Abstract:
A method for reducing defects in the profiles of chemically amplified photoresists used in deep ultraviolet (DUV) and laser lithography. Chemically amplified resists are typically highly sensitive to nitrogen-bearing surface contaminants, and photoresist layers formed on contaminated surfaces exhibit profile defects such as resist footing and T-topping. These defects are reduced by pretreating the surface of a semiconductor device or other structure with a cleansing etchant prior to the formation of the photoresist layer. The cleansing etchant is a solution of sulfuric acid and an oxidizing agent known as “piranha.”

Description:
FIELD OF THE INVENTION 
     The present invention relates to the fabrication of semiconductor devices and more particularly to a method for cleaning a semiconductor substrate prior to the formation of a chemically amplified photoresist layer thereon. 
     BACKGROUND OF THE INVENTION 
     The importance of minimizing contamination during semiconductor fabrication processes has been recognized since the early days of the industry. As semiconductor devices have become smaller and more complex, cleanliness requirements have become increasingly stringent, especially for devices with submicron critical dimensions. Cleanliness is especially important during the application of photoresist layers, because contaminants may lead to defects in the photoresist (“resist”) profile that then result in slopy etching and poor line definition. In particular, if the profile exhibits defects at its base that result in an abnormal (other than 90 degree) intersection between the resist and the substrate, an etch performed with the defective resist will not produce straight-walled structures, and may result in inoperability of the semiconductor structure. 
     Poor resist profiles are especially prevalent in chemically amplified resists (“CA resists”) used for microlithography. Traditional resists based on pheno-formaldehyde polymers (novolac resins) do not work well with DUV and laser lithography because they cannot be tuned to respond to the short wavelengths of DUV and laser light (˜300 nm). Chemically amplified resists contain photoacid generators (PAGs) to increase the photosensitivity of the resist. Development of CA resists depends on the catalytic action of the PAG, and if acid loss (neutralization) occurs, profile abnormalities will result due to areas of incomplete solubility. 
     Acid loss at the top of the resist profile (the resist/air interface) leads to a bulge in the profile known as a T-top or cap due to reactions with bases adsorbed from the air. Acid loss at the bottom of the resist profile (the resist/substrate interface) leads to profile abnormalities such as resist footing on positive resists, and undercutting on negative resists. The acid loss is believed to be due to the presence of nitrogen-containing compounds such as atmospheric ammonia or ammonium ion which neutralize the photoacid, resulting in incomplete solubility of the resist. It is known that the presence of organic compounds such as amines in concentrations as low as 100 ppb adversely affects the stability of some CA resists. 
     Methods for reducing the amount of nitrogen-containing compounds on a CA resist substrate have included the use of a barrier layer such as an anti-reflective coating, treatment with oxygen plasma, and increasing the exposure dose. While use of these known methods produces some improvement over an untreated substrate, significant profile defects are still evident. 
     There is needed, therefore, a method for reducing resist profile defects when a chemically amplified resist such as a DUV resist is formed. In particular, a method for reducing contamination on the substrate before the formation of a CA photoresist layer is needed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for reducing resist profile defects by reducing contamination on a substrate before the formation of a CA photoresist layer. A substrate provided with a first layer to be etched and an anti-reflective coating layer is exposed to a cleansing etchant to remove contaminants prior to the formation of a CA photoresist layer thereon. The cleansing etchant is a solution of sulfuric acid and an oxidizing agent, which may be hydrogen peroxide, ammonium persulfate, potassium persulfate, or the like. The cleansing etchant is known in the chemical arts as “piranha” or Caro&#39;s acid. After treatment with the cleansing etchant for a predetermined time at an elevated temperature, a CA photoresist layer is then applied to the substrate. 
     Advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic cross-sectional view of a semiconductor wafer of the prior art depicting resist profile defects. 
     FIG. 2 is a diagrammatic cross-sectional view of a semiconductor wafer undergoing the process of a preferred embodiment of the invention. 
     FIG. 3 shows the wafer of FIG. 2 at a processing step subsequent to that shown in FIG.  2 . 
     FIG. 4 shows the wafer of FIG. 2 at a processing step subsequent to that shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. 
