Abstract:
A method of making a color filter array on a first substrate having an array of pixels is disclosed. The method includes depositing and patterning a photoresist layer on the substrate layer to form selected openings over pixels in the array; providing a colorant layer having a transferable colorant material on a second substrate and positioning the second substrate such that the transferable colorant material is in transferable relationship with the first substrate; transferring the colorant material to the photoresist layer on the first substrate; and removing the patterned photoresist layer leaving behind the colorant material in the position of the openings over the selected pixels.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to commonly-assigned U.S. patent application Ser. No. 08/648,772 filed May 16, 1996 and entitled &#34;Method of Forming an Organic Electroluminescent Display Panel&#34; by Littman et al; commonly-assigned U.S. patent application Ser. No. 08/788,537, filed concurrently herewith, and entitled &#34;Method of Depositing Organic Layers in Organic Light Emitting Devices&#34; by Tang et al; commonly assigned U.S. patent application Ser. No. 08/788,532 filed concurrently herewith, and entitled &#34;Method of Making Color Filter Arrays by Colorant Transfer and Etch&#34; by L. C. Roberts; commonly-assigned U.S. patent application Ser. No. 08/788,108 filed concurrently herewith, and entitled &#34;Method of Making Color Filter Arrays by Transferring Two or More Colorants Simultaneously&#34; by Roberts et al; and commonly-assigned U.S. patent application Ser. No. 08/787,732, filed concurrently herewith, and entitled &#34;Method of Making Color Filter Arrays by Colorant Transfer Using Chemical Mechanical Polishing&#34; by Roberts et al, the disclosures of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to methods of making color filter arrays which are particularly suitable for use on image sensors. 
     BACKGROUND OF THE INVENTION 
     In making color filter arrays, separate layers of different colors must be formed. Frequently these layers are formed using dye as the colorant material which is imbibed into dye receiving layers which must be of a controlled thickness, and a precise amount of dye must be used to achieve the appropriate color. See, for example, commonly assigned U.S. Pat. No. 4,764,670 to Pace et al and U.S. Pat. No. 4,876,167 to Snow et al which describe such a process. An additional problem associated with this process is that the dye receiving layers swell upon the introduction of dyes, limiting the smallest dimension which can be attainable for use over very small pixels. Another problem with this process is that dyes are susceptible to fading on exposure to light. 
     Color filter arrays may also be fabricated using evaporated colorants which do not involve any receiving polymer. In order to fabricate such color filter arrays over image sensors, a typical process is as follows: A photoresist layer is coated on a semiconductor substrate such as silicon which already has an array of light receiving sites referred to in the art as pixels or photo-sites formed in the substrate. Thereafter, the photoresist is patterned to form openings over the pixels or photo-sites. A colorant, generally an organic dye or pigment, is then deposited over the patterned photoresist layer and, of course, forms a layer of colorant in the openings over the adhesion promoting layer. A lift-off process is now performed and the patterned photoresist layer and the overlying colorant on the photoresist layer are removed from the image sensor. The lift-off solvent is selected so that it will dissolve or soften the photoresist layer without having any effect on the pigment. Alternatively, an ultrasonic bath technique is used. In this technique, the image sensor is immersed into a bath of a solvent and ultrasonic energy is applied to the bath to effectively remove the photoresist layer and the overlying colorant, leaving colorant over the selected pixels. 
     Turning to FIG. 1, which shows a method for making coatings from evaporated organic colorants. A substrate 102 is positioned adjacent to an aperture mask 104. The aperture mask provides an aperture over a portion of the substrate. An organic colorant which is to provide the coating is placed into a source boat 100, which is heated by passing an electric current through it. Alternatively, the boat may be heated by the application of radiant heating from a suitably placed heat source. Upon being heated under reduced pressure, the colorant vaporizes and travels from the source, impinging on mask 105. The portion of colorant vapor which passes through the opening in mask 105 travels along the lines 103, and between those lines, depositing on the substrate 102 and mask 104. 
     There are a number of problems associated with this technique which involves depositing layers in a partial vacuum and is frequently referred to in the art as physical vapor deposition (PVD). In certain cases, it is difficult to control the thickness and uniformity of the colorant disposed over the pixels. The process of vacuum deposition of the colorant typically requires the use of an appropriate placement of sources or masks or moving substrate fixtures to produce a coating which is uniform. However, the colorant material may deposit on the mask and vacuum fixtures to such a degree that it flakes off, creating undesirable contamination and waste of the colorant, and requiring frequent clean-up. In addition, the moving fixtures may generate undesirable particulate materials which may cause contamination of the substrate. 
