METHOD FOR ENCLOSING REFERENCE GASES IN MEMS CELLS

Abstract

In a first aspect, the invention relates to a method for producing a gas-filled reference chamber which is hermetically sealed. Thereby, the gas with which the reference chamber is filled is introduced via an opening in a separate coating chamber only after bonding of the wafers forming the reference chamber. The reference chamber preferably contains MEMS devices.

In another aspect, the invention relates to a photoacoustic gas sensor comprising such a reference chamber within which a MEMS sensor is present.

Claims

1. A method of producing a photoacoustic gas sensor comprising a gas-filled reference chamber within which a microelectromechanical system (MEMS) device and optionally an electronic circuit is present, comprising the steps of: a) providing a first and second wafer, wherein at least the first wafer and/or the second wafer has a cavity and wherein the MEMS device is present on the first and/or second wafer, wherein the MEMS device is a MEMS sensor, b) bonding the first wafer to the second wafer within a bonding chamber to form a volume which can be filled with reference gas, wherein an opening remains on a contact surface of the two wafers after bonding, or an opening is made in the first and/or second wafer before or after bonding, c) flooding a reference gas into the reference chamber via the opening within a coating system, d) sealing the opening of the reference chamber within the coating system e) providing a modulable emitter, f) arranging the reference chamber filled with the reference gas and the modulable emitter, wherein the reference chamber is present in the beam path of the emitter so that the emitter can excite the reference gas in the reference chamber by means of modulably emittable radiation to form sound pressure waves which are detectable by means of the MEMS sensor.

2. The production method according to claim 1, wherein the reference gas comprises corrosive and/or explosive gases.

3. The production method according to claim 1, wherein in order to set a partial pressure of the reference gas within the reference chamber, an inert gas is additionally introduced into the reference chamber via the opening.

4. The production method according to claim 1, wherein the first wafer and the second wafer have contact surfaces which are used for bonding the first wafer to the second wafer, wherein, in order to form the opening, a region on the contact surfaces is not bonded and/or wherein an opening remains on a contact surface of the two wafers after bonding, wherein the opening has a cross-section from 1 ?m.sup.2 to 1000 ?m.sup.2 and a length from 1 ?m to 1000 ?m.

5. The production method according to claim 1, wherein that before or after bonding the first wafer to the second wafer, the opening is formed starting from an outer side to an inner side of the first wafer or the second wafer.

6. The production method according to claim 1, wherein a valve is present at the end of the opening of the first or the second wafer, wherein after bonding of the first wafer with the second wafer the valve is located at the end of the opening starting from the outside of the first wafer or the second wafer and within the volume of the reference chamber.

7. The production method according to claim 1, wherein after bonding the first wafer to the second wafer, the reference chamber in the coating system is flooded with the reference gas, the gas entering the volume of the reference chamber via the opening and via the valve.

8. The production method according to claim 1, wherein after flooding the reference gas into the reference chamber for sealing the opening, a solder is melted.

9. The production method according to claim 1, wherein the opening is sealed by means of a coating process within the coating system.

10. The production method according to claim 1, wherein the coating system is a physical coating system or a chemical coating system, a low-pressure chemical coating system and/or epitaxial coating system.

11. The production method according to claim 1, wherein for sealing the opening-within the coating system, a covering layer is applied at least over a region of the opening, wherein a nitride, silicon carbonitride, silicon oxynitride, titanium nitride and/or tantalum nitride, an oxide, or a metal, is used as material for the covering layer.

12. The production method according to claim 1, wherein for sealing the opening and for forming a cover layer, a process gas is introduced in the coating system, wherein the process gas is introduced into the reference chamber after flooding with a reference gas, or wherein a material for forming the cover layer is selected in such a way that the reference gas can simultaneously serve as the process gas.

13. The production method according to claim 1, wherein the MEMS device comprises a MEMS sensor or a MEMS actuator and/or the electronic circuit comprises a processor, a switch, transistors, and/or transducers.

14. (canceled)

15. A photoacoustic gas sensor comprising: a modulable emitter, a reference chamber filled with a reference gas, wherein a MEMS sensor is present within the reference chamber, wherein the reference chamber is present in the beam path of the emitter so that the emitter can excite the reference gas in the reference chamber by means of modulably emittable radiation to form sound pressure waves which are detectable by means of the MEMS sensor, characterized in that the photoacoustic gas sensor was produced by a method according to claim 1.

16. The photoacoustic gas sensor according to claim 15 wherein the reference chamber forms a sealed system which is filled with the reference gas and a gas to be analyzed is present in the beam path between the emitter and the reference chamber, so that the proportion of the reference gas in the gas to be analyzed can be measured by means of the formation of sound pressure waves in the reference chamber.

