The present invention discloses an optical measurement and/or inspection device that, in one application, may be used for inspection of semiconductor devices. A method is disclosed for extracting information of a device-under-test for an ellipsometer, comprising the steps: providing a plurality of incoming polarized beams using a plurality of polarizers, where each of the beams being polarized at a designated polarizing angle; using a parabolic reflector to focus said plurality of incoming polarized beams on a spot on a DUT; using a parabolic reflector to collect a plurality of beams reflected from said DUT; and analyzing said collected beams using a plurality of analyzers, wherein each of the analyzers having a designated polarizing angle with respect to its respective polarizer.

FIELD OF INVENTION

The present invention relates to the inspection and measurement systems, and in particular, to optical inspection and measurement of devices under test such as semiconductor devices and/or wafers.

BACKGROUND

Spectroscopic ellipsometry is a very power optical measurement technology widely used in semiconductor manufacturing, optical coating and material analysis. The ellipsometer measures the complex ratio of reflectivity of Rp and Rs, where Rp is the reflectivity of the electrical field whose direction is in the plane of incidence and Rs is the reflectivity of the electrical field whose direction is perpendicular to the plane of incidence. Both Rp and Rs are complex number and they are wavelength dependent.

The ellipsometric quantities are defined as:

where

and
Δ=δp−δs

In conventional ellipsometer, in measuring the ellipsometric quantities, i.e. Ψ and Δ, it is necessary to rotate one of the polarizing components (the polarizer, analyzer or compensator) in the system. This limits the speed of the measurement. In some applications, it is desirable to perform measurements for a very small area (such as measuring the thin film thickness on semiconductor wafers). It is necessary to focus the light to a very small area.

Therefore, it is desirable to have spectroscopic ellipsometers capable of focusing on a small focus spot. It is further desirable to have an ellipsometer with minimal moving parts such that measurements can be taken without moving any mechanical parts in the system.

SUMMARY OF THE INVENTION

An object of the present invention is to provide methods and devices for an ellipsometer that can focus on small focus spots.

Another object of this invention is to provide methods and devices for an ellipsometer that can simultaneously polarize, reflect, analyze, and detect several rays.

Another object of this invention is to provide methods and devices for an ellipsometer that can simultaneously polarize, reflect, analyze, and detect several rays without any mechanical moving parts.

Briefly, a method for extracting information of a device-under-test for an ellipsometer, comprising the steps of providing a plurality of incoming polarized beams using a plurality of polarizers, where each of the beams being polarized at a designated polarizing angle; using a parabolic reflector to focus said plurality of incoming polarized beams on a spot on a DUT; using a parabolic reflector to collect a plurality of beams reflected from said DUT; and analyzing said collected beams using a plurality of analyzers, wherein each of the analyzers having a designated polarizing angle with respect to its respective polarizer.

An advantage of the present invention is that it provides methods and devices for an ellipsometer that can focus on small focus spots.

Another advantage of this invention is that it provides methods and devices for an ellipsometer that can simultaneously polarize, reflect, analyze, and detect several rays.

Another advantage of this invention is that it provides methods and devices for an ellipsometer that can simultaneously polarize, reflect, analyze, and detect several rays without any mechanical moving parts.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, an underlying concept of the embodiments of the present invention is explained. Given a parabola110disposed on a y-axis and a z-axis, conceptually, the shape of the parabola can be described be a simple mathematical function, z=ay2, where incoming rays parallel to the z-axis would intersect the z-axis at its focal point “F”, where the focal point is at (0, ¼a), and “a” is a constant. The incoming ray intersects the parabolic surface and it is redirected towards the focal point at the incident plane112(the plane that is perpendicular to the axis of symmetry and passes through the focal point, “F”).

Here, as shown, the incidental incoming light ray114is parallel to the axis of symmetry. The ray hits the parabolic surface and the parabolic reflector, by virtue of its properties, directs the beam towards its focal point and intersects the z-axis at intersection point “F”. After the intersection, the ray hits the parabolic surface again, and the parabolic surface re-directs the ray118back toward its incident direction parallel to the axis of symmetry. Due to the unique characteristic of the paraboloid, reflected ray will be always be parallel to the axis of symmetry if the incoming ray is parallel to the axis of symmetry.