     Some other problems in making color filter arrays by the PVD process are the need to use a large source-to-substrate spacing which requires large chambers and large pumps to reach a sufficient vacuum, and the need for masks which cause low-material utilization and build-up on the mask with the concomitant contamination problems. Very specific off-axis source location relative to the substrate, which is sometimes needed for uniform coating, causes very poor material utilization. Still further, source replenishment problems exist for coating multiple substrates in one pump-down. In addition, when multiple layers are deposited, the process needs to be carefully monitored for the thickness of layers in the multiple colorant coatings in multiple cycles. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method for making color filter arrays which obviates the above difficulties, provides uniform colorant over the pixels, and provides low cost and high quality image sensors. 
     This object is achieved by a method of making a color filter array on a first substrate having an array of pixels, comprising the steps of: 
     a) depositing and patterning a photoresist layer on the substrate layer to form selected openings over pixels in the array; 
     b) providing a transferable colorant layer on a second substrate and positioning such transferable layer in transferable relationship with the first substrate; 
     c) transferring the colorant material to the photoresist layer on first substrate, and 
     d) removing the patterned photoresist layer leaving behind the colorant material in the position of the openings over the selected pixels. 
     ADVANTAGES 
     Advantages of this technique include the effective utilization of evaporant materials with high quality uniformity over large areas. Other advantages include precise control of layer thickness and lower maintenance of deposition vacuum chambers. In addition, the present invention provides an evaporative purification of the colorant during the preparation of the transferable colorant coating, and it requires minimal monitoring for the subsequent deposition process. Still further, it offers the ability to coat at higher pressures and in smaller vacuum chambers which permit faster cycle time and the use of lower-cost vacuum equipment than for standard PVD techniques. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical configuration for conventional physical vapor deposition (PVD); 
     FIG. 2 shows a typical configuration for the thermal transfer of a material from an intermediate substrate to the final substrate, according to the present invention; and 
     FIGS. 3a-g show various steps in a method according to the present invention for making color filter arrays. 
     FIGS. 4a-4f show various steps in a method similar to that shown in FIGS. 3a-3g for making color filter arrays in accordance with the present invention. 
     It will be understood that the drawings are not to scale and have been shown for clarity of illustration. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning first to FIG. 1, an arrangement for conventional PVD is shown, including a heated source 100, containing the material to be deposited, the substrate 102, and masks 104 and 105 which restrict the material vapor to paths 103 and the region in between. 
     In FIG. 2 is shown a configuration for the transfer of a material which has been deposited as a coating on the first substrate 200, onto the second substrate 201 as indicated by the arrows 205 and which is promoted by heating with heat sources 202 as indicated by radiant heat 203 acting through an aperture 204. 
     Turning next to FIG. 3a where a silicon substrate 300 is shown, the substrate has already been processed to provide wells for different dopant materials to form pixels 301. As is well understood to those skilled in the art, the substrate may be a composite of different layers. For a more complete description of the construction of image sensors, see commonly assigned U.S. Pat. No. 5,235,198. 
     As shown in FIG. 3b, an adhesion-promoting layer 302 is formed over the surface of the substrate 300 and the pixels 301. The adhesion-promoting layer can be formed by a number of techniques including spin-coating of an organic polymer or of a spin-on glass, or by chemical vapor deposition. The adhesion promoting layer may be patterned to form recesses in the adhesion promoting layer above the pixels. Alternatively, the adhesion-promoting layer may be applied after formation of the patterned photoresist layer 303. 
     As shown in FIG. 3c, a spin-coated photoresist layer is patterned to provide openings over the pixels 301. Such patterning techniques are well known to those skilled in the art. Typically, the photoresist layer can be imagewise exposed to light, illuminating particular areas of the layer. A development step is then used to form openings over the pixels 301 providing the desired pattern. 
