JP2010029976A - Micro structure formation process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a micro structure formation process capable of reducing a processing failure by inhibiting a dissipation of a mask when forming a structure having a high aspect ratio. <P>SOLUTION: The micro structure formation process to form a micro structure on a substrate made of silicon by dry etching using a plasma, includes the steps of forming: a first mask having an opening on the substrate; a first concavity by etching the substrate using the first mask as an etching mask; a second mask on the first mask after the first concavity is formed; and a second concavity deeper than the first concavity by further etching the first concavity in the thickness direction of the substrate using the second mask as the etching mask. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fine structure forming method for forming a fine structure on a semiconductor substrate by dry etching using plasma, and more particularly to a method for forming a high aspect fine structure having an aspect ratio of 50 or more.

In recent years, development and practical use of MEMS (Micro Electro Mechanical Systems), which are devices that integrate various functions such as mechanical, electronic, optical, and chemical using semiconductor microfabrication technology and the like, are progressing. In manufacturing the MEMS, it is required to process a semiconductor substrate such as silicon three-dimensionally. In particular, dry etching using plasma is required to form a structure having a high aspect ratio. As a dry etching method for forming a high aspect ratio, a method in which anisotropic etching called DRIE (Deep Reactive Ion Etching) (for example, Patent Document 1) is generally performed on a substrate on which a mask material such as a photoresist is formed. It is.
JP 7-503815 A

  However, when a structure having a high aspect ratio (for example, an aspect ratio of 50 or more) is formed, it becomes difficult for ions to reach the groove to be formed as etching progresses, and before a desired processing is performed. Then, part or all of the mask material disappears, and the pattern forming process cannot be continued.

  Although the durability can be improved by increasing the thickness of the photoresist used as a mask material, there is a limit to increasing the thickness of the mask due to the resolution limit of pattern processing by photolithography. On the other hand, it is conceivable to use a metal material having high resistance to dry etching as a mask material, but it is not easy to pattern the metal material with dimensional accuracy. May be damaged.

  In view of the above, an object of the present invention is to provide a fine structure forming method capable of reducing processing defects by suppressing disappearance of an etching mask when a structure having a high aspect ratio is formed by dry etching.

  A fine structure forming method according to the present invention is a fine structure forming method in which a fine structure is formed on a substrate made of silicon by dry etching utilizing plasma, and a first mask having an opening is formed on the substrate. Etching the substrate using the first mask as an etching mask to form a first recess, and forming the first recess and then forming a second mask on the first mask. And a step of further etching the first recess in the thickness direction of the substrate using the second mask as an etching mask to form a second recess deeper than the first recess. It is characterized by that.

  In the fine structure forming method according to the present invention, in the above, at least one of the etching in the first and second recess forming steps is an etching step in which the recess is formed by etching the substrate. The etching is characterized by alternately repeating a deposition step for forming a protective film on the side wall.

  The microstructure forming method according to the present invention is characterized in that, in the second mask forming step, a metal material is deposited from an oblique direction with respect to the substrate.

  The microstructure forming method according to the present invention is characterized in that, in the above, the second mask is formed on a side surface of the first mask and a side wall of the first recess.

  The microstructure forming method according to the present invention is characterized in that, in the above, the opening of the first mask is on the order of microns, and the aspect ratio (depth / opening width) of the first recess is 1 or more. To do.

  The microstructure forming method according to the present invention is characterized in that, in the above, the aspect ratio (depth / opening width) of the second recess is 50 or more.

  In the microstructure forming method according to the present invention, the mask material used for the second mask is selected from Ti, TiN, Cr, Al, Rh, Co, Fe, Mo, Ni, Ta, and Zr. It is characterized by being.

  In this specification, the micron order refers to a range of 0.1 μm to 100 μm. The aspect ratio refers to a value obtained by dividing the depth of the formed recess by the opening width of the recess.

  ADVANTAGE OF THE INVENTION According to this invention, when forming the structure which has a high aspect ratio, the loss | disappearance of a mask can be suppressed and the fine structure formation method which can reduce a process defect can be provided.

