DE102021212947A1 - Micromechanical device and method of manufacture - Google Patents

Abstract

The invention relates to a micromechanical device with a substrate (100) with a main extension plane;
- With a lower polysilicon layer (310) which is arranged above the substrate parallel to the main extension plane and in which a first electrically isolated segment (315) is formed;
- having a middle polysilicon layer (320) which is arranged parallel to the main extension plane above the lower polysilicon layer;
- having an upper polysilicon layer (330) which is arranged parallel to the main plane of extension above the middle polysilicon layer and in which a second electrically isolated segment (335) is formed;
- With a micromechanical functional layer (400), which is arranged parallel to the main plane of extension above the upper polysilicon layer;
- wherein an electrical conductor track (325) is formed in the middle polysilicon layer, which is electrically conductively connected to a part of the micromechanical functional layer,
- wherein the first electrically insulated segment is arranged at least partially under the conductor track and/or the second electrically insulated segment is arranged at least partially above the conductor track.
The invention also relates to a method for producing a micromechanical device.

Description

State of the art

In DE19537814A a method for the production of micromechanical sensors such as acceleration and yaw rate sensors is described. This and similar manufacturing processes are used to create movable silicon structures whose movements are measured by determining changes in capacitance.

It is characteristic of this class of methods and the further discussion that the movable silicon structures 1 are produced in a first step by an etching method in a thick functional layer, trenches 2 being produced in the silicon layer with a high aspect ratio. In a second step, a sacrificial layer 3 is removed from under the thick silicon layer. In this way, silicon structures that can move freely relative to the substrate are obtained. A thin first buried polysilicon layer 5 can be arranged under the movable structures. The polysilicon layer can be used as a suspension 6 for the movable or fixed silicon structures or as an electrode 7 under the movable structures or as a conductor track ( 1 ).

The moving structures are usually hermetically protected with a cap 9 . A cap wafer can be attached to a sensor wafer using different bonding processes. A cavity 10 is usually provided in the cap ( 2 ).

The electrical connection between the hermetically sealed MEMS structure and the outside area is established by conductor tracks 11 in the buried layer, which run under the bonding frame made up of the functional layer.

In order to measure an acceleration or a yaw rate, the deflection of a seismic mass is measured. The seismic masses are formed in the functional layer and suspended by springs. The deflection of the seismic mass in the substrate level takes place via plate capacitors, which are formed in the functional layer, and the change in capacitance is measured.
Deflections perpendicular to the substrate plane (out-of-plane) are measured via a plate capacitor arrangement between the moveable MEMS structure and stationary counter-electrodes, which are formed with the buried polysilicon layer and are anchored to the substrate.

The first distance 12 between the counter-electrode and the substrate is usually chosen to be large. A fixed capacitance forms between the substrate and the counter-electrode, which interferes with the evaluation of the variable capacitance. It is technically difficult to measure small changes on a large fixed base capacitance. This always leads to noise in the measured signal, which usually increases with the ratio between the basic capacitance and the signal change. Therefore, the distance between the counter-electrode and the substrate is usually chosen to be large. Technically, large pitches deposited by thick oxide layers on a silicon substrate are limited. The oxide deposits occur at high temperatures and the different thermal expansion coefficients of silicon and oxide mean that thicker oxide layers on silicon have a tendency to form cracks. Typically, oxide thicknesses of up to 2.5 µm can still be produced on silicon.

In order to obtain a high level of sensitivity in the detection of the out-of-plane movement, the second distance 13 between the movable structure and the counter-electrode is usually chosen to be very small. The distance is created by an oxide layer that serves as a sacrificial layer during the process. Technically, there are almost no limits to small distances, i.e. small oxide thicknesses. Oxide deposits can be deposited as thin as desired.

