Coating apparatus – With indicating – testing – inspecting – or measuring means – With means for visual observation
Reexamination Certificate
2001-11-01
2003-09-16
Lund, Jeffrie R. (Department: 1763)
Coating apparatus
With indicating, testing, inspecting, or measuring means
With means for visual observation
C118S708000, C118S712000, C118S7230VE, C118S7230EB, C118S730000
Reexamination Certificate
active
06620249
ABSTRACT:
The present invention relates to a method and to apparatus for depositing thin layers.
The technical field of the invention is that of manufacturing thin-layer optical devices.
The invention applies in particular to fabricating optical filters essentially constituted by a stack of thin layers deposited in succession on a plane substrate.
BACKGROUND OF THE INVENTION
Such fabrication conventionally makes use of a technique of depositing a material that is to constitute each layer on the substrate or on the stack inside an enclosure in which pressure is maintained at a low value (generally referred to as a “vacuum” enclosure), with the material being deposited by being “evaporated” or “sublimed” from a “source” of said material followed by “condensation” of the same material on a “target” which is constituted by the substrate or by the stack that is being built up; for this purpose, use is made of one or more electron or ion guns.
In order to obtain determined optical performance for the thin layer device, it is necessary to control the thickness of each deposited layer with sufficient accuracy to ensure that thickness error is minimized, and in particular is less than 1%.
For this purpose, various methods have been developed for monitoring the thickness of a layer while it is being deposited.
A “mechanical” method consists, prior to starting the deposition process, in placing a resonant structure such as a quartz crystal inside the vacuum enclosure close to a substrate that is to receive the thin layers; the “evaporated” material is then deposited on said structure as the deposition process advances in a manner that is assumed to be identical to the deposition of the material on the substrate; the deposit on the structure leads to a change in its resonant frequencies; by measuring at least one of these resonant frequencies it is possible to obtain an (indirect) indication concerning the thickness of the layer deposited on the resonant structure, and this thickness is assumed to be identical to the thickness deposited on the substrate; that indirect method of monitoring the thickness of thin layers being deposited does not enable the desired accuracy to be obtained.
There also exist “optical” methods of monitoring the thickness of thin layers that are being deposited, which methods are generally based on measuring the variation over time in the transmission or the reflection of a light beam directed on the stack while it is being built; such optical monitoring techniques include those in which the light beam is monochromatic, those in which the light beam is bichromatic, those in which the light beam is multichromatic, and those in which the light beam is “broad-band”.
Techniques using one or two wavelengths are particularly suitable for monitoring optical thicknesses of quarterwave stacks; conversely, the technique using a broad-band beam is well adapted to stacks in which the optical thicknesses are not quarter wavelengths; in the monochromatic technique, a transmission extremum is sought, generally by computing the derivative with respect to time of transmission, and an order to stop depositing material is issued when the derivative becomes zero; it is also possible to cause deposition to stop when the partial derivative of the transmission factor relative to the optical thickness becomes zero.
In a bichromatic technique, deposition is generally stopped as a function of the measured value of the difference in transmission at the two wavelengths under consideration.
In a broad-band technique, deposition is generally servo-controlled as a function of the difference between a predetermined transmission spectrum and a transmission spectrum as measured during deposition.
In a multichromatic technique, prior to performing deposition, a plurality of wavelengths are determined such that for each of them the transmission of a layer of the stack presents an extremum when the desired optical thickness is reached.
In practice, the thickness deposited on a reference substrate is monitored, and when a plurality of substrates are being deposited on a common support that is set into rotation, the reference substrate is at a distance from said substrates; as a result the accuracy of the monitoring is not sufficient.
U.S. Pat. No. 5,425,964 describes a broad-band optical monitoring method.
U.S. Pat. No. 4,311,725 describes a method and apparatus for measuring and controlling the deposition of a thin layer by combining a mechanical method and an optical or an electrical method: deposition is monitored on the basis of the ratio between a signal delivered by a crystal which is subjected to deposition and a signal representative of the transmittance, the reflectance, or the resistivity of the thin layer being deposited.
Although the known methods and apparatuses for monitoring the deposition of a thin layer can, in certain applications, present performance that is sufficient, other applications, and in particular the manufacture of optical bandpass filters of multilayer structure for processing telecommunications signals, require better performance.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is thus to provide an improved method and apparatus for monitoring vacuum deposition of thin layers.
In one aspect of the invention, an optical property such as transmittance or reflectance is measured on a plurality of thin layers (TL) or a plurality of stacks of thin layers which are being formed by vacuum deposition onto a plurality of substrates (or substrate portions), with measurements being performed by a plurality of light beams, each of said light beams being directed towards a respective one of said substrates; this makes it possible to determine said optical property for each of said TLs or for each of said TL stacks, and to control changes in and/or to interrupt deposition selectively for each of said TLs or said TL stacks.
By substantially simultaneous use of a plurality of monitoring light beams respectively associated with the thin layers being deposited on a plurality of substrates, it is possible to measure and control deposition individually on a plurality of substrates, thus making it possible to obtain improved uniformity in the deposits made in this way.
In a preferred embodiment, the monitoring light beams are obtained by splitting a single “primary” light beam so that the monitoring beams present characteristics which are homogeneous if not identical, thereby simplifying beam processing (after transmission or reflection by the devices being fabricated.
The single primary beam can be split into a plurality of monitoring beams by time division ; this can be achieved quite simply by moving the substrates inside the vacuum enclosure during the deposition process so that each of the substrates (stacks) is brought in succession onto the path of said single beam; in this way, the substrates or stacks to be monitored form a shutter because they are in motion, and the reflected or transmitted beam forms in succession as many monitoring beams as there are substrates (or stacks) that have passed through it.
The substrates or stacks preferably move in substantially continuous manner around a closed outline of circular shape so that all of the substrates are maintained at a substantially constant distance from the source, thereby encouraging continuous and uniform deposition of the material from which the thin layer is being formed; for this purpose, the substrates can be mounted on a support in the form of a circular ring that is set into uniform rotation about its axis of symmetry, throughout the duration of deposition.
In another aspect of the invention, in a method of fabricating a batch comprising a plurality of thin layer optical components by vacuum deposition, in which all of the components are placed on a common support that is mounted to rotate inside the vacuum enclosure, the substrates are placed on the support in such a manner as to ensure that they are situated at substantially the same height (or distance) from the source of material to be deposited, and during dep
Highwave Optical Technologies
Ladas and Parry
Lund Jeffrie R.
Zervigon Rudy
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