Compact spectroscopic ellipsometer

【0001】

[Technical field to which the invention belongs]

The present invention relates to the field of ellipsometry (ellipsometry, ellipsometry), and more specifically to infrared operated ellipsometry.

【0002】

The present invention relates to the field of ellipsometry, more specifically the field of microelectronics, the field of optical characterization of samples, the field of optical control of surface treatments, or the field of thin film growth, eg, semiconductor materials and their interfaces. Has general use in the field of research. The present invention also has applications in surface cleaning, polishing and pretreatment, in particular.

【0003】

[Conventional technology]

Ellipsometry measurements can be performed at a constant wavelength (monochromatic ellipsometry) or at several wavelengths (spectroscopic ellipsometry).

【0004】

Depending on the wavelength range of the light source, such as ultraviolet, visible, near-infrared, and infrared, it is possible to obtain different properties of layers and / or materials, or to explore different materials.

【0005】

In practice, infrared radiation is generally better adapted than visible light to obtain volumetric properties of layers and materials.

【0006】

In general, infrared-operated ellipsometers (ellipsometers, ellipsometers) are designed to carry a sample of a given thickness with a light source that supplies at least one infrared beam; front and back. A sample holder consisting of a sample holder; a detector; a polarizer and a condenser to illuminate a sample placed in the sample holder at an oblique incidence by a beam of polarized light mounted between the light source and the sample holder. It comprises a first optical device consisting of a device; a second optical device mounted between a sample holder and a detector and consisting of a condensing device and an analyzer for condensing the light reflected by the sample.

【0007】

Due to the very strict conditions in semiconductor manufacturing, ellipsometers with high spatial resolution and maximum possible measurement accuracy are required.

【0008】

Using a transparent or translucent sample of a given thickness, such as silicon, the back surface of the sample can confuse ellipsometry measurements by reflecting stray radiation that adversely affects the detection and processing of useful signals.

【0009】

Since the reflectance coefficient of the rear surface of the silicon substrate is not always known, it is difficult to deal with this adverse effect. The absorption coefficient k of the substrate is also not always known. Further, an interference phenomenon can occur on the rear surface. Similarly, infrared diffusion and / or diffraction phenomena can also occur on this posterior surface. Moreover, the rear surface does not have to be parallel to the anterior surface, which can cause additional unnecessary calculations because only the anterior surface measurement is relevant to the user.

【0010】

[Problems to be Solved by the Invention]

Known to eliminate the harmful effects caused on the posterior surface of the sample, in particular to eliminate the use of mechanical means such as means of fogging the posterior surface, i.e., means of neglecting specular reflections radiated on the posterior surface. There is a solution.

【0011】

When the sample thickness is relatively large, the radiation emitted on the front surface can be separated from the radiation emitted on the rear surface. However, such a solution is achievable only for large thickness samples, which limits its application.

【0012】

Other known solutions consist of taking absorption samples (ie, non-transparent samples such as highly doped silicon), which also limits the application of such solutions. ..

【0013】

It is an object of the present invention to eliminate these shortcomings and to provide a high spatial resolution ellipsometer that operates on infrared light and eliminates stray radiation from the posterior surface of the sample.

【0014】

[Means for solving problems]

The present invention includes a light emitting source that supplies at least one infrared beam, a sample holder that carries a sample of a predetermined thickness and has front and rear surfaces, a detector, the light emitting source, and the sample holder. A first optical device equipped with a polarizer and a condensing device for irradiating the sample placed in the sample holder at an oblique incidence using a beam of polarized light mounted between the sample holders and the sample holder. The present invention relates to an ellipsometer device of a type that is attached between the detector and a second optical device that includes a condensing device and an analyzer to condense the light reflected by the sample.

【0015】

According to the general provisions of the present invention, the ellipsometer device can be attached to the reflection path at the focal plane of the condensing device of the second optical device to block stray radiation radiated on the back surface of the sample and of the sample. It also has a blocking device that allows useful radiation emitted in the anterior direction to pass through towards the detector, thereby providing a separation force for the anterior and posterior surfaces of the sample.

【0016】

For example, the blocking device is of a type composed of slots with adjustable dimensions, cutters with adjustable edges, and the like.

