Method and structures for fabricating MEMS devices on compliant layers
Methods and structures for fabricating MEMS devices on compliant layers are provided. In particular, disclosed are methods and structures that can include the use of a sacrificial layer composed of a material having material properties relative to one or more other layers. These methods and structures can reduce final device shape sensitivity to process parameters, deposition temperature differences, specific material, time, and/or geometry. Further, such methods and structures can improve the final as-built shape of released devices, reduce variability in the as-built shape, eliminate decoupling of the deposited layers from the substrate, and reduce variability across a product array, die, or wafer.
1. A method for fabricating a suspended, tri-layer MEMS structure, the method comprising:
providing a substrate;
depositing a sacrificial layer on the substrate;
depositing a first film layer on the substrate and over the sacrificial layer;
depositing a second film layer on the first film layer;
depositing a third film layer on the second layer; and
removing the sacrificial layer by etching to suspend at least a portion of the tri-layer structure in a spaced-apart manner over the substrate;
wherein depositing the sacrificial layer comprises identifying a stiffness of the substrate and of one or more of the first, second, and third film layers, and selecting a material for the sacrificial layer having a stiffness that is greater than the stiffness of the substrate and of the one or more of the first, second, and third film layers.
2. The method of claim 1 , wherein depositing the sacrificial layer comprises selecting a material for the sacrificial layer having a stiffness that is substantially similar to the stiffness of one or more of the substrate and the first, second, and third film layers.
3. The method of claim 1 , wherein depositing the sacrificial layer comprises identifying a deposition temperature of one or more of the first, second, and third film layers, and selecting a material for the sacrificial layer having a characteristic temperature that is greater than the deposition temperature of the one or more of the first, second, and third film layers.
4. The method of claim 1 , wherein depositing the sacrificial layer comprises identifying a coefficient of thermal expansion of one or more of the first, second, and third film layers, and selecting a material for the sacrificial layer having a coefficient of thermal expansion that is substantially similar to the coefficient of thermal expansion of the one or more of the first, second, and third film layers.
5. The method of claim 1 , wherein depositing the sacrificial layer comprises depositing a layer of material having a thickness that is less than the thickness of the first film layer.
6. The method of claim 1 , wherein the sacrificial layer has sacrificial properties and depositing the sacrificial layer comprises identifying an elastic modulus of the first film layer, and selecting a material for the sacrificial layer having an elastic modulus and a thickness in a specific relationship to the elastic modulus and thickness of the first film layer, and wherein the sacrificial properties are related to properties of the first film layer by the relationship
(
1
+
v
sac
)
E
film
2
E
sac
t
film
L
t
sac
L
<<
1
,
where E film is the elastic modulus of the first film layer, E sac is the elastic modulus of the sacrificial layer, t film is the thickness of the first film layer, t sac is the thickness of the sacrificial layer, and ν sac is the Poisson ratio of the sacrificial layer.
7. The method of claim 6 , wherein depositing the sacrificial layer comprises selecting a material for the sacrificial layer having specific material properties that are related to material properties of one or more of the film layers, wherein the specific material properties are selected from the group consisting of the sacrificial elastic modulus, Poisson ratio, coefficient of thermal expansion, thickness, characteristic length, characteristic temperature, yield, and viscoelastic properties.
8. The method of claim 1 , wherein the sacrificial layer is deposited at specified temperatures, CTE, and elastic modulus to elicit yielding of one or more of the first, second, and third film layers.
9. A method for fabricating a suspended, single-layer MEMS structure, the method comprising:
providing a substrate;
depositing a sacrificial layer on the substrate;
depositing a film layer on the substrate and over the sacrificial layer; and
removing the sacrificial layer by etching to suspend at least a portion of the film layer in a spaced-apart manner over the substrate;
wherein depositing the sacrificial layer comprises identifying a stiffness of the substrate and of the film layer, and selecting a material for the sacrificial layer having a stiffness that is greater than the stiffness of the substrate and the stiffness of the film layer.
10. The method of claim 9 , wherein depositing the sacrificial layer comprises identifying a deposition temperature of the film layer, and selecting a material for the sacrificial layer having a characteristic temperature that is greater than the deposition temperature of the film layer.
11. The method of claim 9 , wherein depositing the sacrificial layer comprises identifying a coefficient of thermal expansion of the film layer, and selecting a material for the sacrificial layer having a coefficient of thermal expansion that is substantially similar to the coefficient of thermal expansion of the film layer.
12. The method of claim 9 , wherein depositing the sacrificial layer comprises depositing a layer of material having a thickness that is less than the thickness of the film layer.
13. The method of claim 9 , wherein the sacrificial layer has sacrificial properties and depositing the sacrificial layer comprises identifying an elastic modulus of the film layer, and selecting a material for the sacrificial layer having an elastic modulus and a thickness in a specific relationship to the elastic modulus and thickness of the film layer, and wherein the sacrificial properties are related to properties of the film layer by the relationship
(
1
+
v
sac
)
E
film
2
E
sac
t
film
L
t
sac
L
<<
1
,
where E film is the elastic modulus of the film layer, E sac is the elastic modulus of the sacrificial layer, t film is the thickness of the film layer, t sac is the thickness of the sacrificial layer, and ν sac is the Poisson ratio of the sacrificial layer.
14. The method of claim 13 , wherein depositing the sacrificial layer comprises selecting a material for the sacrificial layer having specific material properties that are related to material properties of the film layer, wherein the specific material properties are selected from the group consisting of the sacrificial elastic modulus, Poisson ratio, coefficient of thermal expansion, thickness, characteristic length, characteristic temperature, yield, and viscoelastic properties.
15. The method of claim 9 , wherein the sacrificial layer is deposited at specified temperatures, CTE, and elastic modulus to elicit yielding of the film layer.