The problem with this arrangement is that with a small distance between the movable structure and the counter-electrode, there is also a small distance between the functional layer in the bonding frame area and the conductor tracks routed there in the buried polysilicon layer. It is also critical that there is an oxide with a high dielectric constant in this region. This creates further, undesired, high additional basic capacitance, which cannot be reduced by simply increasing the distance, since otherwise the signal would also be reduced.

object of the invention

A micromechanical device and a method for its production are sought that allow an electrical signal line to be routed under a bonding frame, which has a low electrical capacitance and at the same time small distances between a movable structure and a stationary counter-electrode in an area with movable MEMS structures allowed to allow large capacitive signal changes.

essence and advantages of the invention

The invention relates to a micromechanical device having a substrate with a main extension plane, with a lower polysilicon layer, which is arranged above the substrate parallel to the main extension plane and in which a first electrically insulated segment is formed, with a middle polysilicon layer, which is arranged above the lower polysilicon layer parallel to the is arranged in the main extension plane, with an upper polysilicon layer, which is arranged over the middle polysilicon layer parallel to the main extension plane, and in which a second electrically insulated segment (335) is formed, with a micromechanical functional layer, which is arranged over the upper polysilicon layer parallel to the main extension plane, wherein an electrical conductor track is formed in the middle polysilicon layer, which is electrically conductively connected to a part of the micromechanical functional layer, the first electrically insulated segment being arranged at least in regions under the conductor track and/or the second electrically insulated segment being arranged at least in regions above the conductor track.

An advantageous embodiment of the device according to the invention provides that a first oxide layer with a first oxide thickness is arranged between the substrate and the lower polysilicon layer, a second oxide layer with a second oxide thickness is arranged between the lower polysilicon layer and the middle polysilicon layer, and between the middle polysilicon layer and a third oxide layer having a third oxide thickness is arranged on the top polysilicon layer.

An advantageous embodiment of the device according to the invention provides that a fourth oxide layer with a fourth oxide thickness is arranged at least in regions between the upper polysilicon layer and the micromechanical functional layer.

It is particularly advantageous that the sum of the first oxide thickness and the second oxide thickness is equal to the sum of the third oxide thickness and the fourth oxide thickness or has a deviation of less than 30% therefrom.
An advantageous embodiment of the device according to the invention provides that a bonding frame is formed in the micromechanical functional layer, which is arranged in a connection area above the conductor track. It is particularly advantageous that the first electrically insulated segment and/or the second electrical segment cover the conductor track in a direction perpendicular to the main extension plane in the connection area under the bonding frame and do not protrude beyond the conductor track by more than 30% of their area.
It is also advantageous that the lower polysilicon layer is structured using trenches, the trench width of which is less than twice the second oxide thickness and/or the upper polysilicon layer is structured using trenches, the trench width of which is less than twice the fourth oxide thickness.

It is advantageous to provide three buried polysilicon layers, which are each separated from one another by oxide layers, under a bonding frame between a substrate and a functional layer.

In the bonding frame, the signals are preferably routed through the middle layer, the polysilicon layer. The lower polysilicon layer and the upper polysilicon layer are provided in the bond frame area as electrically floating areas above and below the middle layer. In the core area, the movement of the seismic mass is preferably provided between the movable functional layer and the upper layer.

The other polysilicon layers serve as buffer layers. In total, a higher oxide thickness can be produced without cracks occurring. If a structure in the poly layer is not electrically connected, i.e. if it has a floating potential, the two oxide layers above and below can be viewed as capacitances connected in series. This arrangement therefore acts as a particularly thick oxide layer, which cannot be produced technically without an intermediate layer.

This means that small gap distances can be used, especially in the sensor core, to enable sensitive sensors, but this no longer leads to an increase in the basic capacitance, which was essentially caused by the conductor path routing under the bonding frame. That means you can now produce sensitive and at the same time low-noise sensors.

The distance to the substrate can also be increased, since an effective distance of more than 2.5 μm can also be produced towards the substrate via a floating structure in the lower layer.

• (A) Bereitstellen eines Substrats und Abscheiden einer ersten Oxidschicht darauf;
• (B) Abscheiden und Strukturieren einer unteren Polysiliziumschicht;
• (C) Abscheiden einer zweiten Oxidschicht;
• (D) Abscheiden und Strukturieren einer mittleren Polysiliziumschicht;
• (E) Abscheiden und Strukturieren einer dritten Oxidschicht;
• (F) Abscheiden und Strukturieren einer oberen Polysiliziumschicht;
• (G) Abscheiden und Strukturieren einer vierten Oxidschicht;
• (H) Abscheiden und Strukturieren einer mikromechanischen Funktionsschicht;
• (I) Ätzen der vierten Oxidschicht als Opferschicht wenigstens unter Teilen der mikromechanischen Funktionsschicht;

