RU2137249C1 - Process of manufacture of micromechanical instruments - Google Patents

Abstract

FIELD: manufacture of microgyros, microaccelerators, pressure microtransducers from silicon-carrying semiconductor structures. SUBSTANCE: process provides for manufacture of micromechanical instruments from silicon-carrying structure and involves application of film made from monocrystalline silicon-carrying material to substrate and its separation from substrate by dielectric layer which is followed by etching of sections of film to form measurement unit, removal of dielectric layer to release mobile elements of measurement unit and by formation of electric contacts for measurement means. Epitaxially grown semiconductor sandwich structure containing silicon carbide film isolated from substrate by layer of aluminium nitride is used in the capacity of starting material. Isolated testing section which linear dimensions take into consideration floating of semiconductor film of testing section during etching of dielectric layer located under it at moment of assured achievement of specified dimensions by left parts of manufactured article is formed additionally to establish moment of termination of operation of dielectric etching. EFFECT: enhanced mechanical, temperature and radiation stability of micromechanical instruments. 2 cl, 4 dwg, 3 tbl

Description

 The invention relates to microelectronics and relates to the manufacturing technology of micromechanical devices, in particular, microgyroscopes, microaccelerometers, pressure microsensors, from silicon-containing semiconductor structures.

 A known method of manufacturing micromechanical devices by explosive etching of a thin epitaxial film and subsequent epitaxial film growth on a substrate coated with an auxiliary layer, forming in the epitaxial film a pattern of elements of the instrument layer with open sides, covering these sides with a carrier layer, etching the auxiliary layer, separating the carrier layer with the instrument layer from the substrate, and then removing the carrier layer in the intermediate medium and transfer the instrument layer from the intermediate Reds on a stationary substrate (U.S. Patent N 5244818, H 01 L 31/18, 1993). In the manufacture of a movable microstructure, a temporary layer is applied to the substrate and parts of this layer are selectively removed, opening holes in the layer to create columns in the microstructure. A second material is deposited on the substrate and the temporary layer, from which the created microstructure is made by selective etching, followed by the removal of individual sections of the temporary layer. Next, a photosensitive polymer (FP) is deposited on the substrate, the temporary layer and the second layer, filling in this case the gaps in the second layer and the temporary layer. After selective removal of the AF regions, a certain amount of this polymer is left in the gaps of the temporary layer to form columns between the substrate and the second layer. Then the temporary layer and unused AF are removed (US patent N 5314572, H 01 L 21/306, 44 C 1/22, 29 C 37/00, C 03 C 15/00, 1994).

 These methods are complex and have low reliability due to the possibility of damage to the microstructure during numerous treatments, especially when the instrument layer is transferred from the initial substrate to a stationary one. In addition, there arises the problem of fixing the instrument layer on a stationary substrate.

 To increase the manufacturability of the production of micromechanical elements, a layer of aluminum nitride (AIN) is deposited on the substrate by sputtering an aluminum target in a vacuum chamber filled with nitrogen and reaction gas. Then, a dielectric layer is deposited on the aluminum nitride layer and, using photolithography, contact windows are opened in the dielectric layer to reveal the surface of aluminum nitride, which is used as a table-etching layer. In conclusion, remove the layer of aluminum nitride from the contact windows (US patent N 5270263, H 01 L 21/465, C 23 C 14/00, 1993).

 The disadvantage of this method is the impossibility of forming a conductive configuration of the formed instrument layer.

 There is also a known method of manufacturing micromechanical devices using an example of a semiconductor pressure sensor by epitaxial growth of a silicon film on a single crystal sapphire substrate, followed by the formation of a diffusion resistor on the film. Using a silicon film as a stop layer, selectively etching with heated phosphoric acid, removes the portion of the substrate that corresponds only to the diffusion resistor, thereby forming a membrane structure. As a mask for etching using a film of silicon dioxide (Japan patent N 5-10830, H 01 L 29/84, 1993).

 However, this method has low accuracy in reproducing the geometric dimensions of micromechanical elements formed by deep etching of sapphire. In addition, etching the substrate reduces the strength of the structure and requires sealing the formed cavities during assembly of the product.

