Method of forming optical thin films on substrate at high...

Details Method of forming optical thin films on substrate at high... Method of forming optical thin films on substrate at high...

2002-07-11

2004-09-28

Stafira, Michael P. (Department: 2877)

Optics: measuring and testing

With plural diverse test or art

C356S630000

Reexamination Certificate

active

06798499

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming a plurality of optical thin-films for an optical device on a substrate at a high accuracy, and to an apparatus therefor.
2. Description of the Related Art
Optical communications using optical fibers have seen rapid development in recent years. Optical devices, such as various filters, used in optical communications are required to achieve high performance, i.e., highly accurate optical characteristics, to meet this development.
In order to satisfy such a requirement, it is essential to accurately control the thicknesses of layers deposited on a substrate during making a multilayer thin film for use in an optical device.
FIG. 20
shows a conventional deposition apparatus used in making optical thin films that require highly accurate thickness control. The deposition apparatus shown in
FIG. 20
is an ion beam sputtering (IBS) apparatus.
The IBS apparatus is controlled by a deposition controller
103
. In the IBS apparatus, a raw material for thin films is arranged on a target
207
of a main unit
100
, is heated by the energy caused by bombardment of ion beams emitted from an ion gun
102
, and is vaporized. Thin-films are made using molecules of plasmas of this raw material and thus exhibit a high density. Moreover, since deposition is performed in a high vacuum, the amount of contaminant is small, and high-quality thin films can be deposited at a high accuracy.
In this IBS apparatus, a multilayer thin-film having a designed thickness is deposited while controlling the thickness of each layer deposited on the substrate using a thickness monitor
101
for measuring the thickness of the layers deposited in the main unit
100
. The thickness monitor
101
is either of a type that measures the thickness using a natural frequency of a crystal oscillator, i.e., a crystal thickness meter, hereinafter referred to as the “crystal monitor”, or of a type that measures the transmittance or the reflectance of the thin-film formed on a substrate, i.e., a thickness sensor, hereinafter referred to as the “optical monitor”.
However, the crystal monitor and the optical monitor described above have the following drawbacks when they are used in making a multilayer thin-film requiring a high accuracy.
The crystal monitor has a high resolution in measuring changes in thickness d of the deposited layers and can accurately control the relative thickness of the deposited layer. However, a measurement error regarding the absolute thickness occurs as the thickness of a thin-film formed on the crystal oscillator changes. Thus, the detected thickness d is different from the actual thickness, which is a problem.
Moreover, since the crystal monitor indirectly measures the optical thickness, i.e., the mechanical thickness, without considering variation in the refractive index, the crystal monitor cannot respond to the changes in the optical thickness. This is because some layers have the same mechanical thickness but different refractive indices depending on the characteristics of the layers.
In contrast, the optical monitor can directly measure the optical thickness, i.e., dp=n·d, that takes into account changes in refractive index n. The optical monitor uses a measuring light having a wavelength &lgr;, a quarter of which is equal to the optical thickness dp of each layer, and processes this measuring light to determine changes in transmittance or the like over time, as shown in FIG.
21
.
The deposition controller
103
detects that a thin-film having a required thickness dp is formed when the changes in the transmittance reach the extrema, such as at a time t
1
or a time t
2
. The deposition controller
103
then stops the operation of the ion gun
102
and ends deposition of thin-films in the main unit
100
.
However, when a film having a small optical thickness dp (=&lgr;/4) is deposited, the measuring light sometimes cannot be set at a suitable wavelength.
Referring now to
FIG. 21
, if a layer having a thickness corresponding to the thickness formed at the time t
3
at a wavelength &lgr;
3
is to be formed, i.e., the optical thickness that does not correspond to &lgr;
3
/4, the output from the optical monitor (the thickness monitor
101
) does not show the extremum of transmittance at the time t
3
.
In contrast, extrema of the transmittance can be observed at the times t
1
, t
2
and t
4
, when the optical layers having thicknesses of &lgr;
1
/4, &lgr;
2
/4, and &lgr;
4
/4, respectively, are formed.
FIG. 21
shows the relationship between time and the transmittance data DT output from the optical monitor. The graph in
FIG. 21
shows that an optical thin film having a thickness dp of &lgr;
1
/4 is formed at the time t
1
, an optical thin film having a thickness dp of &lgr;
2
/4 is formed at the time t
2
, and an optical thin film having a thickness dp of &lgr;
4
/4 is formed at the time t
4
.
Here, &lgr;
1
, &lgr;
2
,&lgr;
3
and &lgr;
4
each represent wavelengths of the measuring light.
Accordingly, in the conventional deposition apparatus, the designated optical thickness must be detected without using extrema, if the optical layer to be deposited has a thickness not suitable to be measured by an optical monitor, resulting in a larger variation, which is a problem.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for forming optical thin films and an apparatus therefor that achieve highly accurate deposition control in which the crystal monitor and the optical monitor function to complement the drawbacks of each other.
In order to achieve this object, a first aspect of the present invention provides an apparatus for forming an optical thin film including: a thin-film forming unit for forming a thin film by depositing a substance on a surface of a substrate; an optical monitor for optically measuring the thickness of the thin film and outputting first thickness data; a crystal monitor for measuring the thickness of the thin film based on a crystal frequency and outputting second thickness data; and a thickness determining unit for controlling deposition by the thin-film forming unit based on one of the first thickness data and the second thickness data by switching the optical monitor and the crystal monitor. The thickness of the deposited film is normally measured with the optical monitor. However, when the thickness of the layer cannot be measured by the optical monitor because the thickness is excessively small or is not suited to be measured by the optical monitor, the crystal monitor is used instead of the optical monitor. Here, the thickness data of the crystal monitor is corrected by the coefficient calculated based on the thickness data of the optical monitor measured up to the point of switching from the optical monitor to the crystal monitor. In this manner, a multilayer thin film constituted from layers having various thicknesses can be formed.
Preferably, the thickness determining unit controls the deposition based on the first thickness data when the thickness of the thin film to be deposited is measurable with the optical monitor, and the thickness determining unit controls the deposition based on the second thickness data when the thickness of the thin film to be deposited is not measurable with the optical monitor. The thicknesses of the layers constituting the multilayer thin film are designed to form a suitable filter, and the designed thickness of each layer is input to the apparatus in advance. Accordingly, when a layer having a thickness not suitable to be measured with the optical monitor is formed, the crystal monitor is used from the beginning of the deposition instead of the optical monitor to control the thickness. Thus, in making multilayer thin film constituted from layers having various thicknesses, the ion gun can be stopped without delay, the thickness of each layer can be accurately controlled, and the deposited layers have designed thicknesses.
Preferably, the thickness determining unit corrects the second thick

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