Infrared transmissive protective window

Abstract: An electro-optic system 10 is described which comprises an infrared sensor 12 and a processing unit 14 protected by a protective infrared transmissive window 16. The window 16 comprises a substrate layer 18 which may comprise gallium arsenide, zinc selenide, zinc sulfide or germanium. A protective layer 22 of gallium phosphide is formed outwardly from the substrate layer 18. An anti-reflective coating 20 is formed inwardly from substrate layer 18 and an outward anti-reflective coating 26 is formed outwardly from protective layer 22. The incorporation of protective layer 22 allows for excellent impact and wear resistance without interfering with the optical characteristics of the protective window 16.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates in general to the field of electro-optical components and more particularly to a infrared transmissive protective window which comprises a layer of gallium phosphide. BACKGROUND OF THE INVENTION The ability to accurately sense infrared radiation has become an important technical consideration in a variety of military and civilian applications. The sensors used to detect the infrared radiation must be protected from environmental hazards. This is usually done by placing the sensors behind infrared transmissive windows or domes. During the lifetime of the electro-optic system, these protective windows and domes suffer from environmental degradation. This is particularly true of windows and domes designed to protect sensors receiving the eight to twelve µm wavelength region of infrared radiation as these protective windows and domes are commonly constructed from zinc sulfide (ZnS), germanium (Ge), zinc selenide (ZnSe) and gallium arsenide (GaAs). The environmental degradation of the protective windows and domes is the result of exposure to rain and other particle wear which can cause impact damage and general erosion of the outer surface of the protective window or dome. This degradation results in a loss of transmission through the component and at times can result in catastrophic failure of the dome resulting in the destruction of the protected electro-optic sensor. The environmental degradation discussed previously is exacerbated if the window or dome is on a high speed platform like a plane or a missile as is common in military applications. For the most part, efforts to construct protective windows and domes having more resistance to environmental degradation have focused on developing protective coatings for the windows and domes. This is due to the limited choice of materials for the domes and windows themselves which allow transmission of infrared radiation in the eight to twelve µm wavelength region. Coatings comprising diamond-like carbon have been developed and used in the past. Diamond-like carbon offers some wear resistance but can only be used as very thin coatings due to the optical absorption of the material. For example, a typical diamond-like carbon coating can only be one to two micrometres in thickness without appreciably affecting the optical characteristics of the dome or window. Other materials such as boron phosphide and germanium carbide are being developed but still have shown unacceptable levels of optical absorption at coating thicknesses sufficient to provide impact and wear resistance for the dome or window. Accordingly, a need has arisen for an improved infrared transmissive window to protect electro-optic components. EP-A-0 429 210 discloses an infrared-transmissive optical element, having a base layer which may be of gallium phosphide, with a protective coating of a refractory oxide joined to the base layer by a bonding layer comprising the materials of the base layer and protective coating in graduated proportions. EP-A-0 317 264 on which the preambles of the independent claims are based, discloses an infrared transmissive optical element having a base layer which may be of gallium phosphide, having an anti-reflective protective coating. The invention provides an infrared transmissive protective window, comprising: a substrate layer comprising an infrared transmissive material; and a protective layer disposed outwardly from said substrate layer; characterised in that the protective layer is a layer of gallium phosphide. According to some specific embodiments of the present invention, the substrate layer may comprise gallium arsenide, zinc sulfide, germanium, or zinc selenide. According to a further embodiment of the present invention, an anti-reflective coating may be formed on the outer surface of the protective layer of gallium phosphide. A further anti-reflective coating may be formed on the inner surface of the substrate layer. The invention also provides an electro-optic system comprising an infrared sensor and an infrared-transmissive protective window according to the invention disposed outwardly from the sensor. The invention further provides a method for forming an infrared transmissive protective window, comprising the steps of: heating a substrate layer comprising an infrared transmissive material; and forming a protective layer outwardly from the substrate layer; characterised in that the protective layer is a layer of gallium phosphide. