Micromachined assembly with a multi-layer cap defining a cavity

• US 20040173886A1
• Filed: 03/07/2003
• Published: 09/09/2004
• Est. Priority Date: 03/07/2003
• Status: Active Grant

• Current Assignee: Cymatics Laboratories Corp.
• Original Assignee: Cymatics Laboratories Corp.
• Inventors: Carley, L. Richard
• Granted Patent: US 7,492,019 B2
• Time in Patent Office: Days
• US Class Current: 257/678
• CPC Class Codes: B81B 2207/015 the micromechanical device and the control or processing electronics being integrated on the same substrate
• CPC Class Codes: B81C 1/00333 Aspects relating to packaging of MEMS devices, not covered by groups B81C1/00269 - B81C1/00325
• CPC Class Codes: B81C 2203/0136 Growing or depositing of a covering layer
• CPC Class Codes: B81C 2203/0145 Hermetically sealing an opening in the lid

Abstract

This invention comprises a process for fabricating a micro mechanical structure in a sealed cavity having a multi-layer high stiffness cap. The high stiffness material used for the cap protects the underlying microstructure from destructive environmental forces inherent in the packaging process and from environmental damage.

Claims

1. A method of creating a sealed cavity on a wafer comprising the steps of:

depositing one or more layers of a sacrificial material on said wafer and shaping said sacrificial material;

depositing a first cap layer on said one or more layers of sacrificial material;

removing said one or more layers of sacrificial material such that said first cap layer and the portion of said wafer underlying said first cap layer define said cavity; and

depositing one or more additional cap layers on said first cap layer, said one or more additional cap layers being formed of a high stiffness material;

wherein said one or more cap layers contact said wafer.

2. The method of claim 1 wherein said high stiffness material is selected from a group consisting of ionic bonded materials, covalently bonded material and or mixed ionic and covalent bonded materials.

3. The method of claim 1 wherein said deposited high stiffness material can withstand pressures above 600 psi and temperatures up to 300 degrees C. when the total thickness of all deposited layers is less than approximately 50 microns.

4. The method of claim 1 wherein said deposited high-stiffness material can withstand a uniform pressure of up to 60 atmospheres without deflecting more than one micron from its original position.

5. The method of claim 1 wherein said high stiffness material is alumina.

6. The method of claim 1 wherein said step of removing said one or more layers of said sacrificial material comprises the steps of:

creating one or more etchant access holes in said first cap layer; and

introducing etchant to said one or more sacrificial layers through said one or more access holes.

7. The method of claim 1 wherein said step of removing said one or more layers of said sacrificial material comprises the steps of:

creating a plurality of pillars through said one or more layers of sacrificial material; and

depositing said first cap layer such that first cap layer is supported by said plurality of pillars after said sacrificial material is removed.

8. The method of claim 1 further comprising the step of removing a portion of the surface of said wafer, such that said wafer is a non-planar surface, wherein the non-planar area of said wafer and said high stiffness cap layers define said cavity.

9. The method of claim 1 wherein at least one of said high stiffness layers is deposited in a sputtering machine in the presence of a bias voltage.

10. The method of claim 9 wherein said one or more high stiffness layers have a relatively low intrinsic stress gradient and good adhesion at the boundary between said one or more layers of high stiffness material and said wafer.

11. The method of claim 10 wherein said step of depositing one or more overlayers of a high stiffness material further comprises the steps of:

depositing a first layer of high stiffness material in the presence of a bias voltage; and

depositing one or more additional layers of high stiffness material.

12. The method of claim 11 wherein said step of depositing one or more additional layers of high stiffness material further comprises the steps of:

depositing a second layer of high stiffness material with no bias voltage; and

depositing a third layer of high stiffness material in the presence of a bias voltage.

13. The method of claim 1 wherein said one or more layers of high stiffness material are deposited by sputtering at a pressure in the range of 2 mTorr to 100 mTorr.