§1. DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is an explanatory view of the fine structure forming step of the present invention.

  First, a positive-type or negative-type photoresist is applied to the surface of the substrate 10 made of Si, which is an object to be etched, exposed through a photomask, and then developed to have a first mask 20 having an opening 25. Is formed (FIG. 1A). The latent image pattern may be drawn directly by drawing using an electron beam. The size of the opening is on the order of microns.

  Next, RIE (Reactive Ion Etching) or DRIE (Deep Reactive Ion Etching), which is dry etching using plasma, is performed on the opening 25 of the first mask 20 to etch the recesses by etching in the thickness direction of the substrate 10. Form.

Here, DRIE will be described. In DRIE, an etching process that digs while eroding the material layer in the thickness direction and a deposition process that forms a polymer wall on the side wall of the dug hole are alternately repeated every few seconds, and etching proceeds only in the approximate thickness direction. It becomes possible. In the case of etching Si, it is known that a fluorine supply etching gas is used as an etching gas and fluorocarbon is used as a deposition gas. In the present embodiment, SF ₆ gas is used in the etching process, and C ₄ F ₈ gas is used in the deposition process. However, the etching gas and the deposition gas are not limited to the above combinations, and may be any gas that is corrosive to the object to be etched in the above gas and a gas that generates a fluororesin polymer on the side wall of the formed recess. Good. Etching is performed by alternately performing the above etching process and the deposition process at short intervals (about several seconds).

  Although it does not specifically limit as a plasma generation source, inductively coupled plasma (ICP type: Inductively-Coupled Plasma), capacitively coupled plasma (CCP type: Capacitive Coupled Plasma), electron cyclotron resonance plasma (ECR type: Electron Cyclotron Resonance Plasma) Microwave excitation surface wave plasma (SWP: Surface Wave Plasma), helicon wave excitation plasma (HWP: Helicon Wave Plasma), etc. are applicable.

Etching by RIE or DRIE is performed to form the first recess 40 (FIG. 1B). The first recess 40 formed at this time has an aspect ratio (the depth (D ₁ ) of the first recess 40 is equal to the width (W) of the opening 25) in consideration of the formation process of the second mask 30 described later. The value obtained by division, that is, D ₁ / W) is preferably 1 or more.

  A second mask 30 is formed by vapor deposition or sputtering on the substrate 10 on which the first recess 40 is formed (FIG. 1C). The mask material used for the second mask 30 is preferably a material having an etching selectivity higher than that of the first mask 20, and a metal material or the like can be used. Among these, it is selected from Ti, TiN, Cr, Al, Rh, Co, Fe, Mo, Ni, Ta, and Zr. The metal material described above is preferable in that it has an ion impact resistance during etching in forming the second recess 50. As the metal material, a material having a low sputtering efficiency is suitable, and the upper limit material of the sputtering efficiency is Cr.

Etching by DRIE is performed on the first recess 40 to form a second recess 50 deeper than the first recess 40 (D ₂ > D ₁ ) (FIG. 1D). Thereby, it is possible to form the fine structure 100 having an aspect ratio (D ₂ / W) of 50 or more, which has been difficult in the past. When forming a structure with a high aspect ratio, as the depth of the recess increases, it becomes difficult for reactive ions to enter, and the etching rate and the mask selection ratio decrease. In the present invention, the processing of the high aspect ratio is facilitated by forming the second mask 30 having a higher mask selectivity on the first mask 20. Note that the first mask 20 and the second mask 30 are easily removed (lifted off) by being immersed in a resist stripping solution after the etching (FIG. 1E).

  Here, the step of forming the second mask 30 will be described in detail. FIG. 2 is a schematic diagram showing a method for forming the second mask 30. In the present invention, it is necessary to suppress the deposition of the metal material of the second mask 30 on the bottom surface portion (the bottom surface of the first recess) 40b. When the metal material is deposited on the bottom surface portion 40b, the metal material functions as a mask and causes processing defects. Therefore, when the second mask 30 is formed, the metal particles reach the substrate 10 with directionality (obliquely). In the actual system, the metal particles that reach the substrate 10 from the vapor deposition source or the sputtering target can be regarded as parallel straight lines in the direction of arrival of the metal particles that can reach the substrate 10. The arrival direction of the substrate 10 and the metal particles 60 forms an angle θ (an angle larger than α described later) as shown in FIG.

