MEMS encapsulated structure and method of making same

Details MEMS encapsulated structure and method of making same MEMS encapsulated structure and method of making same

2002-11-20

2004-10-05

Smith, Matthew (Department: 2825)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Physical stress responsive

C438S125000, C438S126000, C438S127000, C438S055000, C438S025000, C438S026000, C438S723000, C333S219000, C333S235000, C280S735000, C257S214000

Reexamination Certificate

active

06800503

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an encapsulated micro electro-mechanical system (MEMS) and methods of making the encapsulated MEMS. In particular, the present invention generally relates to a MEMS structure having an encapsulating layer of a material with different electrical, mechanical and/or magnetic properties from those of a core material, where the encapsulated MEMS structure may be made by complementary metal oxide semiconductor (CMOS) compatible methods.
2. Description of the Related Art
One example of a commonly used MEMS structure is a radio frequency (RF) switch, such RF switches are used in various microwave and millimeter wave applications, such as, tunable preselectors and frequency synthesizers. Semiconductor RF switches are relatively large and bulky, for example, 400 in
3
for a 16×16 array, making packaging sizes for such an array relatively large. Micro-machined RF switches significantly reduce package sizes for such RF switch arrays, for example, down to approximately 1 in
3
.
FIG. 1
illustrates a commonly used RF switch
100
, formed as a monolithically integrated MEMS switch, including a substrate
102
, a support
104
, and a flexible cantilever beam
106
that is attached at one end to the support
104
. The cantilever beam
106
has an electrical contact
112
at its unsupported end, which contacts an underlying contact
114
on the surface of the substrate
102
. Electrical contact
114
is usually connected to an RF input signal and forms an RF input port of the RF switch, while the electrical contact
112
forms an RF output port.
The RF switch
100
is actuated by electrostatic forces between a field plate
122
formed on the upper surface of the cantilever beam
106
and a grounding plate
124
located on the surface of the substrate
102
. The field plate is connected to a direct current (DC) voltage source, while the grounding plate
124
is connected to ground. As illustrated in
FIG. 1
, when no voltage is applied to the field plate
122
, the electrical contact
112
is separated from electrical contact
114
, defining an open contact or OFF state. However, when an appropriate DC voltage is applied to the field plate
122
, the flexible cantilever beam
106
is deflected by electrostatic forces, causing the electrical contact
112
to contact electrical contact
114
, defining a closed contact or ON state. The closed contact or ON state allows the RF input signal to be electrically connected to the RF output port. When the applied voltage is removed from the field plate
122
, the flexible cantilever arm
106
returns to its open contact or OFF state, due to elastic forces inherent to the material of the cantilever beam
106
.
However, such cantilever beams are subject to mechanical fatigue and stress, when switched on and off a large number of times. Sometimes, due to prolonged mechanical stress, the cantilever beam will deform and may then be subject to stiction at the electrical contact.
In addition, conventional RF switches that use silicon dioxide, polysilicon or even a composite silicon metal alloy as the beam material, are subject to relatively high insertion losses, which result in reduced sensitivity of the RF switch.
Furthermore, conventional MEMS RF switches frequently use polysilicon beams with electroless plated gold or copper. However, the use of electroless gold plating poses problems during conventional CMOS fabrication processes because there is usually no provision for depositing polysilicon or other similar materials, once a back end of line process, such as, the plating of copper, is started. With no provision for a subsequent front end of line process, electroless plating produces a very rough copper structure that is not passivated to prevent oxidation, electro-migration, and diffusion.
SUMMARY OF THE INVENTION
In view of the foregoing and other problems and disadvantages of conventional methods, an advantage of the present invention is to provide an encapsulated MEMS forming an RF switch that may be made of multiple materials, having complementary electrical and mechanical properties, to, for example, reduce metal fatigue and stress during prolonged operation, prevent stiction, and reduce insertion losses.
Another advantage of the present invention is to provide encapsulated MEMS structures that provide various switch structures, such as, encapsulated cantilever beams, encapsulated cantilever beams with one or more electrically-isolated lengths, encapsulated beams fixed at both ends, and encapsulated beams fixed at both ends with one or more electrically-isolated encapsulated lengths. In addition, the encapsulated MEMS may accommodate various numbers of switch contacts and grounding plates and a variety of switch contact and grounding plate configurations on a dielectric layer underlying, for example, an encapsulated beam.
A further advantage of the present invention is to provide an encapsulated MEMS structure that forms an inductive coil, where either the core material or the encapsulating material may comprise a ferromagnetic material in order to enhance inductive performance.
Another further advantage of the present invention is to provide a method of manufacturing the encapsulated MEMS structure that is compatible with CMOS compatible methods.
An additional advantage of the present invention is to provide a method of manufacturing an encapsulated MEMS structure that allows the use of various barrier metals, such as, gold, platinum, palladium, iridium, tungsten, tungsten nitride, tantalum, tantalum nitride, titanium, titanium nitride and nickel, for encapsulating the MEMS structure, which allows the passivation of an encapsulated inner copper layer and prevents oxidation, electro-migration and diffusion of the copper during subsequent processing.
In order to attain the above and other advantages, according to an exemplary embodiment of the present invention, disclosed herein is a method of fabricating an encapsulated MEMS that includes forming a dielectric layer on a semiconductor substrate, patterning an upper surface of the dielectric layer to form a first trench, forming a release material within the first trench, patterning an upper surface of the release material to form a second trench, forming a first encapsulating layer including sidewalls within the second trench, forming a core layer within the first encapsulating layer, and forming a second encapsulating layer above the core layer in which the second encapsulating layer is connected to the sidewalls of the first encapsulating layer.
According to another exemplary embodiment of the present invention, the first encapsulating layer and the second encapsulating layer are made of barrier metals selected from the group of gold, platinum, palladium, iridium, tungsten, tungsten nitride, tantalum, tantalum nitride, titanium, titanium nitride, and nickel, while the core layer is made of a semiconductor dielectric material.
According to another exemplary embodiment of the present invention, the method of fabricating an encapsulated MEMS further includes forming a metal layer between the first encapsulating layer and the core layer.
According to another exemplary embodiment of the present invention, the metal layer includes sidewalls that are connected to the second encapsulating layer.
According to another exemplary embodiment of the present invention, forming the metal layer includes depositing an initial metal layer including sidewalls on the first encapsulating layer, depositing a stop layer on exposed surfaces of at least the first encapsulating layer and the sidewalls of the initial metal layer, removing the stop layer located above the sidewalls of the initial metal layer, and recessing the sidewalls of the initial metal layer and of that portion of the stop layer, which adheres to the sidewalls of the initial metal layer.
According to another exemplary embodiment of the present invention, the metal layer comprises a highly conductive metal from the group of copper, gold and aluminum.
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