 Closest to the claimed one is a method of manufacturing micromechanical devices using the example of manufacturing a microaccelerometer, which involves applying a film of conductive single-crystal silicon separated from a substrate by a dielectric layer onto a substrate, followed by etching of the film and dielectric layer to the substrate to form a measuring unit, creating electrical contacts for the means measuring and partially removing the dielectric layer to release the movable elements of the measuring unit (French Patent N 2700065, H 01 L 49/02, G 01 P 15/08, 15/125, 1994).

 However, devices manufactured by this method have low mechanical, temperature and radiation resistance.

 The objective of the proposed method is to increase the mechanical, temperature and radiation resistance of manufactured products.

 The solution to this problem lies in the fact that in the method of manufacturing micromechanical devices, which involves applying a film of a conductive single-crystal silicon-containing material to the substrate, separated from the substrate by a dielectric layer, followed by etching of the film sections to form the measuring unit, removing the dielectric layer to release the movable elements of the measuring unit and the creation of electrical contacts for measuring instruments, as a film from a conductive single-crystal of siliceous material is applied to the substrate the epitaxial film of silicon carbide (SiC), separated from the epitaxial layer dielectric substrate - aluminum nitride constituting the semiconductor sandwich structure.

 The causal relationship of the technological changes made with the improvement of the quality of manufactured products is that among a significant number of possible pairs of materials of conductive epitaxial films and epitaxial layers of dielectrics with high wear resistance, mechanical strength and close thermal expansion coefficients (which is important to avoid deformation of micromechanical elements ) a pair of "silicon carbide - aluminum nitride" has the best crystal chemical compatibility , which ensures reliable receipt of their sandwich structure.

 In the technical implementation of the proposed method, it is advisable to remove sections of aluminum nitride by etching in heated concentrated phosphoric acid. This etchant is the most selective when processing the used sandwich structure, does not cause gas evolution and does not form ballast compounds that distort the formed structures.

 When forming elements of a micromechanical device that require increased accuracy of reproducing structural elements, the most acceptable is a variant of the method that additionally provides for the formation of an isolated test section of the processed sandwich structure with linear dimensions based on the calculation of the emergence of a semiconductor film of the test section during etching of the section of the dielectric layer located below it at the moment guaranteeing the achievement of the specified dimensions of the formed elements of the device. At this point, the etching of the dielectric is stopped.

 This variant of the method is most acceptable when forming the support of the sensitive element of the microgyroscope, since insufficiently deep etching sharply reduces the sensitivity of measurements, and overexposure to the etching can lead to the separation of the sensitive element from the substrate.

 The AIN layer can be made with windows for contacting the required portions of the SiC layer of the formed microstructure directly with the substrate.

 In FIG. 1 shows a diagram of a micromechanical pressure sensor for example 1.

 In FIG. 2 shows a diagram of a micromechanical accelerometer for example 2.

 In FIG. 3 shows a diagram of a micromechanical gyroscope for example 3.

 In FIG. 4 is a diagram of an isolated test site for determining the etching time of an AIN layer, for example 3.

 The method is illustrated by the following examples.

Example 1. The manufacture of a micromechanical pressure sensor
Layers 2 of AIN and film 3 of SiC are deposited onto a substrate 1 (Fig. 1) of silicon of orientation (III) by sequential deposition in a magnetron sputtering apparatus at a constant discharge current of 0.65 A and a power of 210 W from a target with movable working zones. An aluminum plate is located in the first working zone, and a silicon carbide target in the second.

Precipitation is carried out at a 5% excess of Al and Si flows, respectively. Each layer is deposited for 30 minutes under vacuum at a substrate temperature in the range from 900 to 950 ^o C. In this case, first, AIN layer 2 is deposited on the substrate 1 from the first working zone of the target in a nitrogen-argon medium, then the nitrogen supply to the installation chamber is shut off and depositing the SiC film 3 from the second working zone of the target, etc. An SiC epitaxial sandwich structure (8 μm) on AIN (1 μm) is obtained.

A photoresistive mask is applied to the outer surface of the SiC film 3 by means of which through-holes 4 with a size of 3x3 μm are plasma-chemically etched in this film. Then the mask is removed by dissolving in dimethylformamide and the part of the AIN layer located in the window zone is etched using an aqueous KOH solution (33 wt.%) Heated to 75 ^° C, which enters the etching zone through windows 4 made in a SiC film. Etching of the evacuated membrane cavity 5 in the AIN layer 4 is carried out for 45 minutes. The reaction products and unreacted ingredients are removed by washing in deionized water and the treated structure is dried in an oven for 3 hours at 50 ^° C.