14. The method of claim 12 wherein said one or more layers of high stiffness material are deposited under a pressure of 30 mTorr.

15. The method of claim 1 wherein said first cap layer is deposited at a relatively low pressure in the presence of an inert gas, such that said sealed cavity is filled with said inert gas at a pressure of 10 mT or less.

16. The method of claim 1 wherein said first cap layer is also composed of a high stiffness material.

17. The method of claim 1 wherein said high stiffness material is selected from a group consisting of alumina, titanium oxide, indium tin oxide, zirconium oxide, yttrium stabilized zirconium oxide, titanium nitride, zirconium nitride, cubic boron nitride, aluminum nitride, titanium boride, zirconium boride, titanium carbide, tungsten carbide, vanadium carbide, boron carbide, zirconium carbide, niobium boride, carbide, silicon carbide, strontium titanate, tantalum carbide, cerium oxide, chromium boride, chromium oxide, beryllium oxide, scandium oxide, tungsten and tungsten alloys, magnesium oxide, mullite, diamond, cordierite, ferrite and garnet.

18. The method of claim 1 further comprising the step of depositing an additional layer of material over the outermost of said layers of high stiffness material.

19. The method of claim 18 wherein said additional layer is composed of silicon nitride.

20. The method of claim 1 further comprising the step of forming a microstructure within said sealed cavity.

21. A micro-sealed cavity comprising:

a wafer;

a cap structure covering at least a portion of said wafer, said cap structure contacting said wafer to define a cavity thereunder;

wherein said cap structure is composed of a high stiffness material.

22. The micro-sealed cavity of claim 21 wherein said high stiffness material is selected from a group consisting of ionic bonded materials, covalently bonded material and or mixed ionic and covalent bonded materials.

23. The micro-sealed cavity of claim 21 wherein said cap structure can withstand pressures above 600 psi and temperatures up to 300 degrees C. when the total thickness of all deposited layers is less than approximately 50 microns.

24. The micro-sealed cavity of claim 21 wherein said cap structure can withstand a uniform pressure of up to 60 atmospheres without deflecting more than one micron from its original position.

25. The micro-sealed cavity of claim 21 wherein said high stiffness material is alumina.

26. The micro-sealed cavity of claim 21 wherein said cap structure comprises one or more layers of said high stiffness material.

27. The micro-sealed cavity of claim 26 wherein said cap structure is a multi-layered structure and further wherein at least one of said layers of said multi-layered structure was deposited in the presence of a bias voltage.

28. The micro-sealed cavity of claim 27 wherein said cap structure comprises:

a first layer of high stiffness material deposited in the presence of a bias voltage;

a second layer of high stiffness material deposited with no bias voltage; and

a third layer of high stiffness material deposited in the presence of a bias voltage.

29. The micro-sealed cavity of claim 26 wherein said one or more layers of high stiffness material are deposited under a pressure in the range of 2 mTorr to 100 mTorr.

30. The micro-sealed cavity of claim 29 wherein said one or more layers of high stiffness material are deposited under a pressure of 30 mTorr.

31. The micro-sealed cavity of claim 21 wherein said first layer of high stiffness material is deposited at a low pressure in the presence of an inert gas, such that said micro-sealed cavity is filled with said inert gas at a pressure of 10 mTorr or less.

32. The micro-sealed cavity of claim 21 wherein said cap structure has a low intrinsic stress gradient and good adhesion to said wafer at the point of contact between said cap structure and said wafer.

33. The micro-sealed cavity of claim 27 wherein said cap structure has a low intrinsic stress gradient and good adhesion to said wafer at the point of contact between said cap structure and said wafer.

