Method of optical thin film coating

#This invention relates to thin film deposition systems and more particularly to systems for deposition of optical quality thin films.

#Highly transparent materials used as windows for IR and optical applications are often soluble, soft or fragile and require protective coatings before they may be exposed to atmospheric conditions. More durable window materials are available but generally have high indices of refraction and require antireflection coatings to increase transmittance. Many of the better antireflection coating materials are themselves soft, soluble or fragile and thus often require a mechanically protective coating to protect the antireflection coating. Since the thickness of any type of coating on an optical window determines its optical characteristics it is essential that the coating thickness be closely controlled and uniform over the surface of the window.

#Only thin film deposition by resistive heating or electron beam evaporation techniques has been refined to the extent required to provide optical quality thin films. These refinements include planetary motion substrate holders for improving uniformity and monitors and substrate masks for controlling deposition thickness. But these evaporation techniques are limited to materials which can be easily evaporated and which do not decompose upon heating or which, if they do decompose, recombine reactively to form a suitable thin film coating. Many other materials are desirable as either antireflection coatings or mechanical protective coatings for optical windows but cannot be deposited by traditional evaporation techniques.

#Other objects, features and advantages of the invention will be more readily understood from the following detailed description when read in conjunction with the drawing which illustrates a thin film deposition system constituting the present invention.

#The optical thin film deposition system as shown in the drawing includes a vacuum chamber 2 which may be, for example, a Balzers High Vacuum Corporation, Type BA k550. A resistive evaporation station 3 is included within the chamber. This station includes a pair of electrodes 4 extending through the bottom of the chamber in sealing relationship and a resistive heating boat 5 for holding solid materials to be evaporated. Suitable boats, for example, are those known in the art as filaments, point sources, baskets, crucible heaters and box sources sold by the R. D. Mathis Company of Long Beach, California. Also included with the resistive evaporation station is a shutter or mask 6 attached to a rotatable shaft 7 which extends through the bottom of the chamber. The mask 6 is positioned over the material in boat 5 prior to its evaporation to prevent contamination of the material by vaporized materials from other sources. Mask 6 is also used to control carefully the deposition thickness by being positioned over the source when a desired thickness has been reached. Also positioned on the bottom of chamber 2 is an electron beam evaporation source 8. This source 8 is controlled by connections passing through the bottom of chamber 2 in sealing relationship. Source 8 may be, for example, a Sloan Instruments Corporation multihearth electron beam gun Part No. 126-200. A mask or shutter 10 is also provided for source 8 and is used in the same way as mask 6. A heat source 40 such as an Airco Temescal Model HA-2 quartz heater assembly is also located on the bottom of chamber 2 and positioned to radiate heat toward the top of the chamber where substrates to be coated are located. This heater is used in evaporative deposition processes which require that the surface being coated be hot. A diffusion pump 12 is connected at the rear of chamber 2. A cryogenic baffle 14 covers the top of the opening into pump 12 to block the path of particles traveling from the pump back towards the chamber 2. A standard roughing pump is attached to outlet 13. The roughing pump removes the majority of gases from chamber 2 during a pump down sequence while the diffusion pump 12 provides and maintains high vacuum when needed. A substrate support and movement device 11 is attached to the top of the chamber above the resistive and electron beam evaporation stations 3 & 8. The surfaces to be coated are exposed at the bottom of this support device as is necessary in any optical coating system to prevent contamination by particles settling under the force of gravity. The preferred substrate support comprises six circular support members 15 geared to and driven around a larger circular member 16 to provide planetary motion for the substrates as they are coated. At the center of the planetary motion substrate support is a hollow vertical cylindrical shift 17 with an optical monitor glass slidecovering the bottom of this shaft. A light source 36 provides a light beam through optical window 37 in the center of the bottom of chamber 2. The light beam travels up through the chamber to the optical monitor slide and can be transmitted through the hollow vertical shaft 17 in the substrate support out through an optical window 39 in the top of chamber 2 to a light detector 38 or reflected to a similar detector arrangement below the baseplate. The electrical output of detector 38 is functionally related to the thickness of material deposited on the optical monitor slide and is used to stop deposition when a desired thickness has been reached.

#A gas inlet baffle 18 is attached to one side of the chamber 2 with the top of the baffle close to the substrates and substantially aligned with the plane of rotation of the substrates in the substrate supports 15. A perforated gas inlet line 20 is supported within baffle 18 with the perforations oriented to direct laminar gas flow horizontally across the chamber and the substrates. One end of gas inlet line 20 extends out through a wall of chamber 2 to form an input 22 for connection to a source of reactive gases. Where a mixture of reactive gases is needed for the deposition process and the gas cannot be mixed before being piped into the chamber, additional gas lines 20 and inlets 22 may be added to the baffle 18 as necessary. A screen or perforated cover may also be added to the open side of baffle 18 to improve laminar flow across the substrates and to aid in mixing of inlet gases when two or more inlet lines 20 are used. An exhaust baffle 24 is attached to a wall of the chamber 2 across from the inlet baffle 18 and at substantially the same height from the bottom of the chamber. A perforated exhaust line 26 is supported in exhaust baffle 24 and extends through an outlet 28 in the side of chamber 2 for connection to a high capacity vacuum pump 29. The location and shape of exhaust baffle 24 is not as important as that of inlet baffle 18 in providing a flow of gases across the substrate surfaces. The fact that baffle 24 is positioned on the opposite side of supports 15 increases the probability that gases from inlet tube 20 pass through a reactive zone near the substrate surfaces before being removed from the chamber. The gases react due to high substrate temperature (CVD) or due to an RF field applied via an antenna 30 mounted in the chamber (RPD). Baffles 18 & 24 also provide a convenient mounting for RF antenna 30.