     The terms wafer or substrate used in the following description include any semiconductor-based structure having an exposed layer to be etched using the improved photoresist produced by the process of this invention. Wafer and substrate are to be understood as including silicon-on-insulator, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a wafer or substrate in the following description, previous process steps may have been utilized to forms regions or junctions in the base semiconductor structure or foundation. When referring to aqueous solutions described herein, the term “percent” refers to the percent measured by weight, e.g., a 90% sulfuric acid solution is 90% by weight sulfuric acid. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     Referring now to the drawings, where like elements are designated by like reference numerals, FIG. 1 depicts a semiconductor wafer  20  in an intermediate processing stage of a prior art fabrication process. The wafer  20  comprises a silicon substrate  22  with devices  24  located thereon. The devices  24  are covered by a first layer  26  of material to be etched after the photoresist of the present invention is formed. The first layer  26  may be borophosphosilicate glass (BPSG) or other suitable material. Two layers are present on top of the first layer  26 : an organic or inorganic anti-reflective coating  28 , and a chemically amplified (CA) photoresist layer  30 . The ARC layer  28  prevents DUV light from reflecting off the first layer  26 / 20 / 24  and overexposing the CA photoresist layer  30 . The CA photoresist layer  30  has been exposed to DUV light, and due to contamination, exhibits resist footing  32  and T-topping  34  defects in the resist profile. 
     An embodiment of the present invention for reducing resist profile defects is illustrated by FIGS. 2 through 4. This embodiment uses a cleansing agent called “piranha” or Caro&#39;s acid to cleanse the surface of an anti-reflective coating prior to the formation of a CA photoresist layer thereon. The cleansing agent is a solution of peroxysulfuric acid (also called persulfuric acid) or sulfuric acid and an oxidizing agent, and it is used at an elevated temperature. 
     Referring to FIG. 2, tie process of the present invention begins subsequent to the formation of devices  24 , which may be transistors, capacitors, word lines, bit lines or the like, on a silicon substrate  22  of a silicon wafer  20 , and the formation of a first layer  26  which will be etched using the photoresist produced by the present process. The first layer  26  may be a dielectric material, such as borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), borosilicate glass (BSG), or silicon dioxide, and may be deposited by chemical vapor deposition or other suitable means. 
     FIG. 3 depicts the first step of the process, in which an anti-reflective coating  28  is formed on the top surface of the first layer  26 . The coating  28  may be silicon rich oxide, silicon rich oxynitride, silicon rich nitride, or another material suitable for use to reduce light reflection from the substrate surface into the resist during photolithography. Inorganic or organic anti-reflective coatings may be used, however the process of the present invention is intended for use primarily with inorganic anti-reflective coatings, which are preferred, because of the lesser contamination levels found than organic anti-reflective coatings. A suitable means of forming the anti-reflective coating  28  is to spin it onto the surface of the first layer  26 , or to otherwise deposit it in place. 
     After formation of the anti-reflective coating  28 , the silicon wafer  20  is then subjected to the cleansing process of the present invention. The anti-reflective coating  28  is exposed to a cleansing etchant by suitable wet processing means, such as immersion of the wafer  20  into an etchant bath, or by spraying the etchant onto the anti-reflective coating  28 , or by applying the cleansing etchant as a vapor. The anti-reflective coating  28  is exposed to the cleansing etchant at an elevated temperature for a time sufficient to remove nitrogen-containing contaminants from the anti-reflective coating  28 . The wafer  20  may then be rinsed with deionized water, and spin-dried. 
     As shown in FIG. 4, a CA photoresist layer  30  is then formed by suitable means, such as by spinning it onto the surface of the anti-reflective coating  28 . The photoresist  30  is patterned and developed, yielding a patterned photoresist with a vertical and substantially defect-free profile  36 , with virtually no resist footing. The final structure of the silicon wafer  20  with the improved defect-free photoresist  30  is shown in FIG.  4 . Further steps to utilize the improved photoresist  30  as a pattern, and to create a functional circuit from the wafer  20  may now be carried out. 
     The cleansing etchant is an aqueous solution of peroxysulfuric acid (H 2 SO 5 ) or sulfuric acid (H 2 SO 4 ) and an oxidizing agent. The oxidizing agent may be any strong oxidizer such as hydrogen peroxide, ammonium persulfate, potassium persulfate, or the like. Peroxysulfuric acid or sulfuric acid is used at a concentration of approximately 90 to 99 percent in an aqueous solution. The oxidizing agent is added in a small amount of approximately one percent or less, for example, approximately 100 mL of hydrogen peroxide per 30 L of sulfuric acid. 
     Cleansing is preferably performed at an elevated temperature within the range of approximately 70 to 150 degrees Celsius, for a time within the range of 5 to 50 minutes. More preferably the temperature of the cleansing etchant is within the range of approximately 115 to 135 degrees Celsius, and the etching process has a duration of approximately 15 to 25 