Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1997-06-03
2001-04-17
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192260
Reexamination Certificate
active
06217720
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for depositing complex optical multilayer coatings onto a substrate.
BACKGROUND OF THE INVENTION
In a number of different fields such as telecommunications, scientific instrumentation, optics and so forth, there is a need for more complex optical thin film coatings to meet the requirements of advanced applications. In the past, due to limitations in design techniques, deposition equipment and thickness monitoring instrumentation, it has not always been possible to achieve the desired filter specifications. However, recent advances in all three of these areas have allowed more complex coatings to be designed and fabricated.
For example, in the area of optical thin film design there have been some dramatic improvements over the past few years in the ability to design coatings without any starting design for a given set of materials. Indeed, state of the art of thin film design has advanced to the point where very complex coatings can be found that meet all but the most stringent filter specifications.
The second advance is in energetic deposition methods, which have dramatically changed the field of thin film manufacture over the past twenty years. Previously, and still true to a large extent, most optical coatings have been fabricated using e-beam evaporation or thermal evaporation. However, materials deposited by these processes are generally quite porous, resulting in filters with poor humidity and temperature stability unless they are specially protected. Newer energetic deposition processes, such as ion-assisted evaporation, reactive ion-plating and magnetron sputtering give rise to films that have bulk-like properties which results in filters with good-to-excellent temperature and humidity stability. More importantly perhaps, materials deposited by these energetic techniques have optical constants that are very reproducible from run-to-run. With bulk-like materials and optical constant reproducibility, it becomes more feasible to manufacture complex optical coatings on a routine basis.
The third advance has been in methods used to control or monitor the film thickness during deposition. In particular, optical monitoring techniques have greatly improved over the past ten years. Sophisticated and inexpensive wideband monitors for the visible region are now readily available and infrared photodiode arrays are becoming more common. With these wideband monitors, it is now possible to more accurately determine the thickness of a deposited layer.
In the past, for limited filter quantities, it may not have been economical to design and fabricate small quantities of complex thin film filters even though there is a large market for such custom or prototype coatings. Part of the reason for this is the time it takes to design a coating and the number of trial deposition runs that usually have to be made before a filter is successfully fabricated. If, in addition, the deposition system requires constant operator intervention to ensure the coating is accurately deposited, the cost will be further increased.
In 1991 a project was begun at the National Research Council of Canada (NRCC) to develop an Automated Deposition System (ADS) that could routinely fabricate complex optical coatings automatically without the need for operator intervention during the deposition process.
The original rf-ADS at the NRCC consisted of a cryo-pumped chamber having a rotatable substrate; three rf-sputtering targets; and a wideband optical monitor that will be described in more detail below. The targets and chamber were designed and built by Corona Vacuum Coaters. In the ADS, the sputtering targets and substrates are mounted vertically. The targets are usually metal or semiconductor, so that the system uses rf reactive sputtering for dielectric layers. Typical deposition rates for materials like Nb
2
O
5
and SiO
2
using rf sputtering are ~0.1 nm/s for a target-to-substrate distance of ~12 cm, a total pressure of ~3 mTorr and an oxygen-to-argon flow ratio of ~1.0. The substrate is controlled by a stepper motor that can be used to swing the substrate to the various target positions. The deposition system was controlled by a Techware Systems (known as Brooks Automation (Canada)) PAL68000 controller. An operator can automatically initiate a number of different sequences including pumping down the chamber, starting a deposition run or venting the chamber.
A real-time process control algorithm accurately controls the film deposition thickness for low rate deposition, i.e. deposition rates of the order of 0.1 nm/s. This technique requires a wideband optical monitor that is able to make accurate, absolute, transmittance measurements over a sufficiently wide spectral range. Also, because it is difficult to continuously monitor the deposition of a layer in sputter systems where the target-to-substrate distance is small, this method relies on making one or two transmittance measurements near the end of a layer deposition.
The optical monitor consists of a quartz-halogen lamp source, light delivery optics, and a wideband detector. The collimated light from the source passes through the chamber and is collected by an achromatic lens that focuses the light through a shutter onto the circular aperture of a fiber-optic bundle. At the other end of the bundle, the fibers are arranged to form a slit at the entrance to a monochromator. The light is then dispersed onto a 512-element Hamamatsu photodiode array. The grating was chosen such that optical monitor could measure over a 380 to 860 nm spectral range. In order to make absolute transmittance measurements, it is possible to rotate the substrate in and out of the optical monitor light path. This allows intensity measurements to be made with and without the substrate. These measurements, after subtracting the background, are then normalized to provide the absolute transmittance of the substrate. This measurement process is completely automated.
The last key element in the ADS system is an integrated thin film program that can be used to first design complex multilayer coatings, based on the optical constants of the materials deposited by the ADS, and then can be subsequently used to oversee the manufacture of the coating. This program can determine the current or previous layer thicknesses from the absolute transmittance measurements of the optical monitor. In addition, the program can reoptimize the remaining layers in the multilayer system at any time during deposition in order to achieve the desired filter specifications.
The program is integrated with the deposition controller in such a way that it does not need to know any details concerning the actual deposition system. When it requires a particular layer to be deposited, it is sufficient to pass down the layer material, the desired thickness and a process name. The controller software then interprets this information to determine the target that the substrate should be rotated to; the length of time the substrate should remain in front of the target; and the deposition parameters that should be used during the deposition. By separating the thin film control algorithm and the deposition system in this way, it is possible to completely change the deposition system and processes without affecting the thin film program.
For a given layer during deposition, the thickness process control algorithm essentially has three stages:
I. termination of layer deposition,
II. determination of layer thickness deposited, and
III. reoptimization of remaining layer thicknesses.
With the ADS, stages I and II are combined together. The first stage, concerned with the termination of a layer deposition, can be based on time alone since sputter deposition is being used in this system. Since the uncertainties in the deposition are typically of the order of 1-3%, for a reasonably well-controlled process, this implies that in order not to overshoot the desired layer thickness, the target thickness first specified should be around 95-97% of the desired thickness. Once this
Akiyama Takayuki
Clarke Glenn A.
Dobrowolski Jerzy A.
Ito Takashi
Sullivan Brian T.
Cantelmo Gregg
Jacobson Price Holman & Stern PLLC
National Research Council of Canada
Nguyen Nam
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