membrane is increased, the mechanical strength of the sensor chip is improved, and the deformation or rupture of the membrane area caused by long-term use can be effectively prevented. At the same time, the thermal insulation performance of the membrane is further improved by setting the silicon support layer 3, and the heat generated by the resistor formed by the metal layer 6 is prevented from being transferred to the substrate 1.

[0057] Therefore, this embodiment also derives a MEMS thermal film type flow sensor chip prepared by the above preparation method, which is shown in the last figure of Figure 1 and includes:

[0058] The substrate 1 has a back cavity 8 formed by etching in the middle of the back, and support ends are formed at both ends of the back cavity 8;

[0059] The layer of silicon nitride 2 is deposited on the front and back of the substrate 1, and is used as a protective layer of the substrate 1. At the same time, the layer of silicon nitride 2 on the front is used as a self-stopping layer of the cavity bottom when the back cavity is formed by etching, and is also used as a thermal insulation layer of the metal layer 6. The layer of silicon nitride 2 on the back is used as a protective layer of the cavity opening at both ends when the back cavity 8 is formed by etching. The functions of the layer of silicon nitride 2 are as follows:

[0060] A silicon support layer 3 is grown on the nitride silicon layer 2 on the front surface by a silicon epitaxial growth method, and is used as a thermal insulation layer for the metal layer 6 and for enhancing the structural strength of the suspended membrane;

[0061] A silicon oxide layer 5 is formed on a designated area of the silicon support layer 3 by a thermal oxidation method, and the designated area is located in the middle of the silicon support layer 3;

[0062] A metal layer 6 is formed on the silicon oxide layer 5 (i.e. the designated area of the silicon support layer 3) and the silicon support layer 3 (i.e. the non-designated area of the silicon support layer 3) by sputtering and etching to form a pattern, and is alloyed to form corresponding resistors, the resistors formed on the silicon oxide layer 5 include a heating resistor and bridge resistors located upstream and downstream of the heating resistor, and the resistors formed on the non-designated area of the silicon support layer 3 include an ambient resistor, and in operation, the metal resistors change due to changes in the temperature field, thereby detecting the flow rate.

[0063] A passivation protective layer 7 is formed on the metal layer 6 to protect the metal layer 7.

[0064] Specifically, the thickness of the silicon support layer 3 is preferably 5-10 μm.

[0065] Specifically, the material of the metal layer can be a Pt (platinum) thermal resistance material.

[0066] Specifically, the etching process uses a KOH solution.

[0067] Specifically, the silicon support layer 3 is formed on the nitride silicon layer 2 on the front surface of the substrate 1 by a silicon epitaxial growth method, and includes the following steps:

[0068] The surface of the silicon support layer to be grown (i.e. the surface of the nitride silicon layer 2) is cleaned to remove impurities;

[0069] High-temperature annealing is performed in a hydrogen environment to remove oxides on the surface of the silicon support layer to be grown;

[0070] The surface of the silicon support layer to be grown is preheated in an epitaxial furnace, and the preheating temperature is 1000°C;

[0071] A certain flow of reaction gas and carrier gas is introduced into the furnace, and silicon epitaxial growth is performed under a certain pressure;

[0072] The temperature of the epitaxial furnace is gradually reduced to room temperature, and then the epitaxial furnace is removed for cooling.

[0073] Specifically, the reaction gas is a silicon source gas SiH4, and the flow is 50 sccm; the carrier gas is H2, and the flow is 2000 sccm; during epitaxial growth, the temperature is 800°C, the pressure in the furnace is 50 Torr, and the epitaxial growth rate is controlled at 10 nm / min.

[0074] Specifically, the substrate 1 can be a silicon substrate, and a silicon nitride layer 2 can be grown on the silicon substrate by a low pressure chemical vapor deposition method.

[0075] Specifically, after the metal patterning, an alloying treatment is performed on the metal at a temperature greater than 400°C (preferably 450°C) by a furnace tube process.

[0076] Referring to Figure 2 In the formed groove array, the grooves are equally spaced, the cross section of each groove is a trapezoid with the upper side larger than the lower side, and the bottom angle β of the trapezoid close to the upper side is 86°-89°, so as to facilitate the subsequent thermal oxidation filling process.

