DE102021213259A1 - Process for the production of a cavity SOI substrate and micromechanical structures in it - Google Patents

Abstract

The invention relates to a method for producing a cavity SOI substrate with the steps: (A): providing a substrate and depositing a sacrificial layer on the substrate; (B): etching at least one contact area into the sacrificial layer; (C): depositing a functional layer ;(D): producing at least one etching access to the sacrificial layer; (E): at least partial removal of the sacrificial layer underneath the functional layer; (F): closing the etching access.

Description

State of the art

Cavity SOI substrates are known, which serve as the basis for many MEMS components. An SOI substrate 1 has on a silicon wafer 2 a thin silicon layer 3 which has a very precisely defined thickness (typically 0.5-250 μm) and which is connected to the substrate via a dielectric layer 4, usually an oxide layer of 0.1 -5 µm thickness is connected ( 1a ). In addition, cavities 5 are provided below the thin silicon layer in cavity SOI substrates. In these areas, self-supporting structures can be produced very easily in the thin silicon layer via trenches 7 produced with a trench process ( 1c ). It is not necessary to etch the sacrificial layer in order to create movable structures, as is the case in other processes. Further functional layers 6 can also be applied to the thin silicon layer with simple, planar process steps ( 1b ), which are simply protected with a varnish in the trench step. No subsequent sacrificial layer etch is necessary. Therefore, almost any additional functional layers can be applied to the movable thin silicon layer. It is a wafer process. The micromechanical structures produced in this way can then be very well protected with a cap wafer by means of a bonding process. In order to produce mechanically very well defined MEMS elements, it is important that the thickness of the thin functional layer is very precisely defined.

The manufacturing process for cavity SOI wafers is very complex. An oxide layer is typically grown on a first wafer. A second wafer is bonded onto the first wafer using a very expensive and complex direct bonding process. The second wafer is thinned down to the desired target thickness of the future thin silicon layer. An oxide layer is grown on a third wafer. The oxide layer is removed at defined areas and a depression is etched into the third substrate.

The wafer stack made up of the first and second wafers is bonded to the third wafer using a further direct bonding method. A part of the first wafer is then first removed using a cheap mechanical method. Since there are cavities underneath the first wafer, the first wafer bends through during the mechanical removal and a thicker layer remains above the cavities after the removal. In a next step, the material of the first wafer is completely removed down to the first oxide layer using a slow and expensive but selective etching process. The oxide layer is then removed.

A vacuum is usually enclosed in the cavern, which is why the functional layer of atmosphere is always slightly compressed. The thinner the functional layer is and the wider the cavity underneath, the more the functional layer sags, making further processing difficult or, in extreme cases, the layer can sag so much that it breaks.

In order to create very large movable structures with a thin functional layer, it is obvious to define support areas 8 within the movable structures and later to separate these areas from the movable structures with a trench that defines the movable structures ( 2a ). Further functional layers 6 can be applied to the thin silicon layer ( 2 B ). The disadvantage of this method is that the support structure elements must be very large. On the one hand, each individual bonding area 9 must have a certain minimum size per area in the case of direct bonding, and on the other hand the ring 10 around the support point must have a certain width which at least corresponds to the freedom of movement of the movable structure ( 2c ). In a first approximation, this means that the minimum size of the support points does not scale with the thickness of the functional layer. In contrast, the deflection of a membrane scales with the reciprocal of the third power of the thickness of the membrane. This means that with small layer thicknesses, a large number of support points must be used in order to counteract the deflection. This leads to the fact that it is practically impossible to produce large, movable structures in a thin functional layer in an SOI substrate.