FIG. 3 is an enlarged schematic view of the first recess 40. As shown in FIG. 3A, in order for the metal particles not to reach the bottom surface portion 40b, the angle θ is required to be larger than α, which is the minimum value of the angle formed between the arrival direction of the metal particles and the substrate 10. It is done. Here, α is an angle satisfying tan α = W / D ₁ . Therefore, in order to form the second mask 40 substantially uniformly on the first mask 20 and to suppress the deposition of metal particles on the bottom surface portion 40b, the metal particles are made to reach at a somewhat large angle of less than 90 °. (However, α or more) is preferable. By forming the first concave portion 40 with a certain aspect ratio (1 or more, preferably 2 or more) as described above, the angle α can be reduced, and the degree of freedom of substrate placement in the film forming apparatus can be secured. Deposition on the bottom surface portion 40b can be suppressed.

  As shown in FIG. 3B, the angle θ (60 ° or more, preferably 70 ° or more and less than 90 °) is controlled, and the side surface portion 20a of the first mask 20 and the first recess 40 are controlled. A metal material may be deposited on the side wall 40a. As a result, silicon erosion or receding of the first mask 20 due to side etching (etching in the horizontal direction of the substrate) during subsequent etching can be suppressed, and a more accurate shape can be maintained.

§2. The method of manufacturing MEMS using a fine structure forming method of the present invention below, examples will be described to produce an acceleration sensor 200 (an example of MEMS) using a fine structure formation method of the present invention. FIG. 4 is a diagram showing the appearance of the acceleration sensor. The acceleration sensor 200 is configured by sandwiching a semiconductor substrate 210 in which a sensor main body is formed by an upper glass substrate 220 and a lower glass substrate 230, and they are joined and integrated. The thicknesses of the semiconductor substrate 210, the upper glass substrate 220, and the lower glass substrate 230 are, for example, 300 μm, 600 μm, and 600 μm.

FIG. 5 is a diagram illustrating the upper surface of the sensor main body of the acceleration sensor 200. The sensor body is formed in a semiconductor substrate 210, and the semiconductor substrate 210 can be a silicon substrate or an SOI (Silicon On Insulator) substrate. The outer shape of the acceleration sensor 200 is a substantially rectangular parallelepiped whose X direction is 2.5 mm and Y direction is 2.0 mm.
The semiconductor substrate 210 includes a frame portion 211 that is pierced inside, and a weight portion 212 that functions as an action body in the frame portion 211 and can be displaced with respect to the frame portion 211. The frame portion 211 and the weight portion 212 are connected by a beam portion 213 having flexibility in one axis (X direction in the drawing). The frame portion 211 is provided with a plurality of fixed comb teeth portions 214 that protrude from the inner frame along the outer shape of the sensor. In addition, a plurality of movable comb teeth 215 are arranged on the weight section 212 so as to face the frame section 211 and alternately mesh with the fixed comb teeth 214 with a gap. The width of the comb teeth of the fixed comb tooth portion 214 and the movable comb tooth portion 215 is 22 μm, each gap is 3 μm, and a predetermined number of comb teeth are arranged. In the figure, an example in which three fixed comb teeth and four movable comb teeth are arranged is shown for ease of viewing. In addition, a dimension etc. are examples and are not limited to the above.

  In addition, an opening 217 penetrating the upper and lower sides of the substrate is present in the frame portion 211, and demarcates the frame portion 211, the weight portion 212, and the beam portion 213. A strain detection element 216 such as a piezoresistive element is disposed in the beam portion 213, and the acceleration in the X (sensitive axis) direction is detected by detecting the bending of the beam portion 213 caused by the displacement of the weight portion 212 in the X direction. Can be detected. For example, two strain detection elements are provided for each piezoresistor for each beam portion 213, and a total of four piezoresistors form a bridge circuit (not shown). In these bridge circuits, since the ratio of the input voltage to the output voltage is proportional to the resistance change, that is, the distortion (or deflection) of the beam, the acceleration can be detected.