17. The production method according to claim 1, wherein the MEMS device is a sound pressure detector, wherein the sound pressure detector comprises a capacitively or optically readable, piezoelectric, piezoresistive and/or magnetic bar and/or a capacitive, piezoelectric, piezoresistive and/or optical microphone.

18. The production method according to claim 2, wherein the corrosive and/or explosive gasses comprise methane, propane, propylene, silane, chlorosilane, hydrogen, oxygen or ammonia.

19. The production method of claim 6, wherein the valve is a non-ferrous metal selected from the group comprising lead, gold, indium, copper, platinum, silver, zinc, tin, aluminum and a compound thereof.

20. The production method of claim 10, wherein the physical coating system is a plasma assisted physical coating system or wherein the chemical coating system is a plasma-assisted chemical coating system.

21. The production method of claim 11, wherein the covering layer is applied around the entire reference chamber.

Description

DETAILED DESCRIPTION

[0238] In the following, the invention will be explained in more detail by means of examples, without being limited to them.

[0239] Short Description of the Images

[0240] FIG. 1A-B Schematic overview of a first preferred embodiment of the method according to the invention for forming a reference chamber filled with reference gas in which an opening remains on the contact surfaces of the wafers after bonding.

[0241] FIG. 2A-K Schematic illustration of preferred process steps of the first preferred embodiment of the method for producing a reference chamber filled with reference gas, in which an opening remains on the contact surfaces of the wafers after bonding.

[0242] FIG. 3A-B Schematic overview of a second preferred embodiment of the method according to the invention for forming a reference chamber filled with reference gas, which exhibits a valve.

[0243] FIG. 4A-B Schematic illustration of preferred process steps of the second preferred embodiment of the method according to the invention for producing a reference chamber filled with reference gas, which exhibits a valve.

[0244] FIG. 5A-B Schematic overview of a third preferred embodiment of the method according to the invention for forming a reference chamber filled with reference gas, the opening of which is sealed with a thermal solder.

[0245] FIG. 6A-H Schematic illustration of preferred process steps of the third preferred embodiment of the method according to the invention for producing a reference chamber filled with reference gas, the opening of which is sealed with a thermal solder.

DETAILED DESCRIPTION OF THE ILLUSTRATIONS

[0246] FIGS. 1A-B show a summary illustration of a first variant of the production method according to the invention.

[0247] A first wafer 1 and a second wafer 2 are provided, whereby both the first (upper) wafer 1 and the second (lower) wafer 2 each have a cavity 6. A MEMS device and/or an electronic circuit is present on the first 1 and/or second wafer 2 (for example within the cavities, but not shown).

[0248] The bonding of the first wafer 1 with the second wafer 2 takes place within a bonding chamber for forming a volume 7 which can be filled with reference gas 11, wherein an opening 9 remains on a contact surface 3 of the two wafers after bonding. The first wafer 1 and the second wafer 2 also preferably have contact surfaces 3 for this purpose, which are used for bonding the first wafer 1 to the second wafer 2, with a region on the contact surfaces 3 not being bonded in order to form the opening 9.

[0249] FIG. 1A illustrates on the right side the unbonded lateral region of the reference chamber, which allows an opening 9 to be provided. Flooding of a reference gas 11 into the reference chamber via the opening 9 can be performed as described within a coating system. In FIG. 1A, the flooding step is illustrated as an arrow.

[0250] In the preferred embodiment, the sealing of the opening 9 of the reference chamber within the coating system is performed by a coating process that applies a cover layer 12 at least over a region of the opening, as illustrated in FIG. 1B, preferably around the entire reference chamber. The cover layer 12 may preferably be a nitride that reliably hermetically seals the entire reference chamber.

[0251] FIG. 2A-K shows preferred process steps of a first variant of the method according to the invention for producing a reference chamber filled with reference gas 11, in which an opening remains on the contact surfaces 3 of the wafers after bonding.

[0252] As illustrated in FIG. 2A, in a first process step, the first wafer 1 can be sputtered on the rear side with a first bonding material 4. The first bonding material 4 can preferably be gold. Here, bonding material denotes a material which is preferably suitable for bonding.

[0253] FIG. 2 B illustrates a preferred structuring of the first bond material 4. The left-hand side shows a central cross-section through the reference chamber to be formed. The right-hand side additionally shows a top view. The structured first bonding material 4 forms an almost closed border, which however has a recess on the left side. The recess is not bonded, but serves to form the opening into the reference chamber.

[0254] A structuring of the first wafer 1 is performed by means of a photoresist 8 and etching processes, as illustrated in FIG. 2C to FIG. 2 F.

