Two-dimensional beam deflector

Details Two-dimensional beam deflector Two-dimensional beam deflector

2000-04-24

2003-06-24

Pham, Hoa Q. (Department: 2877)

Optics: measuring and testing

Dimension

Thickness

C356S369000, C359S202100

Reissue Patent

active

RE038153

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to systems and methods for non-destructive quality control in general and to optical systems and methods for measuring the thickness and an index of refraction of thin films, in particular.
BACKGROUND OF THE INVENTION
Optical measuring instruments are typically utilized in the microelectronic industry for non-contact, non destructive measurement of the thickness of thin films. Two main systems are utilized, spectrophotometers (or reflectometers) and ellipsometers. The following U.S. patents represent the prior art:
For ellipsometers: U.S. Pat. Nos. 5,166,752, 5,061,072, 5,042,951, 4,957,368, 4,681,450, 4,653,924, 4,647,207 and 4,516,855.
For spectrophotometers: U.S. Pat. Nos. 5,181.080, 5,159,412, 5,125,966, 4,999,014 and 4,585,348.
The two prior art systems are illustrated in
FIGS. 1A and 1B
, respectively, to which reference is now made. The spectrophotometer utilizes the fact that light beams reflected off thin film boundaries, will interfere one with another. Specifically, the spectrophotometer of
FIG. 1A
measures the reflectance of selected points of a sample
10
as a function of the light wavelength, usually in the visible or near UV spectral ranges. Computer analysis of the detected spectral reflection function, especially its minima and maxima, provides the thickness, and in some cases, also the index of refraction of the measured film.
The spectrophotometer typically includes a transmitter
12
with a light source and appropriate optics, a beam splitter
14
, an objective lens
16
, a tube lens
18
and a receiver
20
which includes optical and electronic means for measurement of light intensity as a function of the input light wavelength. The transmitter
12
produces a collimated light beam
22
which is deflected by the beam splitter
14
and focused on the sample
10
by the objective lens
16
. The reflected beam, labeled
24
, is collected by the microscope imaging optics (lenses
16
and
18
) on to a spectroscopic measurement unit within the receiver
20
.
In order to measure a multiplicity of points on the sample
10
, sample
10
is placed on an x-y stage
26
. X-Y stage
26
is typically very precise and heavy and, as a result, moves very slowly.
The spectrophotometers have difficulty measuring structures with very small reflectance, such as thin films of glass substrates, because the relatively low brightness of traditional white light sources does not provide a sufficient signal-to-noise ration (SNR). Spectrophotometers also have difficulty measuring films with unknown or unrepeatable dispersions of optical constants, such as amorphous silicon.
Despite these limitations, the spectral photometry method is at present widely used in industry because the instrumentation for this method is easily combined with optical microscopes and can utilize conventional microscope optics.
Ellipsometers measure changes in the polarization of light caused by reflectance from the test surface. These changes, characterized as amplitude and phase changes, are very sensitive to the thickness and optical properties of thin films.
A prior art ellipsometer is illustrated in FIG.
1
B. It includes a transmitter
30
which includes a light source and appropriate optics, a polarizer
32
, an optional compensator (phase retarder)
34
, an analyzer
36
and a receiver
38
with a photo-detector and appropriate electronics. The polarizer
32
polarizes the light beam
40
produced by light source
30
. The reflected light beam, labeled
42
, passes through the analyzer
36
before reaching the receiver
38
. If the compensator
34
is used, it may be located either between the polarizer
32
and the test sample
10
or between the sample
10
and the analyzer
36
.
The ellipsometric method requires oblique illumination, i.e. an angle of incidence &THgr; between an incident light beam
40
and a normal
44
to the sample
10
must be greater than zero. The angle between a reflected light beam
42
and the normal
44
is equal to the angle of incidence &THgr;. The angle of incidence &THgr; should be close to the Brewster angle &THgr;
B
of the substrate. In practice, the angle of incidence &THgr; ranges from 45° to 70°.
Because ellipsometers measure two polarization parameters (amplitude and phase), both of which are independent of the light intensity, they are quite accurate and can also measure ultra thin films of the size of 0-100 Å. However, since ellipsometers require oblique illumination as well as a highly collimated light beam, their use for high spatial resolution measurements in dense patterned structures is rather difficult.
There are two basic types of fully automated ellipsometers. Null-ellipsometers (NE) provide the most accurate thickness measurements but they require at least several seconds per measuring point. Rotating-analyzer ellipsometers (RAE) provide very high speed measurements (portions of a second per measuring point), but their sensitivity and accuracy are usually less than those of null ellipsometers.
For all of the prior art instruments, the opto-mechanical apparatus is complicated, large and heavy, and thus, the x-y stage
26
is translated between measurement points, coming to a complete stop before measurement begins. The time between measurements depends on the mass of the x-y stage
26
and on the positioning accuracy requirements and may take at least several seconds (sometimes up to several tens of seconds). This limits the speed with which a thickness mapping can occur, especially during inspection of large size substrates such as 8&Dgr; VLSI silicon wafers,
18″×18″ LCD glass panels, etc.
The footprint, or space on the floor which each machine utilizes, is typically at least twice the size of the x-y stage
26
due to its translation.
Furthermore, the prior art measuring devices are utilized for measuring once a deposition process has been completed. They cannot be utilized for in-process control, since wafer handling and other mechanical movements are not allowed within a vacuum chamber.
Other measuring instruments are also known, one of which is described in U.S. Pat. No. 4,826,321. The '321 patent presents a system similar to an ellipsometer. However, in this system, a mirror is utilized to direct a plane polarized laser beam to the thin film surface at the exact Brewster angle of the substrate on which the thin film lies.
SUMMARY OF THE PRESENT INVENTION
There is provided, in accordance with a preferred embodiment of the present invention, a two-dimensional beam deflector for a thickness measuring device for measuring the thickness of films on a sample with a plurality of different optical systems each performing a different measurement technique. The beam deflector includes a two dimensional translation unit, first and second deflection units and a plurality of optical assemblies. The two-dimensional translation unit translates the beam deflector along a first scanning axis and along a second scanning axis perpendicular to the first scanning axis. The first deflection unit receives a plurality of parallel input beams along parallel input axes which are close to one another and parallel to the first scanning axis. The first deflection unit also deflects the input beams along a plurality of parallel second axes close to each other and parallel to the second scanning axis. The second deflection unit receives a plurality of parallel output beams along parallel third axes close to each other and parallel to the second axes, and deflects the output beams along a plurality of parallel fourth axes close to each other and parallel to the first scanning axis. There is one optical assembly per input beam, each of which provides its input beam towards the sample, receives its output beam from the sample, processes its input and output beams in accordance with its measurement technique, and provides its output beams along the parallel third axes.
Additionally, in accordance with a preferred embodiment of the present invention, the optical assemblies can be at least an ell

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