  The upper glass substrate 220 and the lower glass substrate 230 are provided with gaps 221 and 231 in regions corresponding to the inner frame of the frame portion 211. FIG. 6 is a cross-sectional view of the acceleration sensor (cross section AA in FIG. 5). The weight portion 212 is configured to be displaceable by gaps 221 and 231 provided in the upper glass substrate 220 and the lower glass substrate 230.

The acceleration sensor 200 was produced using the microstructure forming method according to the present invention.
The strain detection unit 216 forms silicon nitride (Si ₃ N ₄ ) or the like on the semiconductor substrate 210 and then patterns it to form a diffusion mask material. Thereafter, a diffusing agent containing boron (B) or the like is applied and heat-treated (approximately 1100 ° C.), whereby a piezoresistive element can be formed. The piezoresistive element can also be formed using an ion implantation method. Thereafter, an insulating film such as a silicon oxide film (SiO ₂ ) is formed, and a contact hole (not shown) is created in the region of the piezoresistive element portion. A bridge wiring (not shown) is formed by patterning, for example, Al by sputtering or vapor deposition. The piezoresistive element and the bridge wiring (not shown) have a good electrical connection with an ohmic contact through a contact hole.

A positive photoresist is formed with a thickness of about 10 μm as a first mask on the semiconductor substrate 210. The first mask is patterned into a shape corresponding to the frame portion 211 (including the fixed comb tooth portion 214), the weight portion 212 (including the movable comb tooth portion 215), the beam portion 213, and the opening portion 217. A first recess having a depth (D ₁ ) of approximately 100 μm was formed by DRIE using the first mask as an etching mask. Next, 1 μm of Cr was deposited on the first mask by vapor deposition on the semiconductor substrate 100 to form a second mask. At this time, θ was set to 60 ° to 70 °. Then, etching was performed by DRIE using the second mask as an etching mask to form a gap between the fixed comb tooth portion 214 and the movable comb tooth portion 215. As described above, the opening having a width of 3 μm corresponding to the gap was etched through the top and bottom of the substrate (thickness: 300 μm) to define the fixed comb teeth 214 and the movable comb teeth 215. At this time, the aspect ratio was 100, and the shape accuracy of the defined fixed comb teeth 214 and movable comb teeth 215 was good.

  The upper glass substrate 220 and the lower glass substrate 230 are glass substrates containing movable ions (for example, Pyrex (registered trademark) glass), and are integrally formed with the semiconductor substrate 210 made of silicon by anodic bonding. The upper glass substrate 220 and the lower glass substrate 230 have gaps 221 and 231 of approximately 5 μm formed by etching, respectively.

  The acceleration sensor 200 is arranged in a multi-sided manner on a wafer having a diameter of 150 to 200 mm, and is separated into pieces using a dicing saw or the like. In this specification, those arranged on a wafer or separated into pieces are collectively referred to as an acceleration sensor 200.

  As described above, a step of preparing a semiconductor substrate, a step of forming a piezoresistive group on the semiconductor substrate, a step of connecting the piezoresistive group by wiring, a step of forming a first mask on the semiconductor substrate, A step of forming a first recess using the first mask as an etching mask, a step of forming a second mask after forming the first recess, and a thickness of the semiconductor substrate using the second recess as an etching mask. The acceleration sensor 200 is manufactured through a process of further etching to form the second recess, a process of forming the frame part and the movable part, and a process of joining the semiconductor substrate and the support substrate. In this embodiment, the processed aspect ratio is 50 or more, and an acceleration sensor having excellent motion characteristics can be provided. Since the specific surface area increases as the aspect ratio increases, the functional superiority of the drive / transmission system or chemical reaction system microcomponent (for example, μ-TAS) increases in addition to the sensor described above.