In addition to the production method via direct bonding of two wafers, other production methods for SOI wafers are also known. With these methods, a functional layer is usually grown over a sacrificial layer and the sacrificial layer is then removed via accesses. Then the access area to the sacrificial layer is sealed. A particularly favorable method is described in the publication DE 102018222377 A1 described. The sacrificial layer can be removed particularly efficiently via etching channels. The basic problem that only very large support construction elements can be produced remains, however, since the undercut width is very poorly defined with sacrificial layer methods. Therefore, only very large support areas can also be constructed.

object of the invention

A manufacturing method for an SOI wafer with a cavity is sought which provides a thin silicon layer with a very precisely defined thickness and which also makes it possible to manufacture large movable structures.

essence and advantages of the invention

1. (A): Bereitstellen eines Substrats und Abscheiden einer Opferschicht auf dem Substrat;
2. (B): Ätzen wenigstens eines Kontaktbereichs in die Opferschicht hinein;
3. (C): Abscheiden einer Funktionsschicht;
4. (D): Herstellen wenigstens eines Ätzzugangs zur Opferschicht;
5. (E): Wenigstens teilweises Entfernen der Opferschicht unter der Funktionsschicht;
6. (F): Verschließen des Ätzzugangs.

The invention relates to a method for producing a cavity SOI substrate with the steps:

1. (A): providing a substrate and depositing a sacrificial layer on the substrate;
2. (B): etching at least one contact area into the sacrificial layer;
3. (C): depositing a functional layer;
4. (D): producing at least one etching access to the sacrificial layer;
5. (E): At least partial removal of the sacrificial layer under the functional layer;
6. (F): Closing the etching access.

This makes it possible to produce cavity SOI substrates with a large-area membrane. Very small support areas can be produced, and the recesses in the functional layer for the support areas can be made extremely small. The contact areas can be made very small with the new process. In the trenching process, the support column is removed down to the level of the substrate. Therefore, the trench in the area of the support column does not have to be enlarged in order to achieve greater freedom of movement. It is given by the etching down to the substrate level.

The manufacturing process for the new support areas is extremely simple. It can be easily integrated into known processes and it is also very inexpensive.

An advantageous embodiment of the method provides that an oxide layer is deposited in step (A).

It is advantageous if, in step (B), a contact area is etched whose width is less than twice the layer thickness of the sacrificial layer. A polysilicon layer is advantageously deposited in step (C). It is advantageous that in step (D) a narrow access to the sacrificial layer is etched using a trench process. In step (E), the sacrificial layer is advantageously etched by means of gas-phase etching, in particular with the participation of hydrogen fluoride, or the sacrificial layer is completely removed directly around the contact area. A further polysilicon layer is advantageously deposited in step (F) in order to close the etching accesses. A functional layer made of polysilicon is particularly advantageously reinforced in this way. It is also advantageous if, after step (F), the surface is planarized using a CMP process.

The invention also relates to a method for producing micromechanical structures in a cavity SOI substrate produced according to the invention, the functional layer being structured in a step (H) using a trench process and being exposed in certain areas.

It is advantageous that at least the functional layer is removed down to the substrate with the trench process in the contact area. It is also advantageous that before step (H) in a step (G) a further functional layer and/or at least one functional element is produced on the surface of the functional layer.

character list

• The 1a until 1c show a first cavity SOI substrate in the prior art and the production of micromechanical structures in it.
• The 2 a until 2 c show a second prior art cavity SOI substrate with supports and the fabrication of micromechanical structures therein.
• 3 shows schematically the method according to the invention for producing a cavity SOI substrate.
• The 4 a until 4 f show a first exemplary embodiment of the method according to the invention for producing a cavity SOI substrate.
• The 5 a until 5 c show the production of micromechanical structures in a cavity SOI substrate, which is produced with the method according to the invention.
• The 6 a until 6 f show a second exemplary embodiment of the method according to the invention for producing a cavity SOI substrate and the production of micromechanical structures therein.

Description of the figures

The 1 a until 1 c show a first cavity SOI substrate in the prior art and the production of micromechanical structures in it. 1a shows a cavity SOI substrate 1 having a silicon wafer 2 with a cavity 5, a dielectric layer in the form of an oxide layer 4 and a thin silicon layer 3 above it. 1b shows Functional elements on the thin silicon layer, which are formed from further functional layers 6. 1c shows self-supporting micromechanical structures in the thin silicon layer, which are defined by trenches 7. FIG.