The invention also relates to a method for producing a micromechanical device with the steps:

• (A) providing a substrate and depositing a first oxide layer thereon;
• (B) depositing and patterning a bottom polysilicon layer;
• (C) depositing a second oxide layer;
• (D) depositing and patterning a middle polysilicon layer;
• (E) depositing and patterning a third oxide layer;
• (F) depositing and patterning a top polysilicon layer;
• (G) depositing and patterning a fourth oxide layer;
• (H) deposition and structuring of a micromechanical functional layer;
• (I) etching the fourth oxide layer as a sacrificial layer at least under parts of the micromechanical functional layer;

An advantageous embodiment of the method provides that in step (H) a bonding frame is structured in the micromechanical functional layer in a connection area.
An advantageous embodiment of the method provides that in step (B) a first electrically insulated segment is structured in the lower polysilicon layer in the connection area.
An advantageous embodiment of the method provides that in step (D) a conductor track is patterned into the middle polysilicon layer in the connection area.
An advantageous embodiment of the method provides that in step (F) a second electrically insulated segment is patterned into the upper polysilicon layer in the connection area.
An advantageous embodiment of the method provides that in step (K) after step (I) a cap is mounted on the bonding frame.

character list

• 1 12 schematically shows a prior art micromechanical device.
• 2 12 schematically shows a prior art encapsulated micromechanical device.
• 3 shows schematically a micromechanical device according to the invention.
• 4 shows a method according to the invention for producing a micromechanical device.

Embodiments of the invention

3 shows schematically a micromechanical device according to the invention.
A first oxide layer 210 with a first oxide thickness 211, a lower polysilicon layer 310, a second oxide layer 220 with a second oxide thickness 221, a middle polysilicon layer 320, a third oxide layer 230 with a third oxide thickness 231 is in a layer sequence on a substrate 100 with a main extension plane , an upper polysilicon layer 330, a fourth oxide layer 240 with a fourth oxide thickness 241 and a micromechanical functional layer 400 are arranged.
A first electrically insulated segment 315 is formed in the lower polysilicon layer 310, which is arranged above the substrate parallel to the main extension plane. An electrical conductor track 325 is formed in the middle polysilicon layer 320 and is electrically conductively connected to a part of the micromechanical functional layer. A second electrically insulated segment 335 is formed in the upper polysilicon layer 330, which is arranged above the middle polysilicon layer parallel to the main extension plane. By etching the sacrificial layer of the fourth oxide layer and partly also of the third oxide layer, parts of the micromechanical functional layer are exposed and made mobile.

A bonding frame 410 is formed in the micromechanical functional layer 400 and is arranged in a connecting region 500 above the conductor track 325 . In this case, the conductor track 325 is arranged between the first electrically insulated segment 315 and the second electrically insulated segment 335 . The electrically isolated segments are therefore at an undefined electrical potential.
A cap 600 , which encloses a cavity 650 , is fastened to the bonding frame by means of bonding material 510 . The structures of the micromechanical functional layer are protected in the cavern 650 .

In an advantageous embodiment of the invention, the segmentation of the insulating areas in the upper and lower polysilicon layer is to be adapted to the respective conductor track in the bonding frame area in such a way that the isolated segments do not protrude beyond the conductor track in the bonding frame area by more than 30%.

In an advantageous embodiment of the invention, the sum of the first and second oxide thicknesses 211, 221 between the substrate and the lower polysilicon layer and the lower polysilicon layer and the middle polysilicon layer are approximately the same or deviate by less than 30% from the sum of the third and fourth oxide thicknesses 231, 241 between the middle polysilicon layer and upper polysilicon layer and to choose between upper poly layer and functional layer.

Furthermore, in a particularly favorable variant, it is proposed that the trenches between the structures in the lower and the upper polysilicon layer in the bonding frame area be made narrower than twice the oxide thickness on the respective layer. This ensures that the respective trench is completely filled with oxide after the oxide deposition and no additional topography is formed on the surface after the oxide deposition.

The inventive idea is not limited to two additional polysilicon layers. In order to achieve even lower basic capacitances, additional polysilicon layers can also be introduced.

