Electrical generator or motor structure – Dynamoelectric – Linear
Reexamination Certificate
2002-01-18
2004-06-29
Ponomarenko, Nicholas (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Linear
C310S015000, C310S054000
Reexamination Certificate
active
06756706
ABSTRACT:
TECHNICAL FIELD
The invention described herein relates to electron beam lithography systems. In particular, the invention relates to methods and apparatus used to cool the temperature of electrical leads in the process chambers of electron beam lithography systems. Most particularly, the invention relates to methods and apparatus used to cool the temperature of electrical power leads supplying the coils of stage linear motors used to move the stages of electron beam lithography systems.
BACKGROUND
As the limits of conventional focusing optics have been pushed out of the realm of usefulness for photolithography, techniques such as e-beam (electron-beam) lithography have come into wider usage. The need to form ever finer features in the layers of semiconductor wafers has driven the continued development of e-beam devices in order to gain higher resolution necessary to form even finer patterns in the layers of semiconductor devices. Among the differences between e-beam lithography and conventional lithography are that focusing an electron beam requires a different type of “optics”, generally involving electromagnetic and/or electrostatic fields to effect focusing and deflection of the beam. Additionally, the nature of e-beam lithography is such that it can only be carried out in a vacuum.
As used herein, the term “lithography” refers to the process whereby a pattern of lines and the like is formed within a layer of a material (e.g., photoresist) on a semiconductor device. The pattern, which represents “converted” material surrounded by “unconverted” material (or vice-versa) is used, in subsequent processing steps, to form corresponding structures in an underlying layer (e.g., polysilicon) on the semiconductor device. Additionally, e-beam lithography can incorporate so-called “direct-write lithography”, which refers to creating patterns directly in the layer, without the intermediary of an imaging mask such as is used in conventional photolithography.
Electron beam projection systems use stages to move various system components (e.g., the reticle or the wafer), as is well known in the art. The stages are an effective means used to move system components in order to achieve extremely accurate photolithographic patterning of semiconductor surfaces.
The motive force applied to the stages is commonly provided by stage linear motors. A typical example of such a motor is disclosed in U.S. Pat. No. 6,140,734 to Hazelton et al. entitled “Armature with Regular Windings and Having a High Conductor Density”. Such electric motors are able to move wafer stages in a very controlled and accurate manner consistent with the precision placement and location of semiconductor wafers and reticles.
Stage linear motors of the type typically required can, and typically do, comprise multiphase electric motors which can require multiple electric wires to provide power to the coils of such motors. Depending on motor design, wire size, and the duty cycle of the motor, the wires will rise in temperature during use. The resulting temperature increase results in resistance in the wires. This problem is exacerbated in vacuum environments, such as the processing chambers of electron beam lithography systems. In order to avoid excessive increases in resistance, it is necessary that the wires be cooled.
The use of stages and stage motors is common in many types of semiconductor processing machines. Ordinarily, the stages of these types of machines are operated in air or other gaseous environments, such that the wires can effectively be cooled by convection. In such cases, the wire temperature rises until the so-called I
2
R losses are balanced by the convective heat transfer rate in the ambient environment. However, in vacuum environments (such as that of a process chamber in an electron beam lithography machine), convective cooling is not possible as a method of wire cooling. In such environments, the heat transfer mechanisms available for cooling are conduction through the insulation of the wires and subsequent radiation to the vacuum chamber walls. Or, alternatively, heat may be conducted along the wires to the ends of the wires where it can be dissipated. Neither of these cooling approaches are very efficient and will cause much higher wire temperatures than is the case where convective heat transfer is available as a cooling means. Making matters worse is the fact that increasing temperature in the wires results in increasing wire resistance, which again further increases power dissipation and heat generation. In such circumstances, a circle of rising temperature and increasing resistance is created.
In addition to the temperature and resistance problems, excess heating causes the wires to reach temperatures high enough such that components in the wire insulation outgas and enter the extremely sensitive vacuum environment. Such a situation is highly undesirable as the outgassed components can have drastic effects on the vacuum processing conditions within the chamber.
Although electron beam lithography systems have numerous process advantages, in particular, their ability to form high resolution patterns in semiconductor wafers, they also present some new difficulties. In particular, there are difficulties in cooling electrical power lines in the high vacuum process environment, e.g., vacuum levels on the order of 10
−6
Torr.
What are needed are methods and apparatus for solving the foregoing heating issues in the power wires of the electron beam lithography systems. The principles of the present invention provide solutions to this and other problems.
FIG. 1
shows a simplified, schematic, perspective view of an arrangement for positioning a semiconductor wafer
100
in an e-beam lithography system. The wafer
100
is mounted on a bed
110
of an X-Y stage. Typically the wafer is secured to the bed using an electrostatic chuck. The bed
110
is movable in an X-direction by operation of a stage linear motor
115
. The wafer
100
is also movable in a Y-direction by a second stage linear motor
125
, which moves a bed
120
in the Y-direction. The beds
110
,
120
are mounted on a rigid platform
130
, which can be secured to a base and with vibration dampers (not shown). In the arrangement shown, the X-Y stage is used to position wafer
100
relative to an optical image produced by an electron beam source
135
, a reticle
140
, and a projection lens assembly
145
in a step-and-repeat wafer exposure apparatus. The position of the wafer
100
can be measured and calibrated using any of a number of positioning devices (e.g., a laser interferometer system). As will be appreciated by those having ordinary skill in the art, the embodiments of the invention can be used with other types of e-beam systems.
FIG. 2
is another simplified schematic view showing a generally used projection exposure apparatus comprising components such as a wafer stage (
110
,
120
,
130
) used for positioning a wafer
100
, an optical system
135
, which generates illumination which is passed through a reticle on a reticle stage
140
through a projection optical system
145
in order to form an image on the wafer
110
. The wafer stage is disposed on a base
140
. Vibration removal devices
141
are provided between the base
140
and a floor surface. The entire assembly is positioned inside a processing chamber
160
.
FIG. 3
is a simplified view of the stage
110
and the stage linear motor
115
. The linear motor
115
comprises a coil element
117
attached to the stage
110
, and a stator element
116
.
FIG. 4
shows a cross-section view of the motor along axis A-A. The stator
116
generally comprises a magnetic track having a slot
116
S formed therein. The coil assembly
117
is positioned such that it lies in the slot
116
S of the stator
116
. By selectively energizing the coils of the coil assembly
117
, a force is impelled towards the stage
110
, which enables the stage to move.
FIG. 5
is an internal section view along axis A-A of the coil assembly
117
. Due to the heat generated by coils during ordinary cours
Beyer Weaver & Thomas LLP
Mohandesi Iraj A.
Nikon Corporation
Ponomarenko Nicholas
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