The 2 a until 2 c show a second prior art cavity SOI substrate with supports and the fabrication of micromechanical structures therein. 2a shows a cavity SOI substrate different from 1 with a support area 8 in the cavern for supporting the thin silicon layer. The support area is connected to the thin silicon layer in a bonding area 9 . 2 B shows functional elements on the thin silicon layer, which are formed from further functional layers. 2c shows self-supporting micromechanical structures in the thin silicon layer, which are defined by trenches. A recess in the functional layer in the form of a ring 10 around the support area enables mobility of the micromechanical structures.

• Schritt (A): Auf einem Substrat wird eine Opferschicht abgeschieden. Bevorzugt wird eine Oxidschicht abgeschieden.
• Schritt (B): Ein oder mehrere Kontaktbereiche werden in die Opferschicht geätzt. Bevorzugt werden bei dicken Opferschichten, also Schichten die dicker als 1 µm sind, schmale Kontaktbereiche angelegt. Besonders günstig sind Kontaktbereiche deren Breite geringer als die doppelte Schichtdicke der Opferschicht sind. Damit kann eine Topographie auf der Opferschicht besonders einfach und effizient vermieden werden.
• Schritt (C): Ein Teil der Funktionsschicht oder die gesamte Funktionsschicht wird abgeschieden. Bevorzugt wird eine Polysiliziumschicht abgeschieden.
• Schritt (D): Ein Ätzzugang zur Opferschicht wird hergestellt. Bevorzugt wird mit einem Trenchprozess ein schmaler Zugang zur Opferschicht geätzt.
• Schritt (E): Die Opferschicht wird zumindest teilweise entfernt. Bevorzugt wird eine Ätzung in der Gasphase mit HF genutzt. Bevorzugt wird die Opferschicht um die schmalen Kontaktebereiche vollständig entfernt.
• Schritt (F) Die Ätzzugänge werden verschlossen. Bevorzugt wird dabei ein Unterdruck vom maximal 100 mbar bezogen auf Raumtemperatur eingeschlossen.

3 shows schematically the method according to the invention for producing a cavity SOI substrate. The procedure has the following main steps:

• Step (A): A sacrificial layer is deposited on a substrate. An oxide layer is preferably deposited.
• Step (B): One or more contact areas are etched into the sacrificial layer. In the case of thick sacrificial layers, ie layers thicker than 1 μm, narrow contact areas are preferably created. Contact areas whose width is less than twice the layer thickness of the sacrificial layer are particularly favorable. A topography on the sacrificial layer can thus be avoided in a particularly simple and efficient manner.
• Step (C): A part of the functional layer or the entire functional layer is deposited. A polysilicon layer is preferably deposited.
• Step (D): An etch access to the sacrificial layer is made. A narrow access to the sacrificial layer is preferably etched using a trench process.
• Step (E): The sacrificial layer is at least partially removed. Etching in the gas phase with HF is preferably used. The sacrificial layer around the narrow contact areas is preferably completely removed.
• Step (F) The etching accesses are sealed. A reduced pressure of at most 100 mbar relative to room temperature is preferably included.

This is preferably done with a further polysilicon deposition. Optionally, the surface is planarized after the deposition with a CMP process (CMP - chemical mechanical polishing).

This completes the actual cavity SOI wafer. Further functional layers and functional elements can now be applied to the flat surface using standard steps.

A step (G) optionally follows for the production of micromechanical structures: application of at least one further functional layer or else at least one functional element to the planar surface.

Step (H): The functional layer is structured with a trench process and individual areas are exposed. In the contact areas, the functional layer is removed at least down to the level of the substrate.

The 4 a until 4 f show a first exemplary embodiment of the method according to the invention for producing a cavity SOI substrate.