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present invention may be acquired by referring to the detailed description and claims when considered in connection with the accompanying figures, wherein: Figure 1 is a graphical representation of the relative transmission characteristics of an uncoated gallium arsenide substrate with a gallium arsenide substrate having a coating of gallium phosphide formed thereon; and Figure 2 is a cross-sectional schematic illustration of an electro-optic system comprising a protective window constructed according to the teachings of the present invention. DETAILED DESCRIPTION OF THE INVENTION According to the teachings of the present invention, an improved infrared transmissive protective window has been developed which comprises a layer of gallium phosphide. Gallium phosphide offers broad infrared transmission over the 0.5 micrometre to 12 micrometre wavelength region. Gallium phosphide further provides for high hardness for wear resistance and high strength for impact resistance. Gallium phosphide further provides for a thermal expansion which is closely matched to the common infrared transmissive component materials which provides for low stresses in the coating and excellent chemical durability. A typical protective dome or window is constructed of an infrared transmissive substrate such as gallium arsenide, germanium, zinc sulfide or zinc selenide on the order of 12.5 mm (half an inch) thick. According to one embodiment of the present invention, a layer of gallium phosphide may be epitaxially grown to a thickness on the order of 25 to 30 micrometres on the outer surface of the infrared transmissive substrate material. The coating of gallium phosphide can be grown using, for example, a conventional thermal chemical vapor deposition process using, for example, tertiary butyl arsenine, trimethyl phosphine, trimethyl gallium as feed gases. By using conventional CVD techniques, stochiometric gallium phosphide may be epitaxally grown to give excellent optical characteristics. Further, the rate of growth of the gallium phosphide coating can be controlled to provide for the formation of polycrystalline gallium phosphide which improves the strength of the gallium phosphide coating. This is accomplished by speeding up the epitaxial growth of the gallium phosphide coating to provide for the formation of grain boundaries within the coating layer. The speeding up of the growth process also provides for economic benefits in that the process can proceed more quickly. Accordingly, conventional chemical vapor deposition techniques provide for exact stochiometric ratios and high purity which provide for excellent optical characteristics of the gallium phosphide layer and fast epitaxial growth provides for polycrystalline formation which increases the strength of the protective coating layer. Figure 1 illustrates the optical transmission characteristics of a 22 micrometre coating of gallium phosphide on a gallium arsenide substrate as it compares to an uncoated gallium arsenide substrate. As can be seen in Figure 1, over the eight to twelve micrometre wavelength region, there is no loss in transmission as a result of the gallium phosphide coating. In fact, due to some impedance matching at the interface between the substrate and protective coating layer, there is some transmission gain over the region of interest. While Figure 1 illustrates the experimental results of a gallium phosphide coating on a gallium arsenide substrate, it should be understood that the gallium phosphide coating could be applied with comparable results to the other common substrate materials - zinc sulfide, germanium and zinc selenide. Gallium phosphide coated substrates have been tested for hardness, infrared transmission, adhesion and rain impact resistance. The hardness of the samples measured 700 using a Vickers micro-indention technique. This measurement is identical to what is measured in bulk single crystalline gallium phosphide which exhibits extremely high wear resistance. The rain impact resistance was measured with a single water drop, two millimeters in diameter, at MACH 0.9 with the gallium phosphide coated gallium arsenide at normal incidence. The test revealed very little damage which is indicative of gallium phosphide. The damage sustained by the gallium phosphide coated substrate was significantly less than the uncoated substrate. The gallium phosphide coatings which have been tested exhibit optical absorption coefficients of less than 1 cm⁻¹. In comparison, other coatings which have been developed such as boron phosphide and germanium carbide alloys exhibit absorption coefficients on the order of 20 cm⁻¹. Gallium phosphide coatings therefore allow for very high impact and wear resistance while exhibiting almost no optical interference with the underlying infrared electro-optic sensors. Figure 2 illustrates an electro-optic system which is indicated generally at 10. Electro-optic system 10 comprises an infrared sensor 12 which is coupled to a processing unit 14. Infrared sensor 12 is protected from environmental degradation by an infrared transmissive protective window, indicated generally at 16. Protective window 16 comprises an infrared transmissive substrate layer 18. As discussed previously, infrared transmissive substrate layer 18 may comprise, for example, gallium arsenide, zinc sulfide, germanium or zinc selenide. It should be understood that the layers shown in Figure 2 are not to scale. The thickness of the entire window 16 would be on the order of 12.5 mm (one-half inch) in thickness. The great majority of the thickness is accounted for by the substrate layer 18. On the inner surface of substrate layer 18, a conventional anti-reflective coating layer 20 is formed. A variety of materials known in the art may be used to form anti-reflective layer 20. As layer 20 resides on the inner surface of the protective window 16, it need not offer any wear or impact resistance. A layer 22 of gallium phosphide is formed outwardly from substrate layer 18. As discussed previously, gallium phosphide layer 22 may be on the order of 25 to 30 micrometres in thickness. An interface region 24 is shown disposed between substrate 18 and gallium phosphide layer 22. Interface region 24 comprises a graded layer