34. The micro-sealed cavity of claim 21 wherein said high stiffness material is selected from a group consisting of alumina, titanium oxide, indium tin oxide, zirconium oxide, yttrium stabilized zirconium oxide, titanium nitride, zirconium nitride, cubic boron nitride, aluminum nitride, titanium boride, zirconium boride, titanium carbide, tungsten carbide, vanadium carbide, boron carbide, zirconium carbide, niobium boride, carbide, silicon carbide, strontium titanate, tantalum carbide, cerium oxide, chromium boride, chromium oxide, beryllium oxide, scandium oxide, tungsten and tungsten alloys, magnesium oxide, mullite, diamond, cordierite, ferrite and garnet.

35. The micro-sealed cavity of claim 21 further comprising an additional layer deposited on the outermost layer of high stiffness material, said additional layer providing a seal of said micro-sealed cavity.

36. The micro-sealed cavity of claim 35 wherein said additional layer is composed of silicon nitride.

37. The micro-sealed cavity of claim 21 further comprising a microstructure defined within said micro-sealed cavity.

38. In a wafer for containing a microstructure, an improvement comprising:

depositing one or more layers of high stiffness material over said microstructure to define a sealed cavity between said wafer and said one or more layers of high stiffness material.

39. The improvement of claim 38 wherein said one or more layers of high stiffness material are deposited directly on a layer of sacrificial material, said sacrificial material being subsequently removed.

40. The improvement of claim 39 wherein said one or more layers of high stiffness material are deposited by a method comprising the steps of:

depositing a first layer of high stiffness material over said sacrificial material;

removing said sacrificial material; and

optionally depositing one or more subsequent layers of high stiffness material over said first layer of high stiffness material.

41. The improvement of claim 40 wherein said first layer of high stiffness material is supported on said wafer and wherein said step of removing said sacrificial material comprises the steps of:

etching one or more holes in said first layer of high stiffness material through which an etching agent is introduced to etch away said sacrificial material;

wherein said one or more subsequent layers of high stiffness material serve to seal said one or more holes in said first layer of high stiffness material.

42. The improvement of claim 38 wherein said high stiffness material is selected from a group consisting of ionic bonded materials, covalently bonded material and or mixed ionic and covalent bonded materials.

43. The improvement of claim 38 wherein said high stiffness material is selected from a group consisting of alumina, titanium oxide, indium tin oxide, zirconium oxide, yttrium stabilized zirconium oxide, titanium nitride, zirconium nitride, cubic boron nitride, aluminum nitride, titanium boride, zirconium boride, titanium carbide, tungsten carbide, vanadium carbide, boron carbide, zirconium carbide, niobium boride, carbide, silicon carbide, strontium titanate, tantalum carbide, cerium oxide, chromium boride, chromium oxide, beryllium oxide, scandium oxide, tungsten and tungsten alloys, magnesium oxide, mullite, diamond, cordierite, ferrite and garnet.

44. The improvement of claim 38 wherein said high stiffness material is alumina.

45. The improvement of claim 38 wherein said one or more layers of high stiffness material can withstand pressures above 600 psi and temperatures up to 300 degrees C. when the total thickness of all deposited layers is less than approximately 50 microns.

46. The improvement of claim 38 wherein said one or more layers of high stiffness material can withstand a uniform pressure of up to 60 atmospheres without deflecting more than one micron from its original position.

47. The improvement of claim 38 further comprising a seal layer over the outermost layer of said high-stiffness material.

48. The improvement of claim 47 where said seal layer is composed of silicon nitride.

49. The improvement of claim 40 wherein said first layer of high stiffness material is supported on said wafer by a plurality of pillars formed through said sacrificial material and wherein said step of removing said sacrificial material comprises the steps of:

exposing said wafer to an etching agent;

allowing said etching agent to etch away said sacrificial material between said plurality of pillars, thereby forming access vias through which said etching agent can etch said sacrificial material disposed under said first layer of high stiffness material;

wherein said one or more subsequent layers of high stiffness material serve to seal said access vias formed between said plurality of pillars.

50. The improvement of claim 40 wherein said first layer of high stiffness material is deposited in the presence of a first bias voltage.