[0255] In FIG. 2 C, a photoresist 8 is applied to the front of the first wafer 1, leaving a region free for further processing. Said region of the first wafer 1 is etched via an etching process, preferably via deep reactive ion etching (DRIE), as shown in FIG. 2 D. The region of the first wafer 1 is then etched in the etching process. However, the first wafer is preferably not etched throughout. In FIG. 2 E, a photoresist 8 is applied to the rear side of the first wafer 1. It can be the same photoresist 8 as in the previous steps, or a different one. Starting from the rear side, an etching process (preferably DRIE) is performed again at the relevant location (see FIG. 2 F) to cut the first wafer 1 at the region previously etched on the front side. In addition, the rear-side etching process creates a cavity 6 in the first wafer 1, which can be used to form the volume 7 in the reaction chamber.

[0256] In FIG. 2 G, the first wafer 1 is bonded to the second wafer 2, on which a second bonding material 5 is located. The second wafer 2 also has a cavity 6 which is complementary to the cavity 6 of the first wafer 1. The second wafer 2 can be provided by analogous etching processes.

[0257] The second bonding material 5 can preferably be aluminum, copper and/or gold. The second bonding material 5 may, but need not, be present in a structured form. To avoid bonding at the opening 9 to be formed, it is sufficient that the first bonding material 4 of the first wafer 1 has a recess on the left side. Preferably, the first wafer 1 and the second wafer 2 are bonded together via thermo-compression bonding (TCbonding for short). However, the region on the left-hand side is not bonded at contact surfaces 3.

[0258] In this preferred embodiment of the process, the region at non-bonded contact surfaces is used as opening 9 to fill the reference chamber with a reference gas 11, e.g. ammonia. Thus, a reference chamber is produced which contains a volume 7 and has an opening 9. Thereby, the opening 9 is located in a lateral region of the reference chamber and is provided by the fact that no bonding process takes place in this region. In FIG. 2 G, the opening 9 can be seen in the lateral region of the reference chamber on the left. To the right of the illustration of the reference chamber, a top view of the first bonding material 4, preferably gold, is shown. Bonding processes take place in a bonding chamber, which is not illustrated.

[0259] In FIG. 2 H, the reference chamber is no longer located in the bonding chamber, but in a coating system, preferably in a PECVD system (Plasma-Enhanced Chemical Vapor Deposition). In this case, any gas with room pressure can initially be located in the volume 7 of the reference chamber, which in FIG. 2 I is pumped out within the coating system, so that a vacuum is then located in the volume 7 of the reference chamber. The person skilled in the art knows that in reality a vacuum is never absolute, but is characterized by a considerably lower pressure compared to atmospheric pressure under normal conditions.

[0260] In FIG. 2 J, the coating system is flooded with the reference gas 11 so that the reference gas 11, e.g. ammonia, enters the volume 7 of the reference chamber via the opening 9. In FIG. 2 K, the reference chamber is hermetically sealed within the coating system via a cover layer 12, in particular by depositing a nitride, as a result of which the opening 9 is also reliably sealed and therefore the reference gas 11 can no longer escape from the reference chamber.

[0261] FIG. 3A-B shows a summary illustration of a second variant of the method according to the invention for producing a reference chamber filled with reference gas 11. The first wafer 1 and the second wafer 2 are provided, both wafers 1 and 2 again exhibiting cavities 6. There is a MEMS device and/or an electronic circuit on the first 1 and/or the second wafer 2, e.g., within the cavities 6. However, these are not shown.

[0262] After bonding the two wafers 1 and 2, an opening 9 is etched into the first 1 or second wafer 2. A valve 14 is located at the end of the opening 9. The two wafers 1 and 2 are bonded together such that the valve 14 is located within the volume 7 of the reference chamber at the end of the opening 9. Preferably, the valve 14 is attached to the first 1 or the second wafer 2 before the bonding process. In contrast, during the bonding process itself, the contact surfaces 3 are used throughout to bond the first wafer 1 to the second wafer 2. After bonding the first wafer 1 with the second wafer 2, the reference chamber is brought out of the bonding chamber into the coating system, where it is flooded with the reference gas 11, the reference gas 11 entering the volume 7 of the reference chamber via the opening 9 and via the valve 14.

[0263] Finally, the cover layer 12, preferably with a nitride deposition, is used to hermetically seal the reference chamber, preferably over the entire region of the reference chamber.

[0264] This manufacturing process is particularly preferred if a process gas different from the reference gas 11 is used to apply the cover layer 12. Advantageously, the valve 14 already seals the reference gas 11 inside the reference chamber before coating with the cover layer 12, so that any gas exchange (from reference gas to process gas) cannot lead to contamination.

[0265] FIG. 4A-I show preferred process steps of the second variant of the method according to the invention for producing a reference chamber filled with reference gas 11.

[0266] In FIG. 4A, the first wafer 1 is coated on the rear side with a first bonding material 4. Preferably, the first bonding material 4 is gold and is coated onto the rear side of the first wafer 1 via a sputtering process.

