Method for the manufacture of a thin film actuated mirror array

Details Method for the manufacture of a thin film actuated mirror array Method for the manufacture of a thin film actuated mirror array

1999-01-19

2001-03-20

Mills, Gregory (Department: 1763)

Etching a substrate: processes

Forming or treating optical article

C216S058000, C438S029000, C156S922000

Reexamination Certificate

active

06203715

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for the manufacture of a thin film actuated mirror array; and, more particularly, to the formation of a tilting mirror using a thin film sacrificial layer made of a poly-Si.
BACKGROUND OF THE INVENTION
Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M×N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electric signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
FIGS. 1A
to
1
E are cross sectional views illustrating a method for manufacturing a thin film actuated mirror array
100
.
The process for the manufacture of the array
100
begins with the preparation of an active matrix
110
including a substrate
111
, an array of switching devices
112
, an array of connecting terminals
113
, a passivation layer
114
and an etchant stopping layer
115
, as shown in FIG.
1
A. Each of the switching devices
112
made of, e.g., metal-oxide-semiconductor field effect transistors (MOSFET), is fabricated on top of the substrate
111
made of, e.g., Si-wafer, by using a common integrated-circuit process. Each of connecting terminals
113
made of a metal, e.g., tungsten (W), is electrically connected to a corresponding switching device in the switching device array
112
. The passivation layer
114
made of, e.g., PSG, and having a thickness of 0.1-2 &mgr;m, is formed on top of the switching device array
112
and the connecting terminal array
113
by using, e.g., a CVD or a spin coating method. The etchant stopping layer
115
made of silicon nitride and having a thickness of 0.1-2 &mgr;m, is deposited on top of the passivation layer
114
by using a low pressure chemical vapor deposition (LPCVD) method or a plasma enhanced chemical vapor deposition (PECVD) method.
In a subsequently step, a thin film sacrificial layer
120
, made of a phosphorus silicate glass (PSG) and having a flat top surface, is formed on top of the active matrix
110
by using a spin coating method. Since the thin film sacrificial layer
120
should endure the high-temperature process in the manufacturing process followed herebelow, the phosphor-silicate glass is chosen as a material for the thin film sacrificial layer
120
. Then, an array of empty cavities
125
is created in the thin film sacrificial layer
120
to expose the active matrix
110
by using a dry or a wet etching method, as shown in FIG.
1
B.
In a next step, an elastic layer
131
, made of a nitride, e.g., silicon nitride, and having a thickness of 0.1-2 &mgr;m, is deposited on top of the thin film sacrificial layer
120
including the empty cavities
125
by using a LPCVD or a PECVD method.
Thereafter, a lower thin film layer
132
, made of an electrically conducting material, e.g., Pt or Pt/Ta, and having a thickness of 0.1-2 &mgr;m, is formed on top of the elastic layer
131
by using a sputtering or a vacuum evaporation method.
Then, a thin film electrodisplacive layer
133
, made of a piezoelectric material, e.g., PZT, or an electrostrictive material, e.g., PMN, and having a thickness of 0.1-2 &mgr;m, is deposited on top of the lower thin film layer
132
by using an evaporation, a sol-gel, a sputtering or a CVD method.
Subsequently, an upper thin film layer
134
, made of an electrically conducting and light reflecting material, e.g., aluminum (Al) or silver (Ag), and having a thickness of 0.1-2 &mgr;m, is formed on top of the thin film electrodisplacive layer
133
by using a sputtering method, thereby forming a multiple layered structure
130
, as shown in FIG.
1
C.
In an ensuing step, the multiple layered structure
130
is patterned by using a photolithography or a laser trimming method to expose the thin film sacrificial layer
120
.
In a subsequent step, an array of via contacts
145
made of a metal, e.g., tungsten(W), is formed by using a lift-off method, thereby forming an array of actuating structures
140
, wherein each of actuating structures
140
includes an upper thin film electrode
144
, a thin film electrodisplacive member
143
, a lower thin film electrode
142
, an elastic member
141
and a via contact
145
, as shown in FIG.
1
D. Each of the via contacts
145
extends from top of the lower thin film electrode
142
to top of a corresponding connecting terminal in the connecting terminal array
113
. Each of the lower thin film electrodes
142
is electrically connected to the corresponding connecting terminal
113
, thereby functioning as a signal electrode and each of the upper thin film electrodes
144
is electrically connected to ground, thereby functioning as a common bias electrode as well as a mirror.
Finally, the thin film sacrificial layer
120
is removed by using a wet etching method using an etchant or a chemical, e.g., hydrogen fluoride (HF) vapor, to thereby form the thin film actuated mirror array
100
, as shown in FIG.
1
E.
One of the problems in using the above-described the method for manufacturing the thin film actuated mirror array is the overall optical efficiency of the resulting thin film actuated mirrors. When each of the thin film actuated mirrors deforms in response to an electric field applied across the thin film electrodisplacive member therein, the upper thin film electrode attached thereto, which also acts as a mirror, also deforms to thereby, instead of creating a planar top surface, create a curved top surface from which the light beams are reflected, reducing the effectiveness of the upper thin film electrode in reflecting the light beams. As a result, the overall optical efficiency of the thin film actuated mirror array decreases.
To overcome the above problem of the method for manufacturing the thin film actuated mirror array, it has been suggested to incorporate in the thin film actuated mirror a tilting mirror that is formed and cantilevered on top of the actuating structures by using a second thin film sacrificial layer.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of the present invention to provide a method for manufacturing a thin film actuated mirror array, each of the thin film actuated mirror having a tilting mirror formed and cantilevered on top of an actuating structure therein using a thin film sacrificial layer made of a poly-Si.
In accordance with one aspect of the present invention, there is provided a method for manufacturing a thin film actuated mirror array, the method comprising the steps of: preparing an active matrix including a substrate, an array of switching devices and an array of connecting terminals; forming a first sacrificial layer including an array of empty cavities; forming an array of actuating structures, each of the actuating structures including an elastic member, a lower electrode, an electrodisplacive member, an upper electrode and a via contact; forming a second sacrificial layer including an array of empty slots; forming an array of mirrors; removing the first and the second sacrificial layer, to thereby form the thin film actuated mirror array.


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