Optics: measuring and testing – By polarized light examination – Of surface reflection
Patent
1989-10-31
1991-10-29
Willis, Davis L.
Optics: measuring and testing
By polarized light examination
Of surface reflection
G01N 2121
Patent
active
050610724
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a differential ellipsometer, more particularly to an ellipsometer for measuring differences in the ratio of complex amplitude reflectance (or the effective complex refractive index) between two or more regions of a sample. The differential ellipsometer may be used in either a transmission or a reflection mode.
BACKGROUND OF THE INVENTION
Ellipsometry is used to determine the state of polarisation of light. When a polarised light beam interacts with an optical system, the polarisation state of the beam may be modified by the optical system. By careful measurements of the initial and final states of polarisation, properties of the optical system under study may be obtained. This is the general use of ellipsometry. Of special relevance to this invention is the use of ellipsometry for measuring the effective complex refractive index of a small region of a sample.
For the conditions that prevail during the epitaxial growth of crystalline materials by molecular beam epitaxy, the crystal surface is clean and unaffected by oxides and other contaminating layers. Under these conditions it is possible to measure the optical properties of the material without the complicating effects of contaminants. In addition for many materials the refractive index varies with the proportion of constituent components and it is therefore possible to use ellipsometry to measure the composition of the material. This may be very useful in the case of the molecular beam epitaxial growth of compounds. The low energy gap semiconducting material cadmium mercury telluride (Cd.sub.x Hg.sub.1-x Te) is such a material. Here `x` is the molar fraction of cadmium telluride in the alloy.
Cadmium mercury telluride for use in infrared optoelectronic devices is prepared in the form of very thin single crystal layers. The composition required for the layers is determined by the portion of the infrared spectrum in which the devices are intended to operate. In order that device production yields be economically viable it is important that the composition of the layers be controlled to a high level of uniformity across the area over which devices are to be made. A variety of epitaxial growth methods are employed to grow single crystal layers of cadmium mercury telluride. All methods known to us produce thin films with variations in composition larger than 0.001 in the molar fraction of cadmium telluride across a distance of approximately 2 centimeters.
DESCRIPTION OF THE PRIOR ART
A method of controlling the compositional uniformity of epitaxial layers is the subject of Australian Patent Application No. 6195186. In the said application an ellipsometer may provide information on the composition at a point on the surface of the film. This information may in turn be used in a closed loop system to control the composition of the growing film at that point. By a variety of possible means, such as scanning the ellipsometer beam across the film, the composition of the film at any number of points may be monitored and hence controlled and the uniformity of the film assured.
In the growth method of said patent application if the compositional information is obtained by ellipsometry, then for each point on the film for which the composition is to be controlled it is necessary to perform one fourier transform per sample period. To obtain the necessary precision of measurement the fourier transform must use a large number of sampled data. Such transforms are time consuming to compute, and may limit the number of points which can be monitored on the film or the frequency with which they can be monitored. In the case of the epitaxial growth of cadmium mercury telluride by molecular beam epitaxy, a fourier transform using at least 1024 sampled data, must be carried out each second for every point of interest on the film. This could require many thousands of transforms to be carried out each second. The computing power required to achieve this level of performance is very costly and it is therefore desirable to reduce the number and compl
REFERENCES:
patent: 4655595 (1987-04-01), Bjork et al.
Smith, "An Automated Scanning Ellipsometer", Surface Science, vol. 56, No. , pp. 212-220, 6/76.
Derwent Abstract Accession No. 87-269624/38, Class S03, SU,A, 1288558 (USSR RADIO ELTRN) Feb. 7, 1987.
Derwent Abstract Accession No. 86-3111148/47, Class S02,SU,A1226042 (Optical Society of America, New York), R. M. A. Azzam, `Simple and Direct Determination of Complex Refractive Index and Thickness of Unsupported or Embedded Thin Films by Combined Reflection and Transmission Ellipsometry of 45.degree. angle of Incidence`,see pp. 1080-1082.
Folkard Margaret A.
Hartley Richard H.
c/o The Secretary, Comonwealth of Australia Department of Defenc
Koren Matthew W.
Willis Davis L.
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