4a shows first a silicon substrate 2 with a sacrificial layer 12 deposited thereon, here an oxide layer. Narrow contact areas 11 are patterned into the sacrificial layer 12 as far as the substrate. A first part of a functional layer 13, here a polysilicon layer, is deposited on the sacrificial layer and in the contact areas ( 4b ). Narrow trenches are etched into the functional layer 13 and etching channels 17 are created ( 4c ).

After that, another part of the functional layer is deposited, whereby the etching channels are closed at the top. Then an etching access 14 is produced through the functional layer 13 to the sacrificial layer 12 ( 4d ). An etchant is supplied through the etch access 14 and the sacrificial layer 12 is at least partially removed. The etch channels 17 help to bring the etchant to the areas to be etched. The sacrificial layer is preferably completely removed in an area 15 around the narrow contact areas 11 ( 4e ). The etching access is preferably closed by depositing another part of the functional layer 16 ( 4f ).

The procedure described can be varied as required. For example, the etching channels 17 are not absolutely necessary. The functional layer could also be produced in one step.
However, before the sacrificial layer is etched, it is advantageous to create etch channels under the first part of the functional layer in order to ensure a quick and defined removal to enable the sacrificial layer. In particular, it is favorable to carry out a first polysilicon deposition 13 on the sacrificial layer and to trench narrow trenches in this layer 17 . A trench width that is narrower than the layer thickness is advantageous in order to produce an etching channel in this area in the following process steps. A layer thickness of 1 μm or more is advantageous. The second polysilicon deposition is then carried out, preferably an epitaxial polysilicon deposition which does not deposit any silicon on the open sacrificial layer areas. This creates cavities in the area of the trenches. An etch access 14 or just a few etch accesses, which are brought close to the cavities, is now sufficient to remove the entire sacrificial layer in the functional area with a relatively short sacrificial layer etch, whose spread over the cavities is controlled, and this in particular also around the support columns removed.
In a further step, the etching access is closed. This is preferably done again with an epitaxial polysilicon deposition. A small indentation may form in the area of the etching accesses.
By using the etch channels, however, the density and the number of etch accesses can be very severely limited and it can thus be ensured, for example, that these areas are not used as active MEMS areas.

In an alternative embodiment of the manufacturing method, the entire surface of the functional layer can also be somewhat removed at the end with a polishing process in order to obtain a very smooth, even surface. The advantage here is that with the new process with the narrow contact areas 11 a large number of support columns can be produced. Additional mechanical pressure is built up during the polishing process, which causes the membrane in classic support columns to deflect very significantly. A polishing process in combination with support columns that have a distance of less than 200 µm is particularly favorable.

The 5 a until 5 c show the production of micromechanical structures in a cavity SOI substrate, which is produced with the method according to the invention.

5a FIG. 1 shows a cavity SOI substrate with a silicon substrate 2, a sacrificial layer 12 and a functional layer 13, which is supported on the silicon substrate over the cavity by means of contact areas 11. FIG.
Further process steps and deposits 26 shown schematically here on the functional layer follow ( 5b ). In the trenching process, a ring structure is not etched around the contact area 11, but the contact area and thus the support column are completely removed by producing a recess 27 ( 5c ). In the process, the underlying substrate 2 is also etched and a base recess 28 is created ( 5c ).

In a somewhat more complex manufacturing process, a very high degree of vertical freedom of movement can also be achieved. This is particularly beneficial for out-of-plane moves that are intended to be higher than the thickness of the sacrificial layer.

The 6 a until 6 f show a second exemplary embodiment of the method according to the invention for producing a cavity SOI substrate and the production of micromechanical structures therein.

