Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2001-03-29
2003-04-15
Ryan, Patrick (Department: 1745)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298040, C204S298110
Reexamination Certificate
active
06547939
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to thin-film deposition systems, and more particularly to shadow masks to control the film deposition rate and uniformity.
Electrical and optical systems often rely on devices with thin films. For example, an optical filter may have a hundred layers of films stacked together, each being a quarter of a desired wavelength. These thin films must be deposited with a uniform thickness to prevent unwanted effects, such as a spreading of the wavelength filtered.
These thin films can be deposited in a vacuum chamber using a sputtering or ion beam method.
FIG. 1
shows a thin-film deposition chamber using a shadow mask. Chamber
10
is part of a larger film-deposition machine. The air is pumped out of chamber
10
to produce a low-pressure, near-vacuum environment. A substrate
20
is placed in the chamber. One or more layers of thin films are deposited on the upper surface of substrate
20
. Substrate
20
is rotated by motor
12
to improve uniformity of deposition.
Ion-beam source
14
generates ion beam
26
that is directed against target
18
. Target
18
is made from an ultra-pure target material that supplies the atoms that form the film being deposited onto substrate
20
. For example, target
18
can be of a silicon-dioxide SiO2 material or of Tantalum (Ta) material to adjust the index of refraction of the deposited film. Target
18
is a consumable item and must be replaced with a new target after films have been deposited.
When ion beam
26
impacts the surface of target
18
, some of the target's atoms are ejected or sputtered off of target
18
. These atoms or ions from target
18
then travel from target
18
to substrate
20
as target beam
28
. When the atoms from target beam
28
impact the upper surface of substrate
20
, they attach to the surface and form a thin film. Over time, the film on substrate
20
becomes thicker and thicker until the desired thickness is reached, and ion-beam source
14
is turned off. Substrate
20
and target
18
can be heated to improve reaction rates.
As target
18
is consumed, and its surface roughness changes, so the rate that atoms or ions are sputtered off its surface can vary. To control the uniformity of film deposited on the substrate, shadow mask
24
can be used to partially block target beam
28
. Shadow mask
24
may consist of several blades. Each blade is made especially for each of the different target materials. A technician can do a test run before each production deposition run to check the uniformity of deposition across the substrate and use the test run results to modify the shadow mask shape to achieve better uniformity. Motor
22
rotates shadow mask
24
to select one of the blades, or to move shadow mask
24
out of position so substrate
20
can be loaded or removed.
While the end of deposition of each layer of film can simply occur after a fixed amount of time, an optical monitoring system improves results. Light source
32
shines light beam
30
through substrate
20
and into endpoint detector
34
. The thin-film deposited onto the surface of substrate
20
interferes with light beam
30
, attenuating the intensity of light beam
30
before it reaches detector
34
. This attenuation varies as the film becomes thicker during deposition. Endpoint detector
34
analyzes one or more wavelengths of light beam
30
and signals an endpoint when a predetermined attenuation is reached. Ion-beam source
14
is then turned off for that layer of film deposition. When non-transparent substrates are used, the light beam can be reflected off the surface of the substrate rather than transmitted through the substrate.
FIG. 2
shows a prior-art shadow mask. Substrate
20
is rotated at a predetermined speed so that the whole substrate
20
can be covered by target beam
28
. Target beam
28
falls on target area
28
′ which is where film deposition occurs. Variations of beam intensity within target area
28
′ are thought to contribute to film non-uniformity.
Shadow mask
24
is a metal blade that obstructs the target beam where shadow mask
24
overlaps target area
28
′. The blade can be shaped so that more obstruction occurs for smaller radii near the center of substrate
20
than for the larger radii near the outer rim of substrate
20
. Areas near the center have a slower linear velocity than regions near the outer rim, and thus lower-radius areas spend more time inside target area
28
′. To compensate, the shape of the blades reduces the film thickness near the center of substrate
20
relative to film thickness near the outer rim of substrate
20
.
Shadow mask
24
can be rotated, allowing other blades
23
,
25
to be selected to overlap target area
28
′. Wider blades reduce the deposition rate, while thinner blades increase the deposition rate. All blades can also be rotated out of position so substrate
20
can be removed or loaded.
While using a shadow mask can improve film uniformity, the degree of control is limited since one of only three shadow mask blades can be pre-selected. The endpoint is determined by the light beam falling on area
30
′ of substrate
20
, which covers only a limited range of radii. Uniformity at different radii is typically checked by a technician who removes substrate
20
from the vacuum chamber and performs a series of measurements on the film. In-situ monitoring of the film at different radii is not provided, and no feedback control mechanism is used. The shadow mask is selected for use during the entire film deposition period and is not normally changed during a run.
Better uniformity of film thickness is desired. This can be achieved by using a multi-radius monitor that controls and varies the shadow mask within a deposition run.
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patent: 63307272 (1988-12-01), None
Chang Shyang
Hsu Jack
Hsueh Paul
Ma Abraham C.
Ma Michael
Auvinen Stuart T.
Cantelms Gregg
Super Light Wave Corp.
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