• (A) Bereitstellen eines Substrats und Abscheiden einer ersten Oxidschicht darauf;
• (B) Abscheiden und Strukturieren einer unteren Polysiliziumschicht;
• (C) Abscheiden einer zweiten Oxidschicht;
• (D) Abscheiden und Strukturieren einer mittleren Polysiliziumschicht;
• (E) Abscheiden und Strukturieren einer dritten Oxidschicht;
• (F) Abscheiden und Strukturieren einer oberen Polysiliziumschicht;
• (G) Abscheiden und Strukturieren einer vierten Oxidschicht;
• (H) Abscheiden und Strukturieren einer mikromechanischen Funktionsschicht;
• (I) Ätzen der vierten Oxidschicht als Opferschicht wenigstens unter Teilen der mikromechanischen Funktionsschicht;

4 shows a method according to the invention for producing a micromechanical device with the steps:

• (A) providing a substrate and depositing a first oxide layer thereon;
• (B) depositing and patterning a bottom polysilicon layer;
• (C) depositing a second oxide layer;
• (D) depositing and patterning a middle polysilicon layer;
• (E) depositing and patterning a third oxide layer;
• (F) depositing and patterning a top polysilicon layer;
• (G) depositing and patterning a fourth oxide layer;
• (H) deposition and structuring of a micromechanical functional layer;
• (I) etching the fourth oxide layer as a sacrificial layer at least under parts of the micromechanical functional layer;

1. 1. Auf einem Substrat wird eine erste Oxidschicht abgeschieden.
2. 2. Es wird eine untere Polysiliziumschicht abgeschieden und strukturiert. Bevorzugt werden im Bondrahmenbereich isolierte Bereiche erzeugt, die derart an die jeweilige Leiterbahn im Bondrahmenbereich angepasst sind, dass die isolierten Bereiche im Bondrahmenbereich die Leiterbahn um nicht mehr als ein Flächenanteil von 30% überragen.
3. 3. Eine zweite Oxidschicht wird abgeschieden.
4. 4. Eine mittlere Polysiliziumschicht wird abgeschieden und strukturiert. Im Bondrahmenbereich werden Leiterbahnen erzeugt.
5. 5. Eine dritte Oxidschicht wird abgeschieden und strukturiert.
6. 6. Es wird eine obere Polysiliziumschicht abgeschieden und strukturiert. Bevorzugt werden im Bondrahmenbereich isolierte Bereiche erzeugt, die derart an die jeweilige Leiterbahn im Bondrahmenbereich angepasst sind, dass die isolierten Bereiche im Bondrahmenbereich die Leiterbahn um nicht mehr als ein Flächenanteil von 30% überragen.
7. 7. Eine vierte Oxidschicht wird abgeschieden und strukturiert.
8. 8. Eine Funktionsschicht wird abgeschieden und strukturiert.
9. 9. Eine Oxid-Opferschichtätzung erfolgt.
10. 10. Eine Kappe wird auf den Bondrahmen gebondet.

In an advantageous variant, the method is carried out as follows:

1. 1. A first oxide layer is deposited on a substrate.
2. 2. A bottom polysilicon layer is deposited and patterned. Insulated areas are preferably produced in the bonding frame area, which are adapted to the respective conductor track in the bonding frame area in such a way that the isolated areas in the bonding frame area protrude beyond the conductor track by no more than a surface proportion of 30%.
3. 3. A second oxide layer is deposited.
4. 4. A middle polysilicon layer is deposited and patterned. Conductor tracks are created in the bonding frame area.
5. 5. A third oxide layer is deposited and patterned.
6. 6. A top polysilicon layer is deposited and patterned. Insulated areas are preferably produced in the bonding frame area, which are adapted to the respective conductor track in the bonding frame area in such a way that the isolated areas in the bonding frame area protrude beyond the conductor track by no more than a surface proportion of 30%.
7. 7. A fourth oxide layer is deposited and patterned.
8. 8. A functional layer is deposited and structured.
9. 9. An oxide sacrificial layer etch is done.
10. 10. A cap is bonded to the bond frame.