51. The improvement of claim 50 wherein said first bias voltage is approximately 90-200 volts.

52. The improvement of claim 50 wherein a second layer of high stiffness material is deposited with no bias voltage.

53. The improvement of claim 52 wherein a third layer of high stiffness material is deposited in the presence of a second bias voltage.

54. The improvement of claim 53 wherein said second bias voltage is approximately 90-200 volts.

55. The improvement of claim 40 wherein all layers of high stiffness material are deposited under a pressure in the range of 2 mTorr to 100 mTorr.

56. The improvement of claim 55 wherein all layers of high stiffness material are deposited under a pressure of approximately 30 mTorr.

57. The improvement of claim 40 wherein said first layer of said one or more subsequent layers of high-stiffness material is deposited at a low pressure in the presence of an inert gas, such that said micro-sealed cavity is filled with said inert gas at a pressure of 10 mTorr or less.

58. The improvement of claim 38 wherein said one or more layers or high stiffness material are deposited on a cap layer, said cap layer defining said cavity.

59. The improvement of claim 58 wherein said one or more layers of high stiffness material are deposited by a method comprising the steps of:

removing said sacrificial material; and

depositing one or more layers of high stiffness material over said cap layer.

60. The improvement of claim 59 wherein said cap layer is supported on said wafer and wherein said step of removing said sacrificial material comprises the steps of:

etching one or more holes in said cap layer through which an etching agent is introduced to etch away said sacrificial material;

wherein said one or more subsequent layers of high stiffness material seal said one or more holes in said cap layer.

61. The improvement of claim 59 wherein said cap layer is supported on said wafer by a plurality of pillars formed through said sacrificial material and wherein said step of removing said sacrificial material comprises the steps of:

exposing said wafer to an etching agent;

allowing said etching agent to etch away said sacrificial material between said plurality of pillars, thereby forming access vias through which said etching agent can etch said sacrificial material disposed under said cap layer;

wherein said one or more layers of high stiffness material serve to seal said access vias formed between said plurality of pillars.

62. The improvement of claim 58 wherein said cap layer is composed of a material selected from the group comprising silicon nitride and aluminum.

63. The improvement of claim 59 wherein said step of depositing one or more layer of high stiffness material comprises the steps of:

depositing a first layer of high stiffness material in the presence of a first bias voltage;

depositing a second layer of high stiffness material; and

depositing a third layer of high stiffness material in the presence of a second bias voltage.

64. The improvement of claim 63 wherein said first bias voltage is approximately 90-200 v and said second bias voltage is approximately 50-100v.

65. The improvement of claim 59 wherein said one or more layers of high stiffness material are deposited under a pressure in the range of 2 mTorr to 100 mTorr.

66. The improvement of claim 65 wherein said one or more layers of high stiffness material are deposited under a pressure of approximately 30 mTorr.

67. The improvement of claim 59 wherein the first layer of said one or more layers of high-stiffness material is deposited at a low pressure in the presence of an inert gas, such that said micro-sealed cavity is filled with said inert gas at a pressure of 10 mTorr or less.

69. The method of claim 61 wherein said material forming said first cap layer is also a high stiffness material.

68. A method of fabricating an encapsulated micromachined assembly, comprising the steps of:

providing a substrate;

depositing a first layer of sacrificial material on said substrate;

forming a microstructure of a desired shape on said first layer of sacrificial material;

depositing a second layer of sacrificial material on said first layer of sacrificial material, said second layer covering said microstructure;

depositing a first cap layer on top of said first and said second layers of sacrificial material;

removing said first and said second layers of sacrificial materials; and

depositing one or more additional cap layers on top of said first cap layer, said one or more additional cap layers being formed of a high stiffness material.

70. The method of claim 68 further comprising the step of depositing a seal layer over the outermost one of said one or more additional cap layers.

71. The method of claim 68 wherein said high stiffness material is selected from a group consisting of ionic bonded materials, covalently bonded material and or mixed ionic and covalent bonded materials.