     As shown in FIG. 3d, where a second substrate 304 is provided. (This substrate typically is stainless steel, but other substrate materials can be used which are heat resistant and flexible.) A layer 305 having a colorant is formed on the substrate 304. Typically, the colorant layer is formed by physical vapor deposition, which provides uniform layers of controlled thickness, containing no materials with higher volatility than the colorant. The layer 305 can be an organic colorant which is transferable upon the application of energy such as heat. In a preferred embodiment of the present invention, the colorant is vaporized by heating under reduced pressure, and condensed on a moving strip of stainless steel foil which is passed over the heated source at a constant rate. 
     In FIG. 3e, the substrate 304 and colorant layer is shown positioned relative to the substrate 300 and the pixels 301 in the substrate. In the process it is desired to transfer the colorant layer 305 onto the substrate and the pixels. As shown in FIG. 3f, the transferred colorant layer is now labeled number 306. In order to provide this transfer, heat is applied to the substrate 304. Typically, the substrate is composed of metals, such as steel or aluminum or of a temperature-resistant plastic such as a polyimide film. Heating is often done by exposing the non-coated side of the substrate 304 to electromagnetic radiation of wavelengths which are absorbed by the substrate or by the colorant coating and are converted into heat by radiationless decay processes. The electromagnetic radiation may be applied over a large area simultaneously as from an extended lamp source, or it may be applied as a scanned beam as with a laser. It is appreciated that imagewise light exposure may be used to heat and transfer only a portion of the colorant coating. Another method used to heat substrate 304 in order to transfer the colorant 305 is to pass an electric current through the substrate, particularly when the substrate used is composed entirely or partially of metal. In still another method, the substrate may be heated by direct contact with an object such as a metal block, a high temperature roller, or other such devices which can be heated or pre-heated to the required temperature and which can transfer heat to the substrate by direct thermal contact. Typical distances and pressures for the transfer of colorant are from about 0.1 mm to about 3 mm at pressures of less than or equal to about 0.1 Torr, up to a distance of about 50 mm at pressures of less than 0.001 Torr. 
     FIG. 3g shows the color filter array after a lift-off process. The lift-off process is needed to remove unwanted portions of the photoresist layer 303 and the portions of the layer 306 on the unwanted portions of the photoresist layer 303, leaving behind colorant layers 307 over the selected pixels. 
     More particularly, the lift-off process is as follows: 
     A photoresist layer is patterned by imagewise exposure to electromagnetic radiation of the appropriate wavelength followed by development to open up areas where a subsequently deposited layer is desired. The subsequent layer is deposited on both the opened areas and the remaining photoresist, followed by the lift-off, in which the photoresist is dissolved or swollen in a solvent, causing it to become detached from the underlying substrate, lifting-off to leave the desired deposit in place. A description of the lift-off process and typical materials used is given in chapter 12 of Semiconductor Lithography, by W. M. Moreau, Plenum Press, N.Y., 1989. 
     In order to make a color filter array with a plurality of colors, the above steps need to be repeated for each new colorant layer that is deposited over pixels. 
     Turning to FIG. 4a where a silicon substrate 400 is shown, the substrate has already been processed to provide wells for different dopant materials to form pixels 401. As is well understood to those skilled in the art, the term substrate includes a composite of different layers. 
     As shown in FIG. 4b, an adhesion promoting layer 402 which can be subsequently etched is formed on the substrate 400 and the pixels 401, and a layer of photoresist 403 is formed above it. 
     As shown in FIG. 4c, the photoresist is exposed and developed to form openings over selected pixels, leaving photoresist 404 over the rest of the layer 402. 
     As shown in FIG. 4d, the openings in the patterned resist layer 404 are used as a mask for an etch of the underlying layer, to produce a pattern of recesses in the underlying layer, now numbered 405. 
     As shown in FIG. 4e, a colorant layer is deposited on the patterned resist layer 404 and the recesses in underlying layer 405 to give layer 406/407. 
     As shown in FIG. 4f, the colorant which is not above the selected pixels 406 and the patterned photoresist layer 404 is removed by a lift off process, to leave the patterned layer 405 and the colorant above the selected pixels, 407. 
     In order to make a three-color color filter array, the above steps need to be repeated for each new colorant layer that is deposited over subsequently selected pixels. In an alternate embodiment of the present invention a second photoresist layer can be deposited and patterned on the adhesion promoting layer. 
