RU2832493C1 - Method of manufacturing mems micromirror matrix - Google Patents

Abstract

FIELD: microelectromechanical devices.

SUBSTANCE: invention relates to methods of manufacturing mobile MEMS devices. Method of manufacturing a MEMS micromirror matrix involves deposition of a metal layer of torsions, photolithography and etching of the layer of torsions, deposition of a reflecting layer, lithography and etching of the reflecting layer, etching of the second and first separation sacrificial layers. Before forming a layer of torsions, a layer of aluminium nitride is deposited on a substrate with layers of metal wiring, including the lower electrodes of the matrix and interlayer insulation from silicon oxide, photolithography and etching of contact windows in the layer of aluminium nitride, plasma-chemical deposition of silicon nitride layer, which is the first separating sacrificial layer, photolithography and etching of contact windows in the layer of silicon nitride and after formation of the topology of the layer of torsions, plasma-chemical deposition of the second separating sacrificial layer of silicon nitride, photolithography and etching of contact windows in the second layer of silicon nitride.

EFFECT: improved geometry of MEMS micromirrors and increased efficiency of their manufacturing.

1 cl, 9 dwg, 1 tbl

Description

The field of application of the proposed method is the production of mobile MEMS devices, including those used in projectors, 3D printers, laser decapsulation units, multi-object spectrometers, teleorientation devices, projection lithography units and maskless lithography units, including X-ray and extreme ultraviolet lithography.

A matrix is an ordered set of MEMS elements organized in a rectangular array [1].

Most MEMS manufacturing routes require very specific equipment for their implementation. The working layers of MEMS devices should not be etched in the same place where the sacrificial layers are etched. For this, they must have high selectivity for this etching or they must be covered with a resistant material.

A method is known based on the use of polycrystalline or amorphous silicon [2]. This is a method for forming a micromechanical device. Polycrystalline or amorphous silicon films are used as sacrificial layers, and metal films are used as structural layers. The sacrificial layers are etched using dry gas-phase etching in xenon difluoride vapor. The method includes applying a protective layer to the substrate, applying a first separating sacrificial layer, photolithography and etching of contact windows in the first sacrificial layer, applying the first structural layer to the sacrificial layer and into the contact windows, photolithography and etching of the first structural layer, including etching of areas intended for the future torsion. Then the second structural layer is deposited, photolithography and etching of the second structural layer is performed, as a result of which torsions are formed. Next, the second sacrificial layer is deposited, photolithography and etching of contact windows in the second sacrificial layer is performed. Next, the third structural layer is deposited, photolithography and etching of the second structural layer are performed, resulting in the formation of reflective elements. After this, the sacrificial layers are etched.

The known method has the following disadvantages. The temperatures of chemical vapor deposition processes of polycrystalline or amorphous silicon are more than 550°C. Such a temperature has a significant negative effect on the wiring metal and metal-semiconductor contacts. Also, polycrystalline silicon has a characteristic developed relief of crystallites on the surface and the layer sprayed on it will repeat this relief, which worsens the perfection of the geometry and the uniformity of the mechanical characteristics between the MEMS elements. Also, the developed morphology of polysilicon, transferred to the reflective layer of metal, will increase the scattering of light, which is undesirable. The developed morphology of polycrystalline silicon can be eliminated using chemical-mechanical polishing, then this will require a significant complication and increase in the cost of the production process.

