Active vibration isolation system having pressure control

Details Active vibration isolation system having pressure control Active vibration isolation system having pressure control

2000-09-15

2003-07-08

Adams, Russell (Department: 2851)

Photocopying

Projection printing and copying cameras

Detailed holder for original

C355S053000, C267S136000, C248S550000

Reexamination Certificate

active

06590639

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an active vibration isolation system (AVIS). Particularly, this invention relates to an AVIS having a direct pressure control on a pneumatic control system of the AVIS to isolate a mass from external disturbances, such as vibration. The AVIS is generally for use in a photolithography process.
2. Description of the Related Art
Photolithography is a process for manufacturing integrated circuits. In a photolithography process, light is transmitted through non-opaque portions of a pattern on a reticle, or photomask, through a projection exposure apparatus, and onto a wafer of specially-coated silicon or other semiconductor material. The uncovered portions of the coating, that are exposed to light, are cured. The uncured coating is then removed by an acid bath. The layer of uncovered silicon is altered to produce one layer of a multi-layered integrated circuit. Conventional systems use visible and ultraviolet light for this process. Recently, however, visible and ultraviolet light have been replaced with electron, X-ray, and laser beams, which permit smaller feature sizes in the patterns.
As the miniaturization of a circuit pattern progresses, the focus depth of the projection exposure apparatus becomes very small. More importantly, it is difficult to accurately align the overlay of circuit patterns of the multi-layered integrated circuit. As a result, a primary consideration for an overall design of the photolithography system includes building components of the system that achieve precision by maintaining small tolerances. Any vibration, distortion, or misalignment caused by external disturbances must be kept at minimum. These external disturbances affecting an individual part collectively alter the focusing properties of the photolithography system.
It has been proposed to provide an active vibration isolation system (AVIS). A conventional AVIS generally includes a pneumatic control system and an electronic control system. The pneumatic control system is capable of generating a large force, but has slow dynamics so that it cannot respond to high frequency disturbances or variations. Hence, the pneumatic control system supports a mass and is used to counteract low frequency internal disturbances and isolate the mass from high frequency ground vibration. The electronic control system compensates for any disturbance that the pneumatic control system does not sufficiently isolate, i.e., low frequency ground vibration and internal disturbances. In a lithography system, the mass may be a stage device and may be supported and isolated by a plurality of AVIS. In addition, more electronic control systems, operating in a horizontal direction, may be provided to isolate the mass from horizontal disturbances.
A conventional AVIS
100
is illustrated in
FIGS. 1 and 2
having a pneumatic control system
102
, shown in
FIG. 1
, and electronic control system
104
, shown in FIG.
2
. Pneumatic control system
102
includes a fluid-filled chamber
110
pressurized to support a mass
120
. Mass
120
may represent an individual part, such as a stage device for holding the wafer, or may also represent the whole photolithography system. Pressurized chamber
110
, generally known as a compliance chamber, acts like an air spring, while mass
120
acts like a piston compressing the fluid when the ground moves up and down or when mass
120
is disturbed or moves. To isolate mass
120
from vibration of the floor, the pressure inside compliance chamber
110
is preferably maintained at a level which counteracts the force of gravity on mass
120
.
A damping system
130
is introduced into pneumatic control system
102
to minimize the movement of mass
120
as it rides on the air spring by allowing the fluid in compliance chamber
110
to pass through some type of resistance or restriction
132
into a damping chamber
134
. The energy dissipated in restriction
132
provides damping of the natural behavior of mass
120
on the air spring. For optimal damping performance, damping chamber
134
may generally need to be as much as eight times the volume of compliance chamber
110
.
In one embodiment, restriction
132
is a small hose connecting compliance chamber
110
to damping chamber
134
. Alternatively, compliance chamber
110
and damping chamber
134
may be constructed of a single large chamber (not shown) with a wall dividing therebetween, the wall having a small hole acting as the restriction. Damping chamber
134
may be connected to a fluid supply
136
via a regulator
138
, which controls the flow of fluid coming into or out of damping chamber
134
. Controlling the flow adjusts the pressure level in damping chamber
134
, which in turn stabilizes the pressure level in compliance chamber
110
.
The electronic control system
104
shown in
FIG. 2
includes an electronic actuator
150
connected to a motion sensor. The motion sensor generally includes a position sensor
152
and a velocity sensor
154
. Position sensor
152
measures and provides a position error signal
156
, while velocity sensor
154
measures and provides a velocity error signal
158
of the isolated mass
120
. In the embodiment of
FIG. 2
, mass
120
is a base
124
over which a stage device
122
moves. Position and velocity error signals
156
,
158
, respectively, enter one or more feedback controllers. The embodiment of
FIG. 2
shows a first feedback controller
160
receiving position error signal
156
and a second feedback controller
162
receiving velocity error signal
158
. First and second feedback controllers
160
,
162
, respectively, in turn generate force signals
164
,
166
, which are used to isolate mass
120
.
Electronic actuator
150
is also connected to a controller (not shown), such as a computer simulating a mathematical model, via a feedforward controller
172
to determine a calculated force signal
168
, which is also used to isolate mass
120
. A summing junction
174
calculates the difference between measured force signals
164
,
166
, and calculated force signal
168
, and delivers the resulting signal
170
to electronic actuator
150
. Electronic actuator
150
generates a force
176
corresponding to resulting signal
170
, which is exerted on mass
120
to isolate it from disturbances that pneumatic control system
102
does not sufficiently isolate.
One problem with this conventional AVIS is that regulator
138
only indirectly controls compliance chamber
110
by controlling the fluid pressure in damping chamber
134
. In the indirect control of compliance chamber
110
, the control is influenced by a loss of the fluid between a measurement point (not shown) and compliance chamber
110
. In addition, it is difficult to use a position information of mass
120
to control the pressure level in compliance chamber
110
since the relationship between the pressure level and the position information is not in exact proportion. Therefore, there is a need for an improved pneumatic control system whereby a direct control over the system is provided to better isolate the mass from external disturbances.
SUMMARY OF THE INVENTION
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, a first aspect of the present invention is a pneumatic control system to support a mass in a vibration isolation system. The pneumatic control system comprises a compliance chamber filled with a fluid to pneumatically support the mass, the fluid having a fluctuating pressure level caused by external disturbances, and a sensor device connected to the compliance chamber for determining a pressure information of th

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