[0077] The maximum distance between the adjacent two grooves is less than or equal to 1 μm, the maximum width of each groove is not greater than 1 μm, and the depth of the groove is the thickness of the silicon support layer; the projection plane (i.e. the perspective from the top of the groove downward) of the groove is a polygon, a circle or a special shape, which can be adjusted according to actual needs. Figure 2

[0078] Referring to Figure 3 , the groove array is filled by thermal oxidation, including:

[0079] Based on the silicon between the adjacent grooves 4 in the groove array, thermal oxidation is performed along the surface of the base, so that the silicon between the adjacent grooves is continuously consumed and the grooves are continuously filled until they are all changed into silicon oxide layers.

[0080] Since the thermal oxidation method needs to consume a certain proportion of silicon during the entire reaction process, the silicon between the grooves will be continuously consumed until it is all changed into silicon oxide layers. At the same time, as the thickness of the silicon oxide on the sidewall of the groove gradually increases, the groove will eventually be filled and sealed.

[0081] As Figure 3 shown, in order to verify the rationality and effectiveness of the setting of the bottom angle β of 86°-89° in the embodiment, a comparative example is set, and the thermal oxidation process of the comparative example is the same as that of the embodiment, and the difference is that the bottom angle β of the trapezoid close to the upper side of the groove is greater than or equal to 90°. As can be seen from the figure, since the oxidation reaction at the top of the groove is fast, the oxidation layer at the top of the groove will be sealed first, while the oxidation reaction at the bottom of the groove is slow and cannot be filled in time, resulting in a gap 10. At the same time, since the top of the groove is sealed, the reaction gas cannot enter the bottom of the groove, so that the sidewall of the groove bottom (the groove spacing) cannot continue to occur oxidation reaction, resulting in a silicon island 20 at the bottom of the groove. The presence of the silicon island 20 will affect the thermal insulation effect of the silicon oxide layer. Therefore, the control of β is very important.

[0082] The thermal oxidation of the embodiment specifically adopts a high-temperature furnace tube process, and the growth rate is generally slow. Therefore, the groove spacing of the embodiment is set to be within 1 μm, so as to effectively control the production efficiency, capacity and cost of the process. ​

[0083] The passivation protective layer 7 is single-layer or multi-layer structure.

[0084] The material of the passivation protective layer 7 is one of silicon oxide, silicon nitride, silicon oxynitride or a composite of several of them, or a polyimide material.

[0085] The preparation method of the embodiment further comprises polishing the surface of the silicon oxide layer to planarize it before sputtering the metal layer on the silicon oxide layer.

[0086] The surface planarization is achieved by removing the surface oxide layer by chemical mechanical polishing (CMP), which eliminates the stress concentration problem caused by the uneven surface or high and low step problem, and further improves the reliability of the device.

[0087] The embodiment also provides a MEMS hot-film flow sensor chip prepared by the preparation method.

[0088] The preparation method of the present application improves the mechanical strength of the sensor chip, which can effectively prevent the deformation or rupture of the membrane area caused by long-term use. The prepared sensor membrane area not only has high support strength but also has precise thickness.

[0089] Those skilled in the art can understand that the above description is only preferred embodiments of the present application and is not used to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method of fabricating a MEMS hot-film flow sensor chip, characterized by, The application relates to a method for manufacturing a micro-bridge resistor, which comprises the following steps: A silicon nitride layer is arranged on the front and back surfaces of a substrate; A silicon support layer is formed on the silicon nitride layer on the front surface of the substrate by using a silicon epitaxial growth method; A groove array is formed on the front surface of the silicon support layer in a specified area by using photoetching and etching, and the bottom surface of the groove is connected to the silicon nitride layer; The groove array is filled with oxidation by using a thermal oxidation method, and the specified area is entirely oxidized to form a silicon oxide layer; A metal layer is sputtered on the silicon oxide layer, and a pattern is formed on the metal layer by using photoetching and etching; the metal pattern is alloyed to form an upstream bridge resistor, a downstream bridge resistor and a heating resistor; a metal layer is sputtered on the non-specified area on the front surface of the silicon support layer, a pattern is formed on the metal layer by using photoetching and etching, and the metal pattern is alloyed to form an ambient resistor; the non-specified area is located on both sides of the specified area; A passivation protective layer is deposited on the metal layer, and a PAD window is formed by photoetching and etching; The middle area of the silicon nitride layer on the back surface of the substrate is photoetched and etched to expose the substrate, the substrate is corroded from the back surface, a back cavity is formed, and the corrosion is stopped at the bottom of the silicon nitride layer on the front surface of the substrate, so that a suspended membrane structure is formed; In the groove array, the grooves are distributed at equal intervals, the cross section of each groove is in the shape of a trapezoid with the upper side being larger than the lower side, and the bottom angle of the trapezoid close to the upper side is 86-89 degrees; the maximum interval between two adjacent grooves is less than or equal to 1 mu m, the maximum width of each groove is not greater than 1 mu m, and the depth of the groove is the thickness of the silicon support layer; the projection surface of the groove is in the shape of a polygon, a circle or a special shape.