  As described above, the acceleration sensor manufacturing method has been described as an application example of the present invention. However, the present invention is not limited to this, and the present invention is not limited to this, but the present invention can be applied to all MEMS having a high aspect structure (sensor MEMS, RF-MEMS, bio MEMS, optical It can be applied to the manufacture of MEMS and the like, and it is possible to provide a functionally superior MEMS. Needless to say, the semiconductor material can be etched to produce a structure having a high aspect ratio.

  A photoresist (AZ-5218E manufactured by AZ Electronic Materials Co., Ltd.) is applied as a first mask to a silicon substrate having a thickness of 625 μm at a thickness of 10 μm, and a mask including a line pattern with an opening width of 3 μm is formed. Was used as an etching mask to form a first recess having a depth of 100 μm by DRIE. As a second mask, 1 μm of Cr was deposited on the photoresist by vapor deposition on a silicon substrate. At this time, θ was set to 65 °. Etching was performed by DRIE using Cr as an etching mask to form a recess having a depth of 300 μm. At this time, the aspect ratio was 100, and the cross-sectional shape accuracy of the second recess was good.

(Comparative Example 1)
A fine structure was manufactured under substantially the same conditions as in the above example except that the second mask made of a metal material was not formed and etching was performed using only the first mask made of a resist. The side surface of the first mask was retreated, and as a result, the upper end of the opening of the fine structure was eroded and the pattern shape was damaged, and the pattern formation processing could not be continued.

(Comparative Example 2)
Except for the point that after depositing 1 μm of Cr on the first mask after forming a first recess of approximately 2 μm depth (D ₁ : aspect ratio 0.6) by DRIE using the first mask as an etching mask The microstructure was manufactured under substantially the same conditions as in the above example. At this time, deposition of metal particles was confirmed on the bottom surface of the first recess, and a processing failure occurred when the second recess was formed.

It is explanatory drawing of the fine structure formation process of this invention. It is a schematic diagram which shows the method of forming into a film the 2nd mask. It is an expansion schematic diagram of a 1st recessed part. It is a figure which shows the external appearance of an acceleration sensor. It is a figure showing the upper surface of the sensor main-body part of an acceleration sensor. It is sectional drawing of an acceleration sensor.

Explanation of symbols

10: Substrate 20: First mask (photoresist)
20a: side part 25: opening 30: second mask (metal material)
40: 1st recessed part 40a: Side wall part 40b: Bottom part 50: 2nd recessed part 60: Metal particle 100: Fine structure

200: Acceleration sensor 210: Semiconductor substrate 211: Frame part 212: Weight part 213: Beam part 214: Fixed comb tooth part 215: Movable comb tooth part 216: Strain detection part 217: Opening part 220: Upper glass substrate 221: Gap 230 : Lower glass substrate 231: Gap

Claims (7)

A microstructure forming method for forming a microstructure on a substrate made of silicon by dry etching using plasma,
Forming a first mask having an opening on the substrate;
Etching the substrate using the first mask as an etching mask to form a first recess;
Forming a second mask on the first mask after forming the first recess;
Further etching the first recess in the thickness direction of the substrate using the second mask as an etching mask to form a second recess deeper than the first recess;
A method for forming a fine structure, comprising:   At least one of the etching steps in the first and second recess forming steps is an etching step in which the recess is formed by etching the substrate, and a deposition step in which a protective film is formed on the side wall of the formed recess. 2. The method for forming a fine structure according to claim 1, wherein the etching is alternately and repeatedly performed.   3. The fine structure forming method according to claim 1, wherein in the second mask formation step, a mask material is deposited from an oblique direction with respect to the substrate.   4. The microstructure forming method according to claim 3, wherein the second mask is formed on a side surface of the first mask and a side wall of the first recess.   The opening of the first mask is on the order of microns, and the aspect ratio (depth / opening width) of the first recess is 1 or more. Microstructure formation method.   6. The microstructure forming method according to claim 1, wherein an aspect ratio (depth / opening width) of the second recess is 50 or more.   7. The mask material used for the second mask is selected from Ti, TiN, Cr, Al, Rh, Co, Fe, Mo, Ni, Ta, and Zr. The method for forming a fine structure according to any one of the preceding claims.

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