Hot-melt sealing glass compositions and methods of making and using the same
View Patent ↗Hot-melt sealing glass compositions that include one or more glass frits dispersed in a polymeric binder system. The polymeric binder system is a solid at room temperature, but melts at a temperature of from about 35° C. to about 90° C., thereby forming a flowable liquid dispersion that can be applied to a substrate (e.g., a cap wafer and/or a device wafer of a MEMS device) by screen printing. Hot-melt sealing glass compositions according to the invention rapidly re-solidify and adhere to the substrate after being deposited by screen printing. Thus, they do not tend to spread out as much as conventional solvent-based glass frit bonding pastes after screen printing. And, because hot-melt sealing glass compositions according to the invention are not solvent-based systems, they do not need to be force dried after deposition.
1. A method of bonding a cap wafer to a device wafer so as to hermetically seal and isolate a MEMS device in a cavity defined therebetween, the method comprising:
providing a hot-melt sealing glass composition comprising:
from about 50% by weight to about 95% by weight of a glass component comprising one or more glass frits, and
from about 5% by weight to about 50% by weight of a polymeric binder system that is a solid at 22.5° C. but which melts at a temperature within the range of from 35° C. to 90° C.,
wherein the polymeric binder system is capable of complete burn-out at a temperature below 450° C., and
wherein the glass component is capable of melting and flowing at a temperature below 500° C.
heating the hot-melt sealing glass composition above the melting point of the polymeric binder system and below a temperature at which the polymeric binder system begins to substantially volatilize, to form a molten paste;
depositing the molten paste onto the cap wafer and/or the device wafer by screen printing;
allowing the deposited molten paste to re-solidify;
positioning the cap wafer and device wafer in relation to each other such that at least the glass component of the hot-melt sealing glass composition is positioned therebetween; and
firing the cap wafer and device wafer at a temperature above the melting point of the glass component to form a hermetic seal between the cap wafer and device wafer that isolates the MEMS device in the cavity defined therebetween.
2. The method of claim 1 , wherein the glass component comprises:
(a) from about 50 to about 96 wt % PbO+ZnO,
(b) from about 1 to about 18 wt % B 2 O 3 ,
(c) from about 2 to about 15 wt % SiO 2 , and
(d) from about 1 to about 10 wt % Al 2 O 3 .
3. The method of claim 2 , wherein the glass component further comprises at least one of (a) and (b):
(a) up to about 5 wt % MgO+BaO and
(b) up to about 10 wt % TiO 2 +Nb 2 O 5 +ZrO 2 .
4. The method of claim 1 , wherein the glass component comprises:
(a) from about 70 to about 96 wt % PbO+Bi 2 O 3 ,
(b) from about 3 to about 30 wt % B 2 O 3 , and
(c) from about 1 to about 5 wt % SiO 2 .
5. The method of claim 1 , wherein the glass component comprises:
(a) from about 50 to about 96 wt % Bi 2 O 3 +ZnO and
(b) from about 1 to about 30 wt % B 2 O 3 .
6. The method of claim 1 , wherein the glass component is lead-free and cadmium-free, and comprises:
(a) from about 20 to about 55 wt % SnO,
(b) from about 25 to about 37 wt % ZnO,
(c) from about 10 to about 45 wt % P 2 O 5 , and
(d) from about 0.5 to about 15 wt % Li 2 O+Na 2 O+K 2 O+Rb 2 O.
7. The method of claim 1 , wherein the polymeric binder comprises a C14 or higher linear primary alcohol.
8. The method of claim 1 , wherein the polymeric binder comprises n-cetyl alcohol.
9. The method of claim 1 , wherein the polymeric binder further comprises acrylic polymers.
10. The method of claim 9 , wherein the acrylic polymers include isobutyl methacrylate polymers.
11. The method of claim 1 , wherein the average particle size of the one or more glass frits in the glass component is from 3microns to 10 microns.
12. The method of claim 1 , wherein the hot melt sealing glass composition does not contain cellulose-ethers.
13. The method of claim 1 , wherein the device wafer is formed of silicon or GaAs.
14. The method of claim 1 , wherein the cap wafer is formed of silicon, GaAs or soda-lime silica glass.
15. The method of claim 1 , wherein the hot melt sealing glass composition does not release volatile organic compounds during the heating, depositing, allowing and positioning steps.
16. The method of claim 1 , wherein organic compounds present in the hot melt sealing composition do not evaporate or volatilize until the firing step.
17. The method of claim 1 , further comprising the step of subjecting the device wafer to further processing and handling between the allowing step and the positioning step.
18. The method of claim 1 , further comprising, between the allowing step and the positioning step, a pre glazing step in which the device wafer and/or the cap wafer on which the hot melt sealing composition has re-solidified is heated to a pre glazing temperature and then allowed to cool.
19. The method of claim 18 , further comprising, between the positioning step and the firing step, a back filling step in which the cavity between cap wafer and the device wafer is filled with a desired gas.