A method for manufacturing micromirrors is known [3]. A micromirror device in which the micromirror is suspended on a torsion suspension and is tilted by means of a progressive connection to a vertical comb electrostatic actuator located under the surface of the micromirror. The progressive linkage system includes a pair of torsion springs that are connected together and operate similarly to a four-link spring-hinged lever system. The progressive connection provides a nonlinear spring stiffness, which allows the micromirror to be tilted to any angle within its range, with virtually no electrostatic instability or hysteresis behavior. A layer of silicon nitride is deposited on the oxidized silicon wafer, then the first structural layer of doped polycrystalline silicon is deposited, photolithography and etching of the first layer of polycrystalline silicon is performed, then the first separating sacrificial layer of silicon oxide is deposited and its planarization is performed by chemical-mechanical polishing, then photolithography and etching of silicon oxide is performed to form windows to the first layer of polysilicon, then the second layer of polysilicon with a thickness of 1 μm is deposited and photolithography and etching of the second layer of polysilicon is performed, torsions, supports and comb actuators are formed in this layer, then the second separating sacrificial layer of silicon oxide with a thickness of 0.3 μm is deposited, photolithography and etching of silicon oxide are performed to form windows to the second layer of polysilicon, then the third layer of polysilicon is deposited and so on. In total, the design contains five layers of doped polysilicon and four sacrificial layers of silicon oxide. Sacrificial layers are etched in a hydrofluoric acid solution, and 0.2 μm of metal is sprayed onto the formed micromirrors through a stencil to improve their reflectivity. The described method has the following disadvantages. This technology involves high-temperature annealing processes for polysilicon (1100°C), chemical vapor deposition at low pressure for polysilicon (580°C) and silicon oxide (750°C), which makes it impossible to use it on wafers with transistor structures and aluminum metal wiring. Liquid chemical etching in a hydrofluoric acid solution for 12 hours is also used for etching, which makes it impossible to use standard metal wiring, except for gold. The process of vacuum deposition through a stencil has significant drawbacks: difficulty in ensuring tight uniform adhesion of the stencil to the plate, the plate at the time of deposition is supposed to have suspended MEMS structures that can easily be damaged when touched by the stencil, therefore, a gap must be provided between the plate and the stencil. The larger the gap between the plate and the stencil, the stronger the dusting in the geometric shadow area. Near the boundary of the geometric shadow area of the mask, the thickness of the deposited material smoothly decreases, thus obtaining a significant unevenness of the thickness. In addition, the minimum dimensions of the elements on the stencils are strongly limited, thus limiting the miniaturization of the MEMS device manufactured using this technology. Polycrystalline silicon has a characteristic morphology, which at a thickness of 2.25 μm will provide high roughness of the micromirror surface and high light scattering. The closest, in technical essence, is a method for manufacturing a digital micromirror device and a method for manufacturing a projection display system using it, adopted as a prototype [4]. A method for manufacturing a digital micromirror device, comprising a set of material layers, including a separating sacrificial layer made of photoresist, including several holes, a control circuit located in a semiconductor substrate, a torsion material layer made of aluminum alloy, located above the separating sacrificial layer and inside the holes, and a torsion support layer made of silicon oxide, located above the torsion layer and inside the holes, forming the topology of the torsion layer on top of the torsion support layer using photoresist, forming the topology of the torsion layer using a torsion support layer etched along a photoresist mask as a hard mask, removing the torsion support layer from the upper surfaces of the etched torsion layer (while it remains on the side surfaces in the contact windows). In the second separating sacrificial layer formed above the torsion layer, windows are formed and above it and in the contact windows a layer of reflective material is formed, after the pattern is formed in the reflective layer, the first and second sacrificial layers are etched, as a result of which the structure is released and becomes movable. This method has the following disadvantages. Photoresist layers are used as sacrificial layers. This excludes the possibility of carrying out liquid chemical treatments, for example, in dimethylformamide, after the formation of the first sacrificial layer. Since the crystals of MEMS products have a large area, the exclusion of liquid treatments from the technological route at the final part of the route, including three photolithographies and four plasma-chemical etchings, sharply increases the requirements for electrovacuum hygiene in production and the introduced defects in each of the processes. Since the movement of MEMS devices manufactured using this technology uses electrostatics, and therefore high voltages, the presence of defects in the form of insulation punctures and short circuits of metal elements is extremely significant. If it is necessary to form, for example, a layer of a multilayer reflective coating for extreme ultraviolet on the reflective layer, then it may be necessary to carry out chemical-mechanical polishing and spraying, combined with ion-beam etching and heating of the substrate, which is impossible if there is a photoresist as a material for sacrificial layers on the plate. Also, to form a reflective layer, it may be necessary to use atomic layer deposition, and even the plasma-activated atomic layer deposition process has an operating temperature of more than 250 ° C, which makes deposition on plates with a photoresist impossible.