     Colorants which are useful in the processes shown in FIGS. 3a-g, 4a-4f include the following: phthalocyanines, such as Pigment Blue 15, nickel phthalocyanine, chloroaluminum phthalocyanine, hydroxy aluminum phthalocyanine, vanadyl phthalocyanine, titanyl phthalocyanine, and titanyl tetrafluorophthalocyanine; isoindolinones, such as Pigment Yellow 110 and Pigment Yellow 173; isoindolines, such as Pigment Yellow 139 and Pigment Yellow 185; benzimidazolones, such as Pigment Yellow 151, Pigment Yellow 154, Pigment Yellow 175, Pigment Yellow 194, Pigment Orange 36, Pigment Orange 62, Pigment Red 175, and Pigment Red 208; quinophthalones, such as Pigment Yellow 138; quinacridones, such as Pigment Red 122, Pigment Red 202, and Pigment Violet 19; perylenes, such as Pigment Red 123, Pigment Red 149, Pigment 179, Pigment Red 224, and Pigment Violet 29; dioxazines, such as Pigment Violet 23; thioindigos, such as Pigment Red 88, and Pigment Violet 38; epindolidiones, such as 2,8-difluoroepindolidione; anthanthrones, such as Pigment Red 168; isoviolanthrones, such as isoviolanthrone; indanthrones, such as Pigment Blue 60; imidazobenzimidazolones, such as Pigment Yellow 192; pyrazoloquinazolones, such as Pigment Orange 67; diketopyrrolopyrroles, such as Pigment Red 254, Irgazin DPP RubinTR, Cromophtal DPP OrangeTR; Chromophtal DPP Flame Red FP (all of Ciba-Geigy); and bisaminoanthrones, such as Pigment Red 177. 
     EXAMPLES 
     In accordance with the above-stated invention, the following has been performed. 
     Example 1 
     Commercially obtained copper phthalocyanine was heated by passing electrical current through the tantalum boat which contained it, while maintaining a reduced pressure of approximately 6×10-5 Torr in a vacuum bell jar. About 0.2 microns of phthalocyanine were deposited onto a section of stainless steel foil, having a thickness of about 25 microns. The coated foil was then mounted about 3 mm distant from a silicon wafer which had been spin-coated with about 1 micron of poly(methyl glutarimide), &#34;PMGI&#34; from Microelectronics Chemical Corp., and then coated with about 1.3 microns of photoresist AZ5214IR (Hoechst Celanese Corp.) which was subsequently patterned and developed, and the non-coated side of the foil was positioned about 25 mm from an array of heat lamps (General Electric, Part no. QH500T3/CL) spaced about 30 mm apart. The assembly was subjected to a vacuum of about 6×10E-5 Torr and the heat lamps were powered for 60 seconds to transfer the phthalocyanine to the silicon wafer. The water was removed from the vacuum chamber and subjected to ultrasound in a tray of acetone for 90 seconds, using a Branson Model 3200 ultrasonic bath. The photoresist was completely removed by this treatment, leaving intact the copper phthalocyanine features in the desired pattern. 
     Example 2 
     Commercially obtained copper phthalocyanine was heated by passing electrical current through the tantalum boat which contained it, while maintaining a reduced pressure of about 6×10E-5 Torr in a vacuum bell jar. About 0.2 microns of phthalocyanine were deposited onto a section of stainless steel foil, having a thickness of about 25 microns. The coated foil was then mounted about 3 mm distant from a glass substrate, and the foil was clamped between two electrodes. The assembly was subjected to a vacuum of about 0.1 Torr, and electric current was passed through the foil (at 30 volts) for about 10 sec., causing the ends of the foil to reach a temperature of about 260 degrees C. and the phthalocyanine to transfer to the glass substrate. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     1a silicon substrate 
     1b pixels 
     3 photoresist 
     12 chapter 
     100 source boat 
     102 substrate 
     103 lines 
     104 mask 
     105 mask 
     200 substrate 
     201 substrate 
     202 sources 
     203 heat 
     204 aperture 
     205 arrows 
     300 substrate 
     301 pixels 
     302 layer 
     303 photoresist layer 
     304 substrate 
     305 layer 
     306 colorant layer 
     307 colorant layers 
     400 substrate 
     401 pixels 
     402 layer 
     404 photoresist 
     405 layer 
     406 pixels 
     407 pixels