72. The method of claim 68 wherein said one or more cap layers can withstand pressures above 600 psi and temperatures up to 300 degrees C. when the total thickness of all deposited cap layers is less than approximately 50 microns.

73. The method of claim 68 wherein said one or more cap layers can withstand a uniform pressure up to 60 atmospheres without deflecting more than one micron from its original position.

74. A MEMS device comprising:

a substrate;

a microstructure formed on said substrate;

a cap covering said microstructure; and

one or more additional layers of high stiffness material covering said cap.

75. The MEMS device of claim 74 wherein the first layer of high stiffness material is deposited in the presence of a first bias voltage.

76. The MEMS device of claim 74 wherein the second layer of high stiffness material is deposited in the absence of a bias voltage.

77. The MEMS device of claim 74 where the third layer of high stiffness material is deposited in the presence of a second bias voltage.

78. The MEMS device of claim 74 further comprising an outer seal layer, said outer seal layer covering the outermost layer of high-stiffness material.

79. The MEMS device of claim 74 wherein said high stiffness material is selected from a group consisting of ionic bonded materials, covalently bonded material and or mixed ionic and covalently bonded materials.

80. The MEMS device of claim 74 wherein said one or more layers of high stiffness material can withstand pressures above 600 psi and temperatures up to 300 degrees C. when the total thickness of all deposited layers is less than approximately 50 microns.

81. The MEMS device of claim 74 wherein said one or more layers of high-stiffness material can withstand a uniform pressure of unto 60 atmospheres without deflecting more than one micron from its original position.

82. A micromachined assembly comprising:

a substrate having a support surface on one side;

a cap layer extending from points on said support surface disposed about a region of interest on said support surface and including a portion overlying said region of interest;

a cap overlayer extending from said support surface and disposed over and contiguous with said cap layer;

whereby said cap layer and said cap overlayer and said support surface define a closed capsule about an interior region containing said region of interest; and

wherein at least one of said cap layer and said cap overlayer is formed of a high stiffness material.

83. A micromachined assembly according to claim 82, wherein said high stiffness of said material is sufficient to allow said cap layer and said cap overlayer to withstand a uniform pressure of up to about 60 atmospheres, without any portion of said cap layer and said cap overlayer deflecting more than one micron compared to their original position.

84. A micromachined assembly according to claim 82, wherein said cap layer and said cap overlayer comprise a thin film that is deposited onto said substrate from a gaseous state.

85. A micromachined assembly according to claim 82 wherein said support surface is substantially planar.

86. A micromachined assembly according to claim 82 further comprising a microstructure disposed on said region of interest of said support surface.

87. A micromachined assembly according to claim 86 wherein said microstructure is a micro-electro-mechanical system (MEMS).

88. A micromachined assembly according to claim 86 wherein said microstructure is a SAW (surface acoustic wave) device.

89. A micromachined assembly according to claim 86, wherein said microstructure is a Film Bulk Acoustic Resonator.

90. A micromachined assembly according to claim 86, wherein said microstructure is a capacitive sense plate adapted to measure ambient pressure due to a variation in the spacing between said support surface and the inner surface of said cap layer.

91. A micromachined assembly according to claim 86 wherein said microstructure is an integrated circuit (IC).

92. A micromachined assembly according to claim 82 wherein said cap layer includes a top portion distalmost from said support surface, said top portion extending in a direction substantially parallel to said support surface and wherein said cap layer includes a lateral portion extending between said top portion and said support surface.

93. A micromachined assembly according to claim 82 wherein said cap layer includes one or more ports extending therethrough in a direction substantially transverse to a normal to said support surface.

94. A micromachined assembly according to claim 93 wherein said cap overlayer is disposed within said one or more ports.

95. A micromachined assembly according to claim 93 wherein said cap overlayer is disposed over and around said one or more ports.