【0017】

According to the first preferred embodiment of the present invention, the ellipsometer device according to the present invention is attached to the irradiation path and can widen the irradiation beam on the condensing device of the first optical device, and the second optical device can be widened. It also has a widening device capable of widening the reflected beam on the condensing device.

【0018】

For example, widening devices are of the type composed of slots with adjustable dimensions, cutters with adjustable edges, and the like.

【0019】

The numerical aperture of the focusing device in the first optical device is preferably selected to obtain a small size irradiation beam on the sample. For example, the size of the irradiation beam on the sample is smaller than 40 microns × 40 microns in the case of a laser type light source.

【0020】

For example, not only the second optical device but also the condensing device of the first optical device includes at least one optical element belonging to a group formed by a concave mirror (for example, an elliptical mirror, a parabolic mirror, a spherical mirror, etc.). ing.

【0021】

The numerical aperture of the condensing device in the second optical device is selected to separate the beams reflected by the front and back surfaces of the sample.

【0022】

According to a second preferred embodiment of the invention, the ellipsometer device according to the invention is mounted downstream of the reflection path of the blocking device according to the direction of light propagation and has a predetermined range of incident angles for measurement by the detector. It further comprises a selector device for the angle of incidence that can select only the radiation reflected by the sample at oblique incidence within.

【0023】

The selector device is preferably of the type composed of slots with adjustable dimensions, cutters with adjustable edges, and the like.

【0024】

In fact, the light source may be a laser type operating at a terahertz frequency, a silicon carbide source, a filament, or a plasma.

【0025】

The polarizer of the first optical device is preferably a type having a grid having a rotation compensating plate or a grid having no rotation compensating plate, or a set having some polarizers having a grid. It is composed of three-dimensional objects.

【0026】

Similarly, the analyzer of the second optical device preferably includes a grid having a rotation compensating plate or a grid having no rotation compensating plate, or two polarizers having a grid. It is composed of an assembly and the like.

【0027】

For example, the detector is of the type composed of mercury-cadmium and / or tellurium cells, liquid nitrogen, and the like.

【0028】

In fact, the sample holder is of a type composed of a table that can move and / or rotate in the XYZ directions, a hanging sample holder, or the like.

【0029】

Other features and advantages of the present invention will become apparent in light of the following detailed description and drawings.

【0030】

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, the light source S provides radiation in the infrared spectrum.

【0031】

For example, the light source S is a silicon carbide type at 1200 ° K. Its spectral range is 1.44-18 microns.

【0032】

As one modification, the light source is of a type composed of a laser operating at a terahertz frequency and a light source including a filament or plasma.

【0033】

In the ellipsometer, the irradiation system includes a Michaelson-type interferometer mounted after the light source and in front of the polarizer to scan the spectral range of the device.

【0034】

The sample holder PE is designed to carry a sample ECH of a predetermined thickness and includes a front FAV and a rear FAR.

【0035】

The sample is, for example, a silicon substrate having a thickness on the order of 400 to 700 microns.

【0036】

The sample holder may be a table that can move in the XYZ directions and / or rotate.

【0037】

The sample holder may also be a hanging sample holder.

【0038】

A first optical device including a polarizer P and a condensing device M1 is provided between the light source S and the sample holder PE. This first optical device makes it possible to irradiate the sample ECH arranged on the sample holder PE with a polarized light beam at an oblique angle.

【0039】

It is advantageous to attach the widening device F1 to the irradiation path. The device can be placed upstream or downstream of the polarizer depending on the direction of light propagation. This widening device spreads the irradiation beam with the mirror M1.

【0040】

In fact, the widening device F1 is of the type consisting of adjustable sized slots, adjustable edge cutters, divergent lenses, and so on.

【0041】

In fact, the numerical aperture of the mirror M1 is chosen to obtain a small size irradiation beam on the sample.

【0042】

For example, the size of the irradiation beam on the sample is smaller than 40 microns x 40 microns within the configuration of the laser source.

【0043】

In fact, the mirror M1 that constitutes the condensing device of the first optical device is an ellipsoidal mirror.

【0044】

As one modification, the mirror M1 may be a parabolic mirror, a spherical mirror, a lens, or a light refraction optical unit or a catadioptric unit.

【0045】

The polarizer P is of a type composed of a grid having a rotation compensating plate or a grid having a grid without a rotation compensating plate. As an example of modification, this polarizer may consist of an assembly having two polarizers having a grid or the like.