Next, on the outer surface of the SiC film 3, a 3 μm nickel layer 6 is applied by thermal spraying in vacuum. In this case, the substrate is placed at an angle of 45 ^° to the direction of flow of the sprayed metal to prevent it from entering the formed membrane cavity in the AIN layer through windows in the film. As a result of this operation, the membrane cavity 5 is sealed, while the applied sealing conductive layer 6, in contact with the flexible SiC membrane formed above the cavity 5, serves as one of the plates of the formed membrane capacitive pressure sensor. The second contact 7 is sprayed externally onto the silicon substrate 1, which serves as another lining of the capacitive sensor. The contacts are connected to the input of a capacitance meter calibrated in pressure units. The pressure supplied from above the sensor is perceived by the membrane cavity, changing the capacity of the sensor by changing the distance between its plates. The resulting microsensor has the following technical characteristics:
- range of measured pressures - from 0 to 500 kPa;
- the error is 0.5%.

For comparison, micromechanical pressure sensors are made, including a SiC film separated from the substrate by an AIN layer, where a BV-100-1 high-temperature oxide-beryllium ceramic plate is used as the substrate (to control the significance of the sign of epitaxial structure, since epitaxial deposition is impossible on a ceramic substrate) . In addition, sensors are made from the epitaxial structures SiC / Al ₂ O ₃ / Si, Si / AlN / Al ₂ O ₃ (French patent N 2700065) and non-epitaxial structure SiO ₂ / AlN / Si (US patent N 5270263).

 Take into account the percentage of manufacturing suitable products in relation to the geometric parameters of the shape and dimensions and evacuation of the membrane cavity (under a microscope), the drift of the electrical characteristics and the sensitivity of the measurement.

The suitable sensors are subjected to a thermal test by holding at a temperature of 700 ^o C for 3 hours and radiation processing at a rate of 1.4 • 10 ¹⁵ neutrons / cm ² and reveal the percentage of storage of suitable products in relation to the above characteristics. The results according to 20-300 tests are given in table. 1 (see the end of the description).

 As can be seen from the table, the yield of suitable micromechanical pressure sensors in the proposed method is 30%, and in the prototype - 27.7% (the difference is not statistically significant). At the same time, the sensors obtained by the proposed method have significantly higher operational characteristics, which is confirmed by 84% preservation of their operability after heat treatment against 37% in the prototype method and 95% preservation of operability against 48% after radiation treatment. The achieved values of these quality indicators also exceed the corresponding values of other methods.

Example 2. The manufacture of a micromechanical accelerometer
A layer 2 of aluminum nitride and a silicon carbide film 3 are epitaxially applied to the sapphire substrate 1 (Fig. 2), in which the through technological windows 4 are etched, as in Example 1. At the same time, the external contour of the flexible device mounted element is etched in the film 3, according to the drawing of the photoresist mask. Then, in the film 3, in the zone of maximum bending stresses (located near the attachment point of the formed hinged element), create a strain-resistant bridge circuit 5 by ion implantation of boron followed by sputtering of nickel-titanium contacts 6 for attachment to the measuring transducer. Then, through the windows 4 etched in the film 3 and the contour of the movable element being formed, part of the AlN layer 2 is etched to form a beam structure of the movable element with a cavity 7.

Etching of sections of the AlN layer in various batches of manufactured devices with volumes from 50 to 200 products is carried out for 30-120 minutes at a temperature of 60 to 95 ^o C using the following reagents: concentrated phosphoric, hydrofluoric and nitric acids, KOH (33% - solution), hydrogen peroxide. Comparative results of their use are given in table. 2 (see the end of the description). As can be seen from the table, the etching of sections of the AlN layer with heated concentrated phosphoric acid provides the maximum yield (32%). The remaining tested reagents damage either the sapphire substrate 1 or the sensitive elements of the device, which sharply reduces the yield of suitable products. In this case, the hydrofluoric acid layer AlN practically does not poison.

 Products are sealed in a protective ceramic case.

