Protection capsule for MEMS devices
View Patent ↗A method of making a MEMS device is disclosed wherein anhydrous HF exposed silicon nitride is used as a temporary adhesion layer to permit the transfer of a layer from a carrier substrate to a receiving substrate.
1. A method of making a MEMS device, comprising:
depositing a silicon nitride layer onto a silicon carrier wafer;
exposing the silicon nitride layer to anhydrous hydrofluoric acid to transform a surface sublayer of silicon nitride layer into ammonium fluorosilicate;
forming multiple protective capsules on said ammonium fluorosilicate sublayer;
bringing said silicon carrier wafer into contact with a MEMS wafer such that said mutiple protective components are aligned with respective mechanical components formed on the MEMS wafer;
applying pressure to said carrier wafer and said MEMS wafer to bond said protective components to said MEMS wafer; and
subsequently heating the bonded wafers to a temperature sufficient to decompose the ammonium silicate and thereby release the carrier wafer from the protective components, whereby the ammonium silicate sublayer is used as a temporary adhesion layer to permit the transfer of protective components from the carrier wafer to the MEMS wafer.
2. The method of claim 1 , wherein the protective components are formed by pattering at least one photopolymer layer.
3. The method of claim 1 , wherein the exposure of the silicon nitride layer to anhydrous hydrofluoric acid takes place in a sub-atmospheric pressure mixture of anhydrous hydrofluoric acid and an organic volatile.
4. The method of claim 3 , wherein the organic volatile is selected from the group consisting of: methanol, acetone, 1-buthanol, 2-buthanol, 1-propanol and 2-propanol.
5. The method of claim 2 , wherein the or each photopolymer layer is a negative tone photopolymer.
6. The method of claim 5 , wherein the negative tone photopolymer is an epoxy-like negative tone photopolymer.
7. The method of claim 5 , wherein the photopolymer thickness ranges between 5 μm and 500 μm.
8. The method of claim 7 , wherein the photopolymer is exposed using a UV source to create the protective components.
9. The method of claim 8 , wherein the UV source is highly collimated to achieve high aspect ratio features.
10. The method of claim 9 , wherein the exposed photopolymer is subjected to a post-exposure heat treatment not exceeding 99° C.
11. The method of claim 2 , wherein the protective components comprise a protective capsule and a sealing ring surrounding the protection capsule and designed to space the protective capsule from moving parts within the MEMS device, and wherein a first layer of photopolymer is applied to the carrier wafer to pattern the protective capsules, and a second layer of photopolymer is applied over the first layer of photopolymer to pattern the sealing ring.
12. The method of claim 11 , wherein each photopolymer is patterned by selective exposure to ultraviolet light, and subsequently developed to reveal the protection capsule and sealing ring.
13. The method of claim 12 , wherein prior to exposure to ultraviolet light, the photolpolymers are subject to a pre-exposure bake at less than 95° C. for stabilization.
14. The method of claim 11 , wherein the silicon nitride layer is exposed to anhydrous hydrofluoric acid at sub-atmospheric pressure.
15. The method of claim 14 , wherein the exposure of silicon nitride to anhydrous hydrofluoric acid occurs in the presence of organic volatiles.
16. The method of claim 11 , wherein after release of the carrier substrate, the MEMS wafer is subjected to vacuum and heat to polymerize the protective capsules.