approximately five micrometres in thickness which comprises gallium, arsenic and phosphorus. Region 24 may be formed by conventional methods during the epitaxial growth of gallium phosphide layer 22 on the outer surface of substrate layer 18. Region 24 is not a defined layer, but rather is a region where the concentration of the materials is gradually changed during the growth of the layer so that there is a gradual change from the gallium arsenide used to form substrate 18 to the gallium phosphide used to form layer 22. This gradual change in materials allows for improved optical characteristics and improved thermal matching of the layer 22 with the substrate layer 18. An outer anti-reflective layer 26 is formed outwardly from layer 22. Anti-reflective layer 26 may comprise, for example, alternating layers of silicon and amorphous carbon. Anti-reflective layer 26 may be on the order of one to two micrometres in thickness. The use of alternating layers of silicon and amorphous carbon provide for the necessary anti-reflective effects as well as providing for additional wear resistance for the window 16. The relative thicknesses of the layers of silicon and amorphous carbon may be dynamically adjusted during the formation of layer 26 according to the complex index of refraction of the materials as they are formed. The complete window 16 provides for protection of infrared sensor 12 without interfering with the transmission of infrared radiation through the window 16. Additionally, the incorporation of a protective coating of gallium phosphide in the window 16 provides for excellent wear and impact resistance without degrading the optical transmission characteristics of the window 16. Although the teachings of the present invention have been described in detail with reference to the embodiments described herein, it should be understood that a variety of substitutions and alterations may be made hereto without departing from the scope of the present invention which is defined solely by the appended claims.

Claims: An infrared transmissive protective window (16), comprising: a substrate layer (18) comprising an infrared transmissive material; and a protective layer (22) disposed outwardly from said substrate layer; characterised in that the protective layer (22) is a layer of gallium phosphide. The window of Claim 1 wherein said infrared transmissive material (18) comprises gallium arsenide, zinc selenide, zinc sulfide or germanium. The window of Claim 1 wherein said protective layer (22) comprises epitaxially formed polycrystalline gallium phosphide. The window of Claim 1 wherein said protective layer (22) comprises an outer surface and further comprising an antireflective coating (26) disposed on said outer surface. The window of Claim 4 wherein said antireflective coating (26) comprises a layer of silicon and a layer of amorphous carbon. The window of Claim 1, comprising an interface layer (24) disposed between said protective layer (22) and said substrate layer (18), said interface layer comprising gallium, phosphorus and said infrared transmissive material. An electro-optic system (10), comprising: an infrared sensor (12); and an infrared transmissive protective window (16) disposed outwardly from said sensor, said window comprising: a substrate layer (18) comprising an infrared transmissive material; and a protective layer (22) disposed outwardly from said substrate layer; characterised in that the protective layer (22) is a layer of gallium phosphide. The system of Claim 7, wherein said infrared transmissive material (18) comprises gallium arsenide, sinc selenide, zinc sulfide or germanium. The system of Claim 7, wherein said protective layer (22) comprises epitaxially formed polycrystalline gallium phosphide. The system of Claim 7, wherein said protective layer (22) comprises an outer surface, and comprising an antireflective coating (26) disposed on said outer surface. The system of Claim 10, wherein said antireflective coating (26) comprises a layer of silicon and a layer of amorphous carbon. The system of Claim 7, comprising an interface layer (24) disposed between said protective layer (22) and said substrate layer (18), said interface layer comprising gallium, phosphorus and said infrared transmissive material. A method for forming an infrared transmissive protective window, comprising the steps of: heating a substrate layer (18) comprising an infrared transmissive material; and forming a protective layer (22) outwardly from the substrate layer; characterised in that the protective layer (22) is a layer of gallium phosphide. The method of Claim 13, wherein said step of heating comprises the step of heating a layer (18) comprising germanium, gallium arsenide, zinc selenide or zinc sulfide. The method of Claim 13, comprising the step of forming an antireflective coating (26) outwardly from the protective layer (22). The method of Claim 15, wherein said step of forming an antireflective coating (26) comprises the steps of: forming a layer comprising silicon; and forming a layer comprising carbon. The method of Claim 16, wherein said step of forming a layer comprising carbon comprises the step of forming a layer comprising amorphous carbon. The method of Claim 13, comprising the step of forming an interface layer (24) disposed between the substrate layer (18) and the protective layer (22) comprising gallium, phosphorus and the infrared transmissive material. The method of Claim 13, wherein said step of forming a protective layer (22) comprises the step of epitaxially growing a layer of polycrystalline gallium phosphide.

Inventors: KLOCEK PAUL, TABOREK PETER, KLOCEK, PAUL, TABOREK, PETER
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1991
Main classification: G01J5
IPC codes: G01J5, C23C16, G02B1