 Under the action of acceleration applied perpendicular to the substrate 1, the movable element bends, which is fixed using a strain gauge circuit 5.

Technical characteristics of manufactured micromechanical accelerometers:
- measurement range - from 0 to 100 m / s ² ;
- error - 0.5% of the measuring range.

 As a result of thermal and radiation tests of suitable products in the modes of example 1, 83 and 89% of devices, respectively, remain operational.

Example 3. The manufacture of a micromechanical gyroscope
On a sapphire substrate 1 (Fig. 3), metallized with plates 2, which also serve as current collection elements, an aluminum nitride layer 3 and a silicon carbide film 4 are epitaxially applied, in which the through technological windows 5 are etched, as in example 1. In this case, in the film 4 the external contour of the formed suspension sensitive element of the microgyroscope is also etched, according to the drawing of the photoresist mask, which provides for the formation of a capacitive bridge between the plates of the movable element and the fixed base. Then, through the windows 5 etched in the film 4 and the contour of the movable element being formed, part of the AIN layer 3 is etched to form the structure of the movable element with the cavity 6 and the central support formed by the non-etched portion of the layer 3.

 Due to the technological difficulty of strictly adhering to the established shape and dimensions of the support of the sensitive element of the microgyroscope, an isolated test section (Fig. 4) with linear dimensions is additionally formed with the linear dimensions based on the emergence of a semiconductor film of the test section during etching located under dielectric layer at the moment, guaranteeing the achievement of the specified dimensions of the support. To this end, test objects are formed in the form of blocks with square (Fig. 4a) and round (Fig. 4b) bases on two free sections of the initial structure. The feasibility of performing a test object in the form of a block of geometric figures consists in visualizing its ascent and averaging the time of separation from the substrate.

 Preliminary determine the characteristic dimensions of the bases A (side of the square or diameter of the round base). In this example, A = 50 μm is optimal (Table 3), providing a maximum yield of suitable products (24%). At A <50 μm, under-etching of the support section occurs, and at A> 50 μm, AlN etching occurs in this area, up to the termination of the sensitive element.

 Etching of sections of the AlN layer is stopped at the time of surfacing of the test section, which is detected by the color change at the point of separation of the film, since the opened portion of the sapphire substrate differs from the dark SiC film.

 The device is sealed, as in example 2.

 When the angular velocity along the roll and / or pitch changes, the Coriolis forces acting on the plates of the moving element vibrating in the plane of the substrate change, which leads to a change in the ratio of capacities in different shoulders of the formed bridge and is taken into account by the transducer connected to the plates.

Technical characteristics of the manufactured micromechanical gyroscope:
- measuring range of angular velocity - from 0 to 100 ^o / s;
- accuracy - 1%.

 As a result of thermal and radiation tests of suitable products in the modes of example 1, 82 and 92% of devices retain their operability, respectively, against 30 and 33% in the prototype method.

From the above examples it can be seen that the proposed method in comparison with the known ones allows to increase the manufacturing accuracy and strength of micromechanical devices, which allows them to be operated in extreme conditions characterized by high temperatures (700 ^o C) and radiation (1.4 • 10 ¹⁵ neutrons / cm ² ).

Claims (3)

 1. A method of manufacturing micromechanical devices from a silicon-containing semiconductor structure, comprising applying to the substrate a film of a conductive single-crystal silicon-containing material separated from the substrate by a dielectric layer, followed by etching portions of the film to form the measuring unit, removing portions of the dielectric layer to release the movable elements of the measuring unit and creating electrical contacts for measuring instruments, characterized in that as a film and From a conductive single-crystal silicon-containing material, an epitaxial film of silicon carbide separated from the substrate by an epitaxial dielectric layer of aluminum nitride, forming a semiconductor sandwich structure, is applied to the substrate.  2. The method according to claim 1, characterized in that the removal of sections of aluminum nitride is carried out by etching in heated concentrated phosphoric acid.  3. The method according to claim 1 or 2, characterized in that to establish the end of the operation to remove portions of the dielectric layer in the semiconductor film, an isolated test section with linear dimensions is additionally formed based on the calculation of the surfacing of the semiconductor film of the test section when the dielectric layer located underneath is removed at the moment , guaranteeing the achievement of the specified dimensions of the remaining parts of the manufactured product.

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