The objective of the invention is to improve the geometry of MEMS micromirrors and the efficiency of their manufacture.

The solution to the problem is a method for manufacturing a MEMS micromirror matrix, including deposition of a metal torsion layer, photolithography and etching of the torsion layer, deposition of a reflective layer, lithography and etching of the reflective layer, etching of the second and first separating sacrificial layers, characterized in that before forming the torsion layer on a substrate with layers of metal wiring, forming, among other things, the lower electrodes of the matrix and interlayer insulation made of silicon oxide, a layer of aluminum nitride is deposited, photolithography and etching of contact windows in the aluminum nitride layer are performed, plasma-chemical deposition of a silicon nitride layer, which is the first separating sacrificial layer, photolithography and etching of contact windows in the silicon nitride layer are performed, and after forming the torsion layer topology, plasma-chemical deposition of the second separating sacrificial layer of silicon nitride, photolithography and etching of contact windows in the second layer are performed. silicon nitride.

The task is solved by using silicon nitride as a sacrificial layer instead of photoresist layers. Instead of a sacrificial layer with a specially processed photoresist (exposure to a large dose of deep ultraviolet and annealing), plasma-chemically deposited silicon nitride is used. Due to this, the problem of insufficient temperature stability of the photoresist at temperatures above 150°C required for many technological operations is solved. The temperature stability of silicon nitride ceases to be a limiting factor. In terms of limiting the operating temperatures of the processes, the main limiting factor is metallization. Thus, temperatures up to 500°C are permissible for aluminum wiring. Thus, the maximum permissible temperature of the processes after deposition of the first sacrificial layer is significantly increased. Silicon nitride is stable at temperatures of plasma-chemical deposition processes, which usually do not exceed 350°C, and temperatures of atomic layer deposition, which usually do not exceed 450°C. This makes it possible to carry out plasma-enhanced chemical vapor deposition, plasma-enhanced atomic layer deposition, and chemical deposition of thin films of substances onto a substrate from the gas phase. Improvement of the MEMS micromirror geometry is achieved by chemical-mechanical polishing (CMP), since silicon nitride is a solid dielectric, technologically acceptable for chemical-mechanical polishing operations and subsequent washing with an abrasive suspension with abrasion products. The use of chemical-mechanical polishing of the upper sacrificial layer makes it possible to improve the geometry of the reflective layer of MEMS micromirrors and the geometry of MEMS micromirrors as a whole. The efficiency of MEMS micromirrors manufacturing is improved due to the fact that silicon nitride has high chemical resistance and is insoluble in organic solvents, which makes it possible to use liquid chemical processing operations in organic solvents and removers, which significantly reduces defects, increases the percentage of good products, improves layer adhesion, reduces the likelihood of metal corrosion due to residual products of plasma-chemical etching, reduces the requirements for the cleanliness class of production facilities and for the introduced defects of processes. In addition, the formation of movable MEMS elements on substrates with transistor structures and standard metal wiring is possible due to the fact that silicon nitride is a standard dielectric in silicon technology and can be plasma-chemically deposited after the formation of metal wiring layers. Due to this, the possibility of forming movable MEMS elements on substrates with transistor structures and standard metal wiring is ensured. The method for manufacturing a matrix of MEMS micromirrors is explained in Figs. 1-9: Fig. 1 - shows sections of structures in operations of the technological route with sacrificial layers of silicon nitride, where:

1 - original plate,

2 - plasma-chemical deposition of silicon oxide,

3 - aluminum spraying,

4 - application of photoresist,

5 - projection photolithography (first layer of layout),

6 - reactive ion etching of metal of the layout 1,

7 - removing photoresist,

8 - plasma-chemical deposition of silicon oxide,

9 - application of photoresist,

10 - projection photolithography (contact windows between the first and second layers of the layout),