96. A micromachined assembly according to claim 93, wherein said cap layer includes:

a top portion distalmost from said support surface, said top portion extending in a direction substantially parallel to said support surface; and

a lateral portion extending between said top portion and said support surface; and

wherein said ports are disposed in said lateral portion of said cap layer.

97. A micromachined assembly according to claim 93, wherein said cap layer includes a top portion distalmost from said device support surface, said top portion extending in a direction substantially parallel to said device support surface;

and wherein said ports are disposed in said top portion of said cap layer.

98. A micromachined assembly according to claim 82, wherein said interior region is substantially filled with a noble gas.

99. A micromachined assembly according to claim 98, wherein said noble gas is at a pressure in the approximate range 0.01-10 Torr.

100. A micromachined assembly according to claim 82, wherein said cap overlayer is a multilayer structure having an innermost layer contiguous with said cap layer.

101. A micromachined assembly according to claim 100, wherein at least the lowermost layer within said multilayer structure is a relatively high energy RF sputtered material.

102. A micromachined assembly according to claim 101, wherein at least two adjacent layers of said cap overlayer are alumina layers.

103. A micromachined assembly according to claim 102 wherein said adjacent alumina layers have material characteristics corresponding to laydown under different conditions.

104. A micromachined assembly according to claim 103 wherein at least one of said adjacent alumina layers comprises an RF-sputtered alumina layer.

105. A micromachined assembly according to claim 82 wherein said cap overlayer is a graded density structure having a relatively high density region contiguous with said cap layer.

106. A micromachined assembly according to claim 105 wherein said cap overlayer is a relatively high energy sputtered material.

107. A micromachined assembly according to claim 106, wherein said relatively high energy sputtered material is selected from a group consisting of alumina, titanium oxide, indium tin oxide, zirconium oxide, yttrium stabilized zirconium oxide, titanium nitride, zirconium nitride, cubic boron nitride, aluminum nitride, titanium boride, zirconium boride, titanium carbide, tungsten carbide, vanadium carbide, boron carbide, zirconium carbide, niobium boride, carbide, silicon carbide, strontium titanate, tantalum carbide, cerium oxide, chromium boride, chromium oxide, beryllium oxide, scandium oxide, tungsten and tungsten alloys, magnesium oxide, mullite, diamond, cordierite, ferrite and garnet.

108. A micromachined assembly according to claim 82 wherein said substrate is a CMOS structure with said device support surface being a passivation layer, and includes CMOS circuit devices defined within said substrate between said device support surface and said base surface.

109. A micromachined assembly according to claim 82 wherein said substrate includes one or more ports extending therethrough in a direction substantially transverse to a normal to said support surface.

110. A micromachined assembly, comprising:

a substrate having a support surface on one side and a base surface on a side opposite said support surface;

a microstructure disposed on said support surface;

a sputter-deposited cap layer extending from points on said support surface and disposed over at least a portion of said microstructure, said cap layer and said device support surface defining a capsule about an interior region containing said microstructure, said cap layer being formed of a material characterized by a high stiffness.

111. A micromachined assembly according to claim 110 wherein said microstructure is a micro-electro-mechanical system (MEMS).

112. A micromachined assembly according to claim 110 wherein said microstructure is at least one of a micro-electro-mechanical system (MEMS), a surface acoustic wave (SAW) device, a Film Bulk Acoustic Resonator, an integrated circuit (IC), and a capacitive sense plate adapted to measure ambient pressure due to a variation in the spacing between said substrate and the inner surface of said cap layer.

113. A micromachined assembly according to claim 110 wherein said cap layer includes one or more ports extending therethrough in a direction transverse to a normal to said support surface.

114. A micromachined assembly according to claim 110, wherein said cap layer is a multilayer structure.

115. A microstructure comprising:

a wafer,

a cap, said cap contacting said wafer around a closed perimeter to define a cavity between said wafer and said cap;

wherein said cap is composed of a plurality of layers.