[0267] In FIG. 4 B, the first bonding material 4 is patterned on the rear side of the first wafer 1. A top view of the first bonding material 4 is illustrated on the right. In FIG. 4 C, a photoresist 8 is applied to the front side of the first wafer 1. In FIG. 4 D, a material layer 13 is applied to the rear side of the first wafer 1. Preferably, a soft metal, especially preferably aluminum, is applied as a material layer 13 as a thin film to the back side of the first wafer 1 via a sputtering process. In FIG. 4 E, starting from the front side of the first wafer 1, an opening 9 is formed via an etching process, preferably via a dry etching process. In this process, the opening 9 is etched up to the material layer 13. Subsequently, in FIG. 4 F, the material layer 13 is structured to form a flexible valve 14.

[0268] In FIG. 4 G, the first wafer 1 is bonded to a second wafer 2, preferably via TC bonding (see above). The second bonding material 5 on the contact surfaces 3 of the second wafer 2 can preferably be gold, copper and/or aluminum. In FIG. 4 H, the reference chamber within a coating system, preferably within a PECVD system, is flooded with the reference gas 11 via the opening 9, wherein the valve 14 opens when the gas is introduced. In FIG. 4 I, the reference chamber is hermetically sealed via a cover layer 12, preferably via a nitride.

[0269] FIG. 5A-B shows a summary of a third variant of the production method according to the invention. Also in this third variant, the two wafers 1 and 2 have cavities 6, in which a MEMS device and/or an electronic circuit can be inserted. The MEMS device and/or the electronic circuit are not shown. The two wafers 1 and 2 are preferably bonded to each other at all contact surfaces 3 in a bonding chamber.

[0270] Preferably, the opening 9 is formed prior to bonding via an etching process, preferably dry etching. After bonding both wafers 1 and 2, a solder 15 is placed near the opening 9. After introducing the reference gas 11, e.g. ammonia, into the volume 7 of the reference chamber within the coating system, the solder 15 is melted so that it flows into the opening 9. The opening 9 is sealed as a result of the melting and the solder flowing into it. The reference chamber is hermetically sealed via a cover layer 12, preferably a nitride.

[0271] FIG. 6A-H shows preferred process steps of the third variant of the method according to the invention for producing a reference chamber filled with gas. In FIG. 6A, the first wafer 1 is provided and coated with the first bonding material 4 on its rear side. Preferably, the first bonding material 4 is gold and is applied to the rear side of the first wafer 1 via a sputtering process. In FIG. 6 B, the first bonding material 4 is structured on the rear side of the first wafer 1.

[0272] A top view of the structuring of the first bonding material 4 is illustrated on the right. In FIG. 6 C, a photoresist 8 is applied to the front side of the first wafer 1. In FIG. 6 D, starting from the front side, an opening 9 is formed in the first wafer 1 via an etching process, preferably via a dry etching process, particularly preferably via reactive ion deep etching. In FIG. 6 E, a second wafer 2 is bonded to the first wafer 1 at the contact areas 3. The second wafer comprises a second bonding material 5 at the contact surfaces 3, which is preferably gold, copper and/or aluminum. Preferably, the two wafers 1 and 2 are bonded together via TCbonding. After the bonding process, in FIG. 6 F, a thermal solder 15 is applied near the opening 9. Then, in FIG. 6 G, the reference chamber within the coating system is flooded with the reference gas 11, e.g., ammonia. Finally, in FIG. 6 H, the thermal solder 15 is melted to flow into and seal the opening 9.

[0273] Furthermore, the reference chamber is coated with a cover layer 12, preferably a nitride, so that the reference chamber is particularly well hermetically sealed.

LIST OF REFERENCE SIGNS

[0274] 1 First wafer [0275] 2 Second wafer [0276] 3 Contact surface [0277] 4 First bonding material [0278] 5 Second bonding material [0279] 6 Cavity [0280] 7 Volume [0281] 8 Photoresist [0282] 9 Opening [0283] 11 Reference gas [0284] 12 Cover layer [0285] 13 Material layer for structuring as a valve [0286] 14 Valve [0287] 15 Solder

BIBLIOGRAPHY

[0288] Bonilla-Manrique, Oscar E., et al. Sub-ppm-Level Ammonia Detection Using Photoacoustic Spectroscopy with an Optical Microphone Based on a Phase Interferometer. Sensors 19.13 (2019): 2890. [0289] Peng, W. Y., et al. High-sensitivity in situ QCLAS-based ammonia concentration sensor for high-temperature applications. Applied Physics B 122.7 (2016): 188. [0290] Schilt, St?phane, et al. Ammonia monitoring at trace level using photoacoustic spectroscopy in industrial and environmental applications. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 60.14 (2004): 3259-3268. [0291] Stemme, Goran, and Edvard Kalvesten. Micromachined gas-filled chambers and method of microfabrication. U.S. Pat. No. 6,124,145. 26 Sep. 2000.