Film thickness measurement apparatus with tilting stage and method of operation

A film measurement apparatus having a stage with a support surface on which a substrate coated with a film may rest. An extended light source faces the stage, and an imager is aimed at the stage to capture the reflection of the light source. The imager includes a receiver upon which an image of at least an extended portion of the substrate may be generated, and a processor in communication with the imager is operable to calculate the thickness of the film at plurality of locations. The stage may be tilted to empirically measure an average illumination and the contrast between interference fringes, avoiding theoretical estimates provided by Fresnel'equation.

FIELD OF THE INVENTION 
This invention relates to inspection apparatus and methods for thin film 
measurement, and more particularly to thin film reflectance techniques. 
BACKGROUND AND SUMMARY OF THE INVENTION 
The manufacture of semiconductor chips typically involves the repeated 
imaging of multiple pattern layers on a wafer. Each image is printed on a 
film or coating to generate the desired pattern. To provide high quality 
manufacturing to tight tolerances, it is desirable to monitor and control 
film thickness. 
Current techniques for measuring the film thickness over an entire wafer 
require a multitude of sequential point-by-point thin film reflectance 
measurements to cover an extended area of the wafer. For each point, the 
wafer is positioned, the point is illuminated, and the resulting combined 
intensity of the first and second surface reflections is measured. 
Depending on the thickness of the film, the reflections may interfere 
constructively or destructively, so that the measured signal may fall 
within a range between maximum and minimum possible intensities. 
To calculate the film thickness, it has been necessary to determine 
reflectance from the film surface and the substrate surface. This approach 
requires that the optical indices of the film and substrate materials be 
known or estimated. The indices are then used in Fresnel's equation to 
calculate normalized reflected intensity values for the particular 
materials. However, in practice, these indices may vary slightly due to 
manufacturing variations, introducing errors into the film thickness 
calculation. 
The disclosed embodiments provide improvements over existing systems by 
providing a film measurement apparatus having a stage with a support 
surface on which a substrate coated with a film may rest. An extended 
light source faces the stage, and an imager is aimed at the stage to 
capture the reflection of the light source. The imager includes a receiver 
upon which an image of at least an extended portion of the substrate may 
be generated, and a processor in communication with the imager is operable 
to calculate the thickness of the film at a plurality of locations. The 
stage may be tilted to empirically measure an average illumination and the 
contrast between interference fringes, avoiding theoretical estimates 
provided by Fresnel's equation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENTS 
FIG. 1 shows a thin film measurement system 10 having a light source 12, a 
movable wafer support stage 14, a camera 16, and a computer 20. The light 
source 12 provides an extended diffuse source of substantially 
monochromatic light that shines downward onto a wafer 22 resting on the 
stage 14. The camera 16 is positioned on the opposite side of the stage 
from the light source so that it may view and record the image of the 
light source specularly reflected from the wafer. With the light source 
subtending a greater angle than the wafer as viewed from the camera, and 
being positioned at about the same angular elevation above the wafer 
surface as the camera, the entire surface of the wafer appears specularly 
illuminated to the camera. 
The light source includes a box 24 mounted on a support 26. Six fluorescent 
tubes 30 individually filtered or coated to transmit primarily green and 
UV light in the manner of a "black light" are evenly arrayed within the 
box. A window 32 covers a square opening in a major face of the box, and 
is about 40 cm on each side. The window includes a diffuser plate 
providing 120 degrees diffusion with less than .+-.5% uniformity variation 
to eliminate potential hot spots for each light tube 30. The window also 
includes a green filter, preferably transmitting at 546 nm with a 
bandwidth of about 20 nm. The light source is tilted so that the window 
faces somewhat downward, with a line perpendicular to the center of the 
window intersecting the center of the wafer 22. 
The stage 14 includes a base 34 and a motorized movable wafer support 36 
connected to and controlled by the computer 20 via a motor controller 40. 
The wafer support may be tilted about an axis perpendicular to the plane 
of the figure to vary the angle of incidence of light from the light 
source and may be laterally translated relative to the base for proper 
positioning, The wafer support 36 has a flat upper surface 42 upon which 
the wafer rests. The upper surface may include a number of small holes 
connected to a vacuum pump (not shown) to selectably secure a wafer to the 
stage for measurement. 
The camera 16 has a lens 44 that focuses an image of the wafer 22 onto a 
film plane 46 occupied by a CCD 48 having a matrix of pixels. The CCD 
converts the image of the illuminated wafer into a bit map of data, with 
each pixel being assigned an intensity value corresponding to the apparent 
level of illumination of a small point or region of the wafer. The bit map 
data is transmitted to the computer 20 via line 50, so that the computer 
may make calculations based on the data as will be discussed below, and 
store or display the results. The stage may be contained within a clean 
enclosure (not shown), with the light source and/or the computer 
positioned outside the enclosure to minimize contamination of the wafer. 
As shown in FIG. 2, the wafer includes a substrate 52 coated with an 
optical film 54 of thickness "d". Examples of films that may be measured 
include photo-resist, polysilicon, SiO.sub.2, metallic thin film, and the 
like. The film has an upper surface 56 and a lower surface 60 that 
directly contacts the substrate without an air gap. Both film surfaces 56 
and 60 are sufficiently optically flat and smooth to provide a 
substantially specular reflection of incident light. Because the thickness 
may vary from point to point, it is necessary to measure the thickness at 
each point in a matrix of closely spaced points. 
