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  T-gate formationUSPTO Application #: 20060223245Title: T-gate formation Abstract: Methods of forming T-gate structures on a substrate are provided that use only UV-sensitive photoresists. Such methods provide T-gate structures using two lithographic steps using a single wavelength of radiation. (end of abstract)
Agent: S. Matthew Cairns Edwards & Angell, LLP - Boston, MA, US Inventors: Rudy Pellens, Frank Linskens USPTO Applicaton #: 20060223245 - Class: 438167000 (USPTO) Related Patent Categories: Semiconductor Device Manufacturing: Process, Making Field Effect Device Having Pair Of Active Regions Separated By Gate Structure By Formation Or Alteration Of Semiconductive Active Regions, Having Schottky Gate (e.g., Mesfet, Hemt, Etc.) The Patent Description & Claims data below is from USPTO Patent Application 20060223245. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit under 35 U.S.C. .sctn. 19(e) of U.S. Provisional Application No. 60/539,932, filed Jan. 29, 2004, the entire contents of which are incorporated herein by reference. BACKGROUND OF THE INVENTION [0002] The present invention relates generally to the manufacture of electronic devices. More particularly, the present invention relates to the fabrication of T-gate structures used in the manufacture of electronic devices. [0003] A T-gate is a gate conductor structure for a semiconductor device in which the top of the gate conductor structure is wider than the base of the gate conductor structure. Such T-gates include, without limitation, structures that are substantially T-shaped, mushroom-shaped, and Y-shaped. [0004] In general, gate structures such as T-gates have been advantageously used in several technologies. For example, metal semiconductor field effect transistors ("MESFETs"), high electron mobility transistors ("HEMTs") (variant of gallium arsenide field effect transistor technology) mainly used in satellite broadcasting receivers, high speed logic circuits and power modules have employed gate structures with bases smaller than the contact area. These types of gate structures are required in field effect transistors for operation in ultra-high frequency ranges. The narrow base of a T-gate structure provides a short channel length which results in increased speed and decreased power consumption. Parasitic resistances and capacitances that limit device speed are also reduced. The top portion of a T-gate is made wide so that the conductance of the T-gate remains high, for example, for high switching speeds. [0005] Recent advances in CMOS transistor architecture make use of T-gate structures where the polysilicon gate electrode is narrowed in the gate regions and wider on top of the gate. This is due to the ever increasing demand for scaling down semiconductor devices and scaling down power consumption requirements. [0006] Electron-beam ("e-beam") is the most commonly used technique for T-gate fabrication. FIGS. 1A-1D illustrate a process for forming a T-gate using e-beam. Typically, substrate 1 is coated with a layer of first poly(methyl methacrylate)-based photoresist 2, a layer of second poly(methyl methacrylate)-based photoresist 3, and a layer of third poly(methyl methacrylate)-based photoresist 4. Photoresist layers 2 to 4 are then exposed to e-beam and developed to provide a patterned photoresist stack having generally T-shaped profile 5 as shown in FIG. 1B. A layer of a conductive material 6 is then deposited on the entire surface inclusive of the surface of substrate 1 exposed by the patterning of the photoresist layers, see FIG. 1C. Photoresist layers 2 to 4 are then removed, lifting-off the conductive material layer on the surface of photoresist layer 4 in the process, to provide T-gate structure 7 on substrate 1 as shown in FIG. 1D. [0007] However, such e-beam techniques suffer from certain drawbacks. For example, e-beam lithography suffers from poor linewidth control in the multi-layered stacks used in typical T-gate processes because the exposing e-beam must pass through relatively thick resist films (e.g., about one micron). Further, e-beam exposure is a direct write process which is both slow and expensive. [0008] Other methods of forming T-gates have been developed. Certain of these methods utilize a number of sacrificial inorganic layers which require various etching steps and harsher removal processes than photoresist-based processes. Other methods utilize multiple photoresist layers, however, these multiple photoresist layers are imaged at different wavelengths. For example, U.S. Pat. No. 6,387,783 (Furukawa et al.) disclose a process for forming T-gates using a hybrid first photoresist that is imaged using x-rays and a second photoresist that is imaged using I-line radiation. The use of such different wavelengths requires different exposure tools, which increase the costs and complexity of the process. Accordingly, a need exists for improved methods of forming T-gate structures. SUMMARY OF THE INVENTION [0009] The inventors have found that T-gate structures may be formed using conventional UV exposure tools using a single wavelength and with fewer processing steps. [0010] The present invention provides a method for forming a T-gate on a substrate including the steps of: a) providing a substrate; b) optionally disposing an organic planarizing layer on the substrate; c) disposing a layer of a UV-sensitive first photoresist; d) patterning the first photoresist by exposing the first photoresist to UV radiation through a mask and developing the exposed first photoresist to define a first opening for a base of the T-gate; e) transferring the pattern to the planarizing layer if present; f) rendering the pattern insensitive to the UV-radiation; g) disposing a layer of a UV-sensitive second photoresist, the second photoresist being negative-acting; h) patterning the second photoresist by exposing the second photoresist to the UV radiation through a mask and developing the exposed second photoresist to define a second opening for a cap of the T-gate over the first opening; and i) depositing a conductive material within the first and second openings to form a T-gate. [0011] The present invention further provides a method for forming a T-gate structure comprising the steps of: a) providing