Reference List

1

movable silicon structures

2

dig

3

sacrificial layer

5

polysilicon layer

6

suspension

7

electrode

9

cap

10

cavern

11

trace

12

first distance between electrode and substrate

13

second distance between movable structure and electrode

100

substrate

210

first oxide layer

211

first oxide thickness

220

second oxide layer

221

second oxide thickness

230

third oxide layer

231

third oxide thickness

240

fourth oxide layer

241

fourth oxide thickness

310

lower polysilicon layer

315

first electrically isolated segment

320

middle polysilicon layer

325

trace

330

top polysilicon layer

335

second electrically isolated segment

400

micromechanical functional layer

410

bond frame

500

connection area

510

bonding material

550

bond pad

600

cap

650

cavern

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 19537814 A [0001]

Claims (13)

Micromechanical device with a substrate (100) with a main extension plane; - With a lower polysilicon layer (310) which is arranged above the substrate parallel to the main extension plane and in which a first electrically isolated segment (315) is formed; - having a middle polysilicon layer (320) which is arranged parallel to the main extension plane above the lower polysilicon layer; - having an upper polysilicon layer (330) which is arranged parallel to the main plane of extension above the middle polysilicon layer and in which a second electrically isolated segment (335) is formed; - With a micromechanical functional layer (400), which is arranged parallel to the main plane of extension above the upper polysilicon layer; - wherein an electrical conductor track (325) is formed in the middle polysilicon layer, which is electrically conductively connected to a part of the micromechanical functional layer, - wherein the first electrically insulated segment is arranged at least partially under the conductor track and/or the second electrically insulated segment is arranged at least partially above the conductor track. micromechanical device claim 1 , characterized in that between the substrate (100) and the lower polysilicon layer (310) a first oxide layer (210) with a first oxide thickness (211) is arranged, between the lower polysilicon layer (310) and the middle polysilicon layer (320) a second Oxide layer (220) with a second oxide thickness (221) is arranged and between the middle polysilicon layer (320) and the upper polysilicon layer (330) a third oxide layer (230) with a third oxide thickness (231) is arranged. micromechanical device claim 2 , characterized in that a fourth oxide layer (240) with a fourth oxide thickness (241) is arranged at least in regions between the upper polysilicon layer (330) and the micromechanical functional layer (400). micromechanical device claim 3 , characterized in that the sum of the first oxide thickness and the second oxide thickness is equal to or less than 30% deviation from the sum of the third oxide thickness and the fourth oxide thickness. Micromechanical device according to one of Claims 1 until 4 , characterized in that in the micromechanical functional layer (400) a bonding frame (410) is formed, which is arranged in a connection area (500) over the conductor track (325). micromechanical device claim 5 , characterized in that viewed in a direction perpendicular to the main plane of extent in the connection area (500) under the bonding frame (410), the first electrically insulated segment (315) and/or the second electrical segment (335) cover the conductor track (325) and thereby do not protrude more than 30% of their area over the conductor track. micromechanical device claim 1 , characterized in that the lower polysilicon layer (310) is structured by means of trenches, the trench width of which is less than twice the second oxide thickness (221) and/or the upper polysilicon layer (330) is structured by means of trenches, the trench width of which is less than that double fourth oxide thickness (241). A method for producing a micromechanical device, comprising the steps: (A) providing a substrate and depositing a first oxide layer thereon; (B) depositing and patterning a bottom polysilicon layer; (C) depositing a second oxide layer; (D) depositing and patterning a middle polysilicon layer; (E) depositing and patterning a third oxide layer; (F) depositing and patterning a top polysilicon layer; (G) depositing and patterning a fourth oxide layer; (H) deposition and structuring of a micromechanical functional layer; (I) etching the fourth oxide layer as a sacrificial layer at least under parts of the micromechanical functional layer; Method for manufacturing a micromechanical device claim 8 , characterized in that in step (H) a bonding frame is structured in the micromechanical functional layer in a connection area. Method for manufacturing a micromechanical device claim 9 , characterized in that in step (B) a first electrically isolated segment is patterned into the lower polysilicon layer in the connection region. Method for manufacturing a micromechanical device claim 9 , characterized in that in step (D) in connection tion area, a conductor track is structured in the middle polysilicon layer. Method for manufacturing a micromechanical device claim 9 , characterized in that in step (F) a second electrically isolated segment is patterned into the upper polysilicon layer in the connection area. Method for manufacturing a micromechanical device claim 9 , characterized in that in a step (K) after step (I), a cap is mounted on the bonding frame.

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