Trenches are made in the substrate 2 and filled with a sacrificial layer 12 in this production process. The outer surface of the substrate is also covered with the sacrificial layer. In the area of the subsequent support column, a trench is provided in particular, which delimits a silicon area 19 which approximately corresponds to the geometry of the support column or is even chosen to be somewhat narrower. In the remaining areas 20 between the support columns 19, the substrate material is completely or partially removed. For this purpose, for example, an access can be etched into the sacrificial layer and the material can be removed with an isotropic SF6 etch. The etching access can then be closed again with a further sacrificial layer deposition. 6a shows the cavity SOI substrate in this preliminary stage.

The rest of the process flow is analogous to 4 . In the sacrificial layer 12, a contact area 11 is introduced over the silicon area 19, which extends to the silicon ( 6b ). A functional layer 13 is deposited in the contact region 11 and on the sacrificial layer 12 ( 6c ). Thereafter, an etching access 14 is created through the functional layer 13 and the sacrificial layer 12 is etched ( 6d ). The cavities in the substrate can be used in a favorable manner to distribute and control the etching medium during the etching of the sacrificial layer. The etching access is then closed again with the deposition of a further part of the functional layer 16 ( 6e ). Further process steps and deposits 26 shown schematically here on the functional layer follow ( 6f ). In the trenching process, the support column is now removed, preferably with a recess 21 far into the substrate 2 . The support column is preferably removed in such a way that the recess extends into the substrate to a depth which corresponds to more than half the thickness of the sacrificial layer ( 6f ).

With the etching in the contact area, electrical contacts can also be produced between the substrate and the functional layer. Furthermore, local suspensions of structures formed from the functional layer can advantageously also be formed with this step. In addition, etching stop structures for the sacrificial layer etching can also be formed by the etchings in the contact area.

Reference List

1

SOI substrate

2

silicon wafer

3

thin layer of silicon

4

dielectric layer, oxide layer

5

cavity

6

further functional layers

7

dig

8th

support area

9

bonding area

10

ring

11

contact area

12

sacrificial layer

13

first part of the functional layer

14

Etching access to the sacrificial layer

15

environment of the contact area

16

another part of the functional layer

17

etching channel

26

separation

27

recess

28

base recess

19

silicon area

20

remaining area

21

recess

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102018222377 A1 [0006]

Claims (11)

Method for manufacturing a cavity SOI substrate with the steps: (A): providing a substrate and depositing a sacrificial layer on the substrate; (B): etching at least one contact area into the sacrificial layer; (C): depositing a functional layer; (D): producing at least one etching access to the sacrificial layer; (E): At least partial removal of the sacrificial layer under the functional layer; (F): Closing the etching access. Method for producing a cavity SOI substrate claim 1 , characterized in that in step (A) an oxide layer is deposited. Method for producing a cavity SOI substrate claim 1 , characterized in that in step (B) a contact area is etched, the width of which is less than twice the layer thickness of the sacrificial layer. Method for producing a cavity SOI substrate claim 1 , characterized in that in step (C) a polysilicon layer is deposited. Method for producing a cavity SOI substrate claim 1 , characterized in that in step (D) a narrow access to the sacrificial layer is etched with a trench process. Method for producing a cavity SOI substrate claim 1 , characterized in that in step (E) the sacrificial layer is etched by means of gas phase etching, in particular with the participation of hydrogen fluoride, and/or that the sacrificial layer is completely removed immediately around the contact area. Method for producing a cavity SOI substrate claim 1 , characterized in that in step (F) a further polysilicon layer is deposited. Method for producing a cavity SOI substrate claim 1 , characterized in that after step (F) the surface is planarized with a CMP process. Method for producing micromechanical structures in a cavity SOI substrate, produced according to one of Claims 1 until 8th , characterized in that in a step (H) with a trench process, the functional layer is structured and partially exposed. Process for the production of micromechanical structures claim 9 , characterized in that with the trench process in the contact area at least the functional layer is removed down to the substrate. Process for the production of micromechanical structures claim 9 , characterized in that before step (H) in a step (G) on the surface of the functional layer, a further functional layer and / or at least one functional element is produced.

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