11 - reactive ion etching of SiO2 interlayer insulation,

12 - removal of photoresist,

13 - aluminum spraying,

14 - application of photoresist,

15 - projection photolithography (second layer of layout),

16 - reactive ion etching of metal layout 2,

17 - removal of photoresist,

18 - plasma deposition of silicon oxide,

19 - chemical-mechanical polishing,

20 - aluminum nitride spraying,

21 - application of photoresist,

22 - photolithography (contact windows from the layout to the torsion),

23 - reactive ion etching of aluminum nitride in chlorine plasma,

24 - reactive ion etching of silicon oxide in fluorine plasma,

25 - removal of photoresist,

26 - plasma-chemical deposition of silicon nitride (the first separating sacrificial layer),

27 - application of photoresist,

28 - projection photolithography (contacts from the layout to the torsion),

29 - reactive ion etching of silicon nitride,

30 - removal of photoresist,

31 - spraying of aluminum alloy (torsion bar material),

32 - application of photoresist,

33 - projection photolithography (torsion layer),

34 - reactive ion etching of torsion material in chlorine plasma,

35 - removal of photoresist,

36 - plasma-chemical deposition of silicon nitride (second separating sacrificial layer),

37 - projection photolithography (contacts from the layout to the torsion),

38 - reactive ion etching of silicon nitride,

39 - removal of photoresist,

40 - aluminum sputtering (reflective layer material),

41 - application of photoresist,

42 - projection photolithography (mirror layer),

43 - reactive ion etching of torsion material in chlorine plasma,

44 - removal of photoresist,

45 - plasma-chemical etching of silicon nitride in isotropic mode in SF6 (etching of the sacrificial layer).

Fig. 2 - shows the thicknesses of the layers shown in Fig. 1:

H0 - silicon wafer,

H1 - insulating dielectric 1,

H2 - metal wiring 1,

H3 - insulating dielectric 2,

H4 - metal wiring 2,

H5 - insulating dielectric 3,

H6 - sacrificial layer 1,

H7 - metal torsion bar,

H8 - sacrificial layer 2,

H9 - metal mirror,

H10 - positive photoresist.

Fig. 3 - an example of the topology of the first layer of metal wiring corresponds to the top view of Fig. 1, item 7,

Fig. 4 - an example of the topology of contact windows from the first layer of the layout to the second corresponds to the top view of Fig. 1, item 12,

Fig. 5 - an example of the topology of the second layer of metal wiring corresponds to the top view of Fig. 1, item 17,

Fig. 6 - an example of the topology of the contact window layer from the second metal layer to the torsion layer corresponds to the top view of Fig. 1, item 25,

Fig. 7 - an example of the torsion layer topology corresponds to the top view of Fig. 1, item 35,

Fig. 8 - example of the topology of the contact window layer from the torsion layer to the reflective layer

corresponds to the top view of Fig. 1, item 39,

Fig. 9 - an example of the topology of the reflective layer corresponds to the top view of Fig. 1, item 44.

The method for manufacturing a matrix of MEMS micromirrors is implemented as a result of the following technological operations:

on a plate with a metal layout, in the layer topology of which there are areas of pulling electrodes and areas for contacts to the layer of torsions and interlayer insulation made of silicon oxide, on which planarization is performed by chemical-mechanical polishing (Fig. 1, item 1-19), a layer of aluminum nitride is deposited by magnetron sputtering or atomic layer deposition (Fig. 1, item 20);

photolithography and reactive ion etching of contact windows in the aluminum nitride layer are performed, thereby opening the contacts from the upper layer of the wiring metal to the torsion layer formed further (Fig. 1, items 21-25); plasma-chemical deposition of a silicon nitride layer, which is the first separating sacrificial layer, is performed (Fig. 1, item 26); photolithography and etching of contact windows in the silicon nitride layer are performed, thereby opening the contacts from the upper layer of metal to the torsion layer formed further (Fig. 1, items 27-30);

the deposition of a metal layer of torsions is carried out, while the spraying is carried out on such equipment and by such a method that ensure high conformity of the sprayed film, that is, the proximity of thicknesses on horizontal and inclined surfaces (Fig. 1, item 31), due to which the dusty side walls of the contact windows after etching of the sacrificial layers will remain as supports for the suspended structure; photolithography and reactive ion etching of the metal are carried out to form the topology of the torsion layer (Fig. 1, items 32-35);