Referring toFIG. 2, a presently preferred embodiment of the present invention is disclosed. Here, a side view of an optical head is presented where the optical head comprising of a parabolic reflector201and a polarizer205and an analyzer206. The parabolic reflector can be manufactured as a paraboloid and cut in such a manner such that its focal point203is at the surface of the object202to be measured or tested, the device-under-test (“DUT”). The function of the parabolic reflector201is to focus the incoming beam204to the surface of the DUT202at the focal point203and collects the reflected beam from the focal point203.FIG. 3illustrates a top-front view the parabolic reflector201with the polarizer205and analyzer206disposed at the opening of the parabolic reflector.

In the preferred embodiment of the present invention, in performing ellipsometric measurements, several polarizers and analyzers are arranged in such a manner that Rp and Rs as well as the incident power of incoming beam can be measured simultaneously.FIG. 4shows one of such arrangements in illustrating a top view of an optical head. Here, there are eight polarizers and analyzers,405and406,407and408,409and410, and411and412, for incoming rays and reflected rays. Their polarizing axes are parallel to each other except polarizer406whose axis is perpendicular to the rest. In following the illustrated four rays (4a/4b,10a/10b,11a/11band15a/15b), the principle of this device can be demonstrated. Here the letter “a” represents incoming rays and the letter “b” represents outgoing rays.

Each pair of the rays defines a plane of incidence. Ray4apasses through polarizer405and becomes polarized along the direction of the arrow shown. This direction is perpendicular to the incident plane201. After reflecting off the parabolic reflector and the focal point, the outgoing ray4bpasses through another polarizer406. This ray is then received by a detector whose output is proportional to the intensity of the ray.

The principle can be further illustrated in more rigorous mathematical expression illustrated below. Let's use Jones representation for polarization.

Incident beam electrical field:

The electrical field for reflected beam:
{right arrow over (E)}D=−1(A)(A)−1(P)P(P){right arrow over (E)}0

where(A) and(P) are the rotation matrices for analyzer with angle A and polarizer with angle P. The angle is measured clockwise from the plane of incidence along the propagation of the ray.

The intensity on the detector is proportional to

It can be shown that

Here
Rp=RpsampleRpMRpM
and

For ray4aand4b,

For ray10aand10b,

For ray15aand15b,

The mirror effect can be calibrated out with a known quantity sample.

Other reflecting optics can also be used to realize the ellipsometric measurements, such as those illustrated inFIG. 5andFIG. 6. InFIG. 5, an incoming ray passes through a polarizer502, reflects off a lens disposed at504and another lens at506to focal on a focal point at403and a DUT520. After reflecting off the DUT520, the ray reflects off lens at510and lens at504to pass through an analyzer512. InFIG. 6, an incoming ray601passes through a polarizer602, passes through a lens604and focuses at focal point606and a DUT620. The ray reflects off the DUT, passes back through the lens604and an analyzer608. Note that polarization analyzers can be used, where a polarization analyzer redirects incoming light into two orthogonally polarized directions.

FIG. 7illustrates an alternative embodiment where the incident ray is linearly polarized at P=45°, and four analyzers are arranged at A=45°, −45°, 0°, and 90°. There is the incident ray701passing through a polarizer for P=45°702, which can be any lens or a parabolic mirror, reflecting off a parabolic reflector704at706to focus at focal point708, and reflecting off a device-under-test720. The reflected ray again reflects off the parabolic reflector704at710, passes to a beam splitter712to split off to a first analyzer for A=45° and −45°714and a second analyzer for A=0° and 90°716.

for A=−45° and P=45°720

for A=90° and P=45°724

and for A=0° and P=45°722

FIG. 8illustrates yet another alternative embodiment where the incident ray is linearly polarized at P=45°, and four analyzers are arranged at A=45°, −45°, 0°, and 90°. There is the incident ray801passing through a polarizer for P=45°802, which can be any lens or a parabolic mirror, reflecting off a parabolic reflector804at806to focus at focal point808, and reflecting off a device-under-test820. The reflected ray again reflects off the parabolic reflector804at810, passes to a first beam splitter812to split off to a second beam splitter814and a third beam splitter816. From the first beam splitter, the ray passes to analyzers for A=45°822and A=−45°824; and from the second beam splitter816, the ray passes to analyzers for A=0°818and A=90°820.

While the present invention has been described with reference to certain preferred embodiments, it is to be understood that the present invention is not limited to such specific embodiments. Rather, it is the inventor's contention that the invention be understood and construed in its broadest meaning as reflected by the following claims. Thus, these claims are to be understood as incorporating not only the preferred embodiments described herein but all those other and further alterations and modifications as would be apparent to those of ordinary skilled in the art.