An exemplary point 62 on the film defines a normal line 64 perpendicular to 
the film. A first ray 66 from the light source 12 is incident on the film 
at angle ".alpha." from normal to the surface, and is split into a 
reflected ray 70 and a refracted ray 74. The refracted beam 74 reflects 
off the film-substrate interface and exits the film's upper surface 56 at 
a point 63 that is laterally shifted from point 62 by a distance that is 
very small in comparison to the resolution of the CCD 48. Focused by lens 
44, these two parallel rays combine at the image plane of the camera to 
form a point on a single pixel of the CCD 48. 
With a film of local thickness "d" and having optical index "n", and 
illuminated by a beam of wavelength ".lambda." at an angle of incidence 
".alpha.", the resulting ray will have an intensity I, with I.sub.1 being 
the intensity contributed if the light from the first ray 70 were measured 
alone, and I.sub.2 being the intensity contributed if the light from the 
second ray 74 were measured alone. Due to interference effects, the value 
of the detected intensity I is not simply the sum of I.sub.1 and I.sub.2, 
but is a function of I.sub.1, I.sub.2, .alpha., d, and .lambda.: 
EQU I=I.sub.1 +I.sub.2 +2(I.sub.1 I.sub.2).sup.1/2 cos 
(4.pi./.lambda.)d(n.sup.2 -sin.sup.2 .alpha.).sup.1/2! 
To solve for thickness d, it is necessary to experimentally determine the 
values of I.sub.1 and I.sub.2, since these can not be independently 
discerned within the combined beam. Also, the index n must be 
experimentally determined or known. 
To determine these values such that d may be established as a function of 
I, an initial experiment is conducted for each wafer. Since the indices of 
refraction of the film and substrate do not vary appreciably over a single 
wafer, a only a limited portion of the wafer need be tested. The wafer is 
illuminated normally by the light source, and tilted through a range of 
angles as the reflected intensity from the tested portion is monitored and 
recorded. The intensity varies with angle due to the interference between 
the ray components. The intensity function yields a periodic cosine 
squared curve with maxima corresponding to constructive interference, and 
minima corresponding to destructive interference. 
An average is taken of the minima and maxima to calculate a DC component 
providing the I.sub.1 +I.sub.2 terms noted in the intensity equation 
above: 
EQU I.sub.1 +I.sub.2 =.sup.1/2 (I.sub.max +I.sub.min). 
A contrast or fringe visibility factor V is calculated as: 
EQU V=(I.sub.max -I.sub.min)/(I.sub.max +I.sub.min). 
The factor: 
EQU 2(I.sub.1 I.sub.2).sup.1/2 
in the intensity equation above is solvable, as it is equal to the DC 
component multiplied by V. 
The phase factor in the intensity equation is: 
EQU COS (4.pi./.lambda.)d(n.sup.2 -sin.sup.2 .alpha.).sup.1/2 !. 
Thus, having measured I.sub.max and I.sub.min, d may be calculated for each 
point or pixel on the wafer as a function of the I measured by each pixel 
of the CCD. 
For applications in which the film thickness may be greater than .lambda., 
multiple fringes will be generated, and is will be necessary to determine 
the order of the fringe being measured. 
The integral fringe order m is derived from the equation: 
EQU n.sup.2 =m.sup.2 (sin.sup.2 .alpha..sub.1 -sin.sup.2 .alpha..sub.2)+2m 
sin.sup.2 .alpha..sub.1 +sin.sup.2 .alpha..sub.1 !/(2m+1), 
where .alpha..sub.1 and .alpha..sub.2 are the incidence angles 
corresponding to two adjacent intensity minima measured as the wafer is 
tilted. 
The wafer is returned to the horizontal position so that the entire 
surface, or at least any portion desired to be measured, is reflecting an 
image of the light source. The CCD then essentially simultaneously records 
an I value for each pixel, or makes the recording in a rapid raster or 
comparable sequence without movement of the wafer or any of the other 
system components. 
The total thickness at a given point is: 
EQU d=(m+.epsilon.).lambda./2n.sup.2 -sin.sup.2 .alpha.!.sup.1/2. 
The fractional fringe order .epsilon. is given by: 
EQU .epsilon.=.sup.1 /(.sub.2.pi.)! cos.sup.-1 (I-I.sub.1 +I.sub.2 
!)/(2(I.sub.1 I.sub.2).sup.1/2)! 
The .alpha. value for each pixel may be preestablished and mapped when the 
system is set up and calibrated, since the camera does not translate 
relative to the stage, and the stage may returned to the same horizontal 
position for each wafer tilt. 
The computer calculates the thickness of the film for each corresponding 
pixel, using the I.sub.max and I.sub.min levels determined experimentally 
for the wafer being measured, and using the .alpha. value map established 
for each pixel in the given setup. The resulting thickness data may be 
displayed or stored by the computer, and used to correct the film 
manufacturing process, or to reject certain out of tolerance regions of 
wafers produced by the process. 
In the preferred embodiment, the stage 14 and motor controller are units 
from Velmex, of East Bloomfield, N.Y. In an alternative embodiment, the 
extended diffuse light source may be replaced by an extended collimated 
light source, such as a laser beam projected through a diverging lens, to 
a large fresnel lens having a focal point coincident with the focal point 
of the diverging lens, in the manner of a Newtonian or Keplerian 
telescope. 
While the invention is described in terms of a preferred embodiment, the 
following claims are not intended to be so limited.