a substrate; b) disposing an organic planarizing layer on the substrate, disposing a layer of a UV-sensitive first photoresist on the organic planarizing layer, patterning the first photoresist by exposing the first photoresist to UV radiation through a mask and developing the exposed first photoresist to define a first opening for a base of the T-gate, transferring the first opening to the organic planarizing layer, and optionally removing the first photoresist to provide a patterned organic planarizing layer; c) disposing a layer of a UV-sensitive second photoresist wherein the second photoresist is negative-acting; d) patterning the second photoresist by exposing the second photoresist to the UV radiation through a mask and developing the exposed second photoresist to define a second opening for a cap of the T-gate over the first opening; e) depositing a conductive material within the first and second openings to form a T-gate; and f) removing the second photoresist. Preferably, the organic planarizing layer is an antireflective coating layer. [0012] Additionally, the present invention provides a method for forming a T-gate structure comprising the steps of: a) providing a substrate; b) disposing a layer of a UV-sensitive first photoresist on the substrate; patterning the first photoresist by exposing the first photoresist to UV radiation through a mask and developing the exposed first photoresist to define a first opening for a base of the T-gate; and curing the patterned first photoresist; c) disposing a layer of a UV-sensitive second photoresist on the cured first photoresist wherein the second photoresist is negative-acting; d) patterning the second photoresist by exposing the second photoresist to the UV radiation through a mask and developing the exposed second photoresist to define a second opening for a cap of the T-gate over the first opening; e) depositing a conductive material within the first and second openings to form a T-gate; and f) removing the first and second photoresists. BRIEF DESCRIPTION OF THE DRAWING [0013] FIGS. 1A-1D are a schematic cross-sectional illustration of a conventional process for T-gate formation using e-beam. [0014] FIGS. 2A-2F are a schematic cross-sectional illustration of a process for forming a T-gate according to one embodiment of the invention. [0015] FIGS. 3A-3H are a schematic cross-sectional illustration of a process for forming a T-gate according to another embodiment of the invention. [0016] FIG. 4 is a scanning electron micrograph of a T-gate structure made by the present invention. DETAILED DESCRIPTION OF THE INVENTION [0017] As used throughout this specification, the term "T-gate" refers to any gate conductor structure for an electronic device in which the top of the gate conductor structure is wider than the base of the gate conductor structure. Such T-gate structures may have a variety of shapes including, without limitation, T-shaped, mushroom-shaped, and Y-shaped. The articles "a" and "an" refer to the singular and the plural. All numerical ranges are inclusive and combinable in any order except where it is clear that such numerical range is constrained to add up to 100%. Like reference numerals refer to like elements. [0018] T-gate structures are formed on a substrate according to the present method using two lithographic processes, the two lithographic processes being performed using the same wavelength of radiation. A negative-acting photoresist is typically used as the photoresist in the second lithographic process. An advantage of the present invention is that a single exposure tool can be utilized instead of multiple tools, thus reducing costs. Accordingly, the present invention provides a method for forming a T-gate structure including the steps of: a) providing a substrate; b) optionally disposing an organic planarizing layer on the substrate; c) disposing a layer of a UV-sensitive first photoresist; d) patterning the first photoresist by exposing the first photoresist to UV radiation through a mask and developing the exposed first photoresist to define a first opening for a base of the T-gate; e) transferring the pattern to the planarizing layer if present; f) rendering the pattern insensitive to the UV-radiation; g) disposing a layer of a UV-sensitive second photoresist, the second photoresist being negative-acting; h) patterning the second photoresist by exposing the second photoresist to the UV radiation through a mask and developing the exposed second photoresist to define a second opening for a cap of the T-gate over the first opening; and i) depositing a conductive material within the first and second openings to form a T-gate. The remaining photoresist layers are then removed and function as lift-off layers by removing the conductive material deposited on the surface of the second photoresist. The wavelength of the UV radiation used to image the first and second photoresists is the same. [0019] A wide variety of substrates may be used in the present invention. Suitable substrates are those used in the manufacture of electronic devices. Exemplary substrates include, without limitation, gallium arsenide ("GaAs"), silicon ("Si"), indium gallium arsenide ("InGaAs"), aluminum gallium arsenide ("AlGaAs"), strained silicon, silicon germanium ("SiGe"), and mixtures thereof. Other suitable substrates are well known to those skilled in the art. For example, the substrate may include an InGaAs/AlGaAs/GaAs film stack grown on a semi-insulating GaAs substrate. Such films may be grown by a variety of means, such as by molecular beam epitaxy ("MBE"), metalorganic chemical vapor deposition ("MOCVD"), physical vapor deposition ("PVD"), liquid phase epitaxy ("LPE"), chemical beam epitaxy ("CBE") and atomic layer deposition ("ALD"). These film growth techniques are well known to those skilled in the art. The substrates may include one or more additional layers of materials. The choice of such substrates will depend upon the particular electronic device desired and is well within the ability of those skilled in the art. Continue reading... Full patent description for T-gate formation Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this T-gate formation patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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