116. The microstructure of claim 115 wherein individual ones of said plurality of layers in said cap are composed of different materials

117. The microstructure of claim 115 wherein individual ones of said layers in said cap have been deposited under one or more different deposition parameters.

118. The microstructure of claim 117 wherein said deposition parameters are selected from a group consisting of temperature, bias voltage and pressure in a sputter deposition apparatus.

119. The microstructure of claim 117 wherein the first of said plurality of layers to be deposited is deposited using said parameters selected to result in a good adhesion between said first layer and said wafer.

120. The microstructure of claim 117 wherein said plurality of layers is deposited using said parameters selected to promote conformity from layer to layer.

121. The microstructure of claim 120 wherein at least one of said layers are deposited in the presence of a relatively high bias voltage between a sputter source and said wafer.

122. The microstructure of claim 115 wherein the first of said plurality of layers to be deposited is deposited over a layer of sacrificial material, said sacrificial material being subsequently removed.

123. The microstructure of claim 115 wherein said plurality of layers have been sputter deposited.

124. The microstructure of claim 115 wherein at least one layer of said plurality of layers is composed of a material selected from a group consisting of ionic bonded materials, covalently bonded materials and mixed ionic and covalently bonded materials.

125. The microstructure of claim 115 wherein at least one layer of said plurality of cap layers is composed of a relatively high stiffness material selected from a group consisting of alumina, titanium oxide, indium tin oxide, zirconium oxide, yttrium stabilized zirconium oxide, titanium nitride, zirconium nitride, cubic boron nitride, aluminum nitride, titanium boride, zirconium boride, titanium carbide, tungsten carbide, vanadium carbide, boron carbide, zirconium carbide, niobium boride, carbide, silicon carbide, strontium titanate, tantalum carbide, cerium oxide, chromium boride, chromium oxide, beryllium oxide, scandium oxide, tungsten and tungsten alloys, magnesium oxide, mullite, diamond, cordierite, ferrite and garnet.

126. The microstructure of claim 115 wherein said cap can withstand pressures above 600 psi and temperatures up to 300 degrees C. when the total thickness of said plurality of layers is less than approximately 50 microns.

127. The microstructure of claim 115 wherein said cap can withstand a uniform pressure of up to 60 atmospheres without deflecting more than one micron from its original position.

128. The microstructure of claim 115 further comprising a micro-electro-mechanical device disposed in said cavity.

129. A method of creating a sealed micro cavity on a wafer comprising the steps of:

depositing one or more layers of a sacrificial material on said wafer;

depositing a plurality of cap layers over said sacrificial material, individual ones of said plurality of cap layers being defined by a change in one or more deposition parameters or a change in material between one layer and the next.

130. The method of claim 129 further comprising the step of removing said sacrificial material.

131. The method of claim 129 wherein said deposition parameters are selected from a group consisting of temperature, bias voltage and pressure.

132. The method of claim 131 wherein at least one of said layers is deposited using a high bias voltage.

133. The method of claim 129 wherein at least one layer of said plurality of cap layers is composed of a material selected from a group consisting of ionic bonded materials, covalently bonded materials and mixed ionic and covalently bonded materials.

134. The improvement of claim 129 wherein at least one layer of said plurality of cap layers is composed of a high stiffness material selected from a group consisting of alumina, titanium oxide, indium tin oxide, zirconium oxide, yttrium stabilized zirconium oxide, titanium nitride, zirconium nitride, cubic boron nitride, aluminum nitride, titanium boride, zirconium boride, titanium carbide, tungsten carbide, vanadium carbide, boron carbide, zirconium carbide, niobium boride, carbide, silicon carbide, strontium titanate, tantalum carbide, cerium oxide, chromium boride, chromium oxide, beryllium oxide, scandium oxide, tungsten and tungsten alloys, magnesium oxide, mullite, diamond, cordierite, ferrite and garnet.