plasma-chemical deposition of a layer of silicon nitride is performed, which is the second separating sacrificial layer (Fig. 1, item 36);

photolithography and reactive ion etching of contact windows in the silicon nitride layer are performed, thereby opening contacts from the torsion metal to the reflective layer being formed further (Fig. 1, items 37-39); the reflective layer is deposited (Fig. 1, item 40); photolithography and etching are performed to form the topology of the reflective layer (Fig. 1, items 41-44);

the sacrificial layers of silicon nitride are etched by plasma-chemical etching in sulfur hexafluoride plasma to release the moving parts of the MEMS micromirror (Fig. 1, item 45). If it is required to form a layer with special reflective properties on the surface of the mirrors, for example, a multilayer Bragg mirror for extreme ultraviolet from layers of molybdenum and silicon, which are etched in a fluorine plasma, then these layers must be deposited after aluminum deposition (Fig. 1, item 40). After this, photolithography of the mirror layer must be performed as in (Fig. 1, item 42), but with a higher exposure dose. Next, reactive ion etching of the Bragg mirror layers and removal of the photoresist must be performed. After this, a protective coating must be deposited, for example, aluminum nitride, which will prevent etching of the layer during etching of the sacrificial layers in a fluorine plasma. After etching the sacrificial layers, the protective layer can be removed using ion beam etching.

Table 1 shows approximate characteristics and data of the layers.

The matrix structure in accordance with the claimed method of manufacturing a matrix of MEMS micromirrors is realized due to the presence of horizontal (Fig. 3) and vertical (Fig. 5) buses.

To test the method of manufacturing the MEMS micromirror matrix, a pilot batch was manufactured on 150 mm diameter wafers using projection photolithography with minimum dimensions of 0.7 μm. The laboratory studies confirmed the high technical characteristics of the reflectivity of the MEMS micromirror matrix and the possibilities of dynamic control. MEMS micromirrors have two stable states with minimal consumption in the mode of being in one of them. This allows reducing the power consumption and increasing the switching frequency, since the switching time depends only on the moment of inertia of the MEMS micromirror and the value of the control potential. Estimates show that for MEMS micromirrors with characteristic dimensions of about 20 μm, the switching time is about 10 μs. The claimed method for manufacturing a matrix of MEMS micromirrors allows improving the geometry of MEMS micromirrors by using chemical-mechanical polishing, since silicon nitride is a solid dielectric, technologically acceptable for chemical-mechanical polishing operations and subsequent washing with an abrasive suspension with abrasion products. This provides the ability to improve the geometry of the reflective layers and MEMS micromirrors as a whole. Improvement in the efficiency of manufacturing a matrix of MEMS micromirrors is ensured by reducing defects, increasing the percentage of good products, improving the adhesion of layers, reducing the likelihood of metal corrosion due to residual products of plasma-chemical etching and reducing the requirements for the cleanliness class of production facilities and for the introduced defects of processes.

Sources of information:

1. Gehner A, Doleschal W, Elgner A, Kauert R, Kunze D, Wildenhain M. Active-matrix-addressed micromirror array for wavefront correction in adaptive optics. In: Motamedi ME, Goering R, editors. Moems Miniaturized Syst. II 2001; vol. 4561:265-75.

2. US Patent No. 2001040675.

3. US Patent 7529016.

4. US Patent No. 7450297 - prototype.

Claims (1)

A method for manufacturing a MEMS micromirror matrix, comprising depositing a metal torsion layer, photolithography and etching the torsion layer, sputtering a reflective layer, lithography and etching the reflective layer, etching the second and first separating sacrificial layers, characterized in that before forming the torsion layer on a substrate with layers of metal wiring, including forming the lower electrodes of the matrix and interlayer insulation made of silicon oxide, a layer of aluminum nitride is deposited, photolithography and etching of contact windows in the aluminum nitride layer is performed, plasma-chemical deposition of a silicon nitride layer, which is the first separating sacrificial layer, photolithography and etching of contact windows in the silicon nitride layer is performed, and after forming the torsion layer topology, plasma-chemical deposition of a second separating sacrificial layer of silicon nitride, photolithography and etching of contact windows in the second silicon nitride layer are performed.