【0046】

The second optical device is mounted between the sample holder PE and the detector D. This second optical device consists of a condensing device M2 and an analyzer A for condensing the light reflected by the sample.

【0047】

The numerical aperture of the focusing device M2 in the second optical device is selected to separate the beam reflected by the front FAV and rear FAR of the sample.

【0048】

The condensing device M2 of the second optical device consists of a concave mirror (elliptical surface, paraboloid, or spherical surface), a lens, and optical elements belonging to a group formed by a refraction optical unit or a catadioptric unit.

【0049】

For example, the numerical aperture of the condensing device M2 in the second optical device is on the order of 2.5 °.

【0050】

The analyzer A of the optical device is of a type composed of a grid having a rotation compensating plate or a grid having no rotation compensating plate, or an assembly having two polarizers having a grid. Is. This analyzer A is located downstream of the reflection path of the second mirror M2.

【0051】

The detector D is of a type composed of a mercury-cadmium-tellu cell, liquid nitrogen, or the like. This detector is compatible with infrared operation.

【0052】

It is advantageous to provide another mirror M3 between the mirror M2 and the detector D that can focus the reflected beam on the detector. The mirror M3 may be of the same type as the mirror M2.

【0053】

It is advantageous to couple the selector device F3 for the angle of incidence to the mirror M3. The selector device F3 allows only the radiation reflected by the sample to be selected for oblique incidence within a predetermined range of incident angles for measurement by detector D.

【0054】

For example, the selector device F3 is of a type composed of slots with adjustable dimensions, cutters with adjustable edges, and the like.

【0055】

According to the present invention, the blocking device F2 mounted on the reflection path is provided on the focal plane of the focusing device M2 in the second optical device. This blocking device F2 is capable of blocking stray radiation RPs emitted by the sample's posterior FAR and allows useful radiation RUs emitted by the sample's anterior FAV to pass towards detector D.

【0056】

Such a blocking device F2 makes it possible to obtain separation force for the front FAV and rear FAR of the sample.

【0057】

It is advantageous that the blocking device F2 is of a type composed of slots with adjustable dimensions, cutters with adjustable edges, and the like.

【0058】

It is advantageous for the selector device F3 to be positioned in front of the mirror M3. The reason is that when it is placed in front of the mirror M2, the diffraction due to the selector device F3 reduces the separating power of the blocking device according to the present invention.

【0059】

In a preferred embodiment of the present invention, an optical fiber is arranged between the light source S and the first optical device P. Similarly, another optical fiber is arranged between the second optical device M3 and the detector D.

【0060】

FIG. 2 shows the intensity curve C1 and the intensity differential curve C2 of the beam as the position function of the cutter F2 with respect to the normal of the beam.

【0061】

Curves C1 and C2 show the effective separation of beams from the anterior and posterior surfaces of the sample.

【0062】

Applicants have noticed that when an ellipsometer according to the drawing described in FIG. 1 is used, the ellipsometer will perform infrared ellipsometry measurements on the front and back of the sample in a semiconductor material such as silicon. To have a completely satisfactory value of separation to perform.

【0063】

For example, the separation power is a light source with a wavelength on the order of 12 microns, a silicon substrate with a thickness on the order of 500 microns, a mirror M1 with an effective numerical aperture of 2.5 °, and an effective numerical aperture on the order of 2.5 °. It is on the order of 400 microns using a mirror M2 with a mirror M2 and an image in slot F1 on the order of 300 microns.

【0064】

In fact, the practice of the present invention here depends on the size of the spot, the quality of the optics, the thickness of the silicon substrate, and the angle of incidence. For example, with a 70 ° angle of incidence and a mirror M2 with a magnification factor of 4.21, the separating power of the blocking device is on the order of 600 microns.

[Simple explanation of drawings]

FIG. 1 is a general view of an infrared operated ellipsometer according to the present invention.

FIG. 2 is a diagram showing a separating force of a blocking device according to the present invention as a position function with respect to a light beam.

[Explanation of symbols]

A Analyzer D Detector ECH Sample FAR Rear FAV Front F1 Widening device F2 Blocking device F3 Selector device M1 Focusing device M2 Focusing device M3 Selector device P Polarizer PE Sample holder RP Stray radiation RU Useful radiation S Light source