Optical waveguides – Optical waveguide sensor
Reexamination Certificate
1998-05-29
2001-08-21
Lee, John D. (Department: 2874)
Optical waveguides
Optical waveguide sensor
C118S688000, C118S712000, C250S559270, C385S128000
Reexamination Certificate
active
06278809
ABSTRACT:
TECHNICAL FIELD
The present invention relates to apparatus for characterizing thin films deposited on opto-electronic and photonic devices, and more particularly to apparatus for in situ monitoring, measurement and control of the optical properties of such films, without the need for bulk optics.
BACKGROUND OF THE INVENTION
Thin film deposition plays a key role in the fabrication of almost all opto-electronic or photonic devices, regardless of the means by which such films are created, such as, for example, molecular beam epitaxy (MBE), ion beam assisted deposition (IBAD), sputtering, chemical vapor deposition (CVD), or metal organic chemical vapor deposition (MOCVD).
Although the semiconductor industry represents a large market, many industrial and consumer goods also employ thin films. These include anti-reflection and scratch-resistant coatings for optics (including eyewear), the manufacture of photovoltaic cells, thermal control coatings for residential- and office-window materials, and deposition of thermal- and/or wear-resistant coatings on turbine blades, tools, and bearing surfaces. This list is hardly inclusive; the number of consumer products requiring deposition of a thin-film (or application of a thin-film coating) is estimated to be far larger than the volume of high-tech electronic and opto-electronic goods.
Whatever the application, monitoring and precisely controlling film thickness is key to maximizing the yield of high-quality, affordable parts. Accurate, real-time information on structure, quality, and film composition permits adjustment of process parameters to reliably and repeatably deliver films with the desired properties. In situ measurements now employed for certain film deposition configurations include RHEED (reflection high energy electron difraction), TOF (time of flight) ion beam surface analysis, quartz crystal monitors, and optical probe techniques, including ellipsometry and interferometry.
By far the most commonly used technique, and the only one permitting a wide-band thickness measurement, is the quartz oscillator, which performs a very indirect measurement of film thickness. Conversion from the oscillator's frequency shift to weight of the coating, then to its thickness, is prone to numerous errors arising from subtle changes in material properties (strain, density, age of crystal, temperature of sensor). Also, quartz oscillators provide no index of refraction information. Furthermore, to avoid shadowing the workpiece, crystal monitors must necessarily be placed a few inches away from the part to be coated. This results in a difference in deposition rates between the workpiece and monitor, which can fluctuate randomly from run-to-run and lead to unpredictable changes in indicated thickness.
Interferometry, ellipsometry, and other optical-probe techniques have been under development for many years and can provide a large amount of information on as-grown films. These techniques are among the most common diagnostics for post-deposition examination and evaluation of films. However, when employed as real-time diagnostics, apparatus employed in phase- and polarization-sensitive measurements must be reproducibly positioned on or about the reactor to within a fraction of a wavelength of light, not an easy adjustment to make in an industrial setting. All of these processes typically proceed “blindly,” without real-time knowledge of the growing layer's characteristics. Reliable, affordable information on the structure, quality, and composition of the growing film would be welcomed and would surely lead to lower cost, swifter development of new devices, and improved quality.
Furthermore, no remote optical diagnostic can escape the need to probe the deposition region through a window; in real-life deposition systems, material is deposited not just on the substrate but also on chamber walls, fixtures, liners, and most particularly on windows. This is no small detail; windows through which interferometric or ellipsometric measurements are made must be coating-free. Although this condition can be met in a (very expensive and very slow-growing) molecular beam epitaxy (MBE) system, even with shields and shutters it is virtually impossible to achieve in the relatively high-throughput CVD reactors and physical deposition systems characteristically employed in production situations. In these systems, the environment is inherently “dirty”, and it is extremely difficult to keep windows from becoming coated.
In addition to these difficulties, many important coating processes require very high temperature substrates; the growth rate and ultimate film thickness achievable by these processes can be effectively limited not by deposition conditions, but by the inability of present optical monitoring techniques to function reliably above 800° C.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a system for in situ characterization of thin films deposited on a substrate. The system comprises:
a) a deposition chamber which includes an input port for introducing one or more process reactants into the chamber, an output port for exhausting by-products from the chamber, and one or more controllers for monitoring and controlling the environment within the chamber;
b) a support within the deposition chamber for supporting a substrate upon which a film is to be deposited;
c) at least one optical fiber mounted within the deposition chamber so that the film deposited on the substrate is also deposited onto at least a portion of the optical fiber;
d) a light source disposed outside of the deposition chamber and coupled to the optical fiber for transmitting light through the optical fiber to the film deposited on the fiber and establishing a light reflectance pattern from the film on the optical fiber;
e) a detector for detecting light reflected from the film on the optical fiber and for generating a signal representative of the reflected light; and
f) a signal analyzer for analyzing the signal to characterize the film substantially in real time and to provide a feedback signal for controlling the environment of the chamber and the temperature of substrate to achieve desired properties of the film.
The controllers in the deposition chamber preferably control one or more of the temperature, pressure, composition and flow rate of the process reactants.
The support for the substrate preferably includes a temperature control element for controlling and regulating the temperature of the substrate before, during and after the deposition process.
The optical fiber is preferably mounted within the deposition chamber so that the film deposited on the substrate is also deposited onto an end of an optical fiber.
The light source preferably includes a source of monochromatic light and a source of polychromatic light. The two types of light are coupled into the optical fiber in a predetermined ratio. The source of monochromatic light preferably includes a light-emitting diode, such as a laser diode, and the source of polychromatic light preferably includes broadband white light.
The signal analyzer preferably includes a signal processor for smoothing the signal against fluctuations in intensity of input light. It can also include a display element for displaying the signal.
In a preferred embodiment, the optical fiber includes a plurality of deposition surfaces or end facets which are renewable or replaceable after each deposition cycle so as to present a fresh facet for deposition. The optical fiber can include, for example, a replaceable tip. In a preferred embodiment, the optical fiber is adapted for use at temperatures in excess of 1100° C. and can be made of, for example, sapphire.
The films deposited onto the substrate and the optical fiber can be formed by a deposition process selected from the group consisting of chemical vapor deposition, metal organic chemical vapor deposition, sputtering, ion beam assisted deposition, and molecular beam epitaxy. The properties of the film which can be characterized include the rate of film growth, the
Johnson Edward A.
Morse Theodore F.
Ion Optics, Inc.
Lee John D.
McDermott & Will & Emery
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