2. The method of claim 1, wherein the MEMS hot-film flow sensor chip is prepared by the steps of: The method for forming the silicon support layer on the silicon nitride layer on the front surface of the substrate by using a silicon epitaxial growth method comprises the following steps: The surface of the silicon support layer to be grown is cleaned to remove impurities; The surface of the silicon support layer to be grown is annealed at high temperature in a hydrogen environment to remove oxides; The surface of the silicon support layer to be grown is preheated in an epitaxial furnace, and the preheating temperature is 1000 DEG C; A certain flow of reaction gas and carrier gas is introduced into the furnace, and silicon epitaxial growth is carried out under a certain pressure; The temperature of the epitaxial furnace is gradually reduced to room temperature, and then the epitaxial furnace is taken out and cooled.

3. The method for fabricating a MEMS hot-film flow sensor chip according to claim 2, characterized in that, The reaction gas is a silicon source gas SiH4, and the flow is 50 sccm; the carrier gas is H2, and the flow is 2000 sccm; during the epitaxial growth, the temperature is 800 DEG C, the pressure in the furnace is 50 Torr, and the epitaxial growth rate is controlled at 10 nm / min.

4. The method for fabricating a MEMS hot-film flow sensor chip according to claim 1, characterized in that, The thickness of the silicon support layer is 5-10 mu m; The substrate is a silicon substrate, and the silicon nitride layer is grown on the silicon substrate by using a low-pressure chemical vapor deposition method.

5. The method for fabricating a MEMS hot-film flow sensor chip according to claim 1, characterized in that, The method for filling the groove array with oxidation by using a thermal oxidation method comprises the following steps: Based on the silicon between the adjacent grooves in the groove array, the surface of the base is continuously oxidized, so that the silicon between the adjacent grooves is continuously consumed, and the grooves are continuously filled until the silicon is completely changed into a silicon oxide layer.

6. The method of claim 1, wherein the MEMS hot-film flow sensor chip is prepared by the steps of: After the metal patterning, the metal is alloyed at a temperature greater than 400 DEG C by using a furnace tube process.

7. The method for fabricating a MEMS hot-film flow sensor chip according to claim 1, characterized in that, The passivation protective layer is a single-layer or multi-layer structure; The material of the passivation protective layer is one or a composite of several of silicon oxide, silicon nitride and silicon oxynitride, or is a polyimide material.

8. The method for fabricating a MEMS hot-film flow sensor chip according to claim 1, characterized in that, Before sputtering the metal layer on the silicon oxide layer, the method further comprises: polishing the surface of the silicon oxide layer to planarize it by using a chemical mechanical polishing process.

9. A MEMS hot-film flow sensor chip, characterized by The preparation method according to any one of claims 1-8, comprising: a substrate, a back cavity is formed in the middle of the back surface by etching, and support ends are formed at both ends of the back cavity; a silicon nitride layer deposited on the front surface and the back surface of the substrate, the silicon nitride layer on the front surface serving as a self-stopping layer for the bottom of the back cavity when the back cavity is formed by etching and as a thermal insulation layer for the metal layer, and the silicon nitride layer on the back surface serving as a protective layer for both ends of the opening of the back cavity when the back cavity is formed by etching; a silicon support layer grown on the silicon nitride layer on the front surface by a silicon epitaxial growth method, serving as a thermal insulation layer for the metal layer and for enhancing the structural strength of the suspended membrane; a silicon oxide layer formed on a designated area of the silicon support layer by a thermal oxidation method, the designated area being located in the middle of the silicon support layer; a metal layer formed on the silicon oxide layer and the silicon support layer by sputtering, etching, patterning, and alloying treatment to form a corresponding resistor; a passivation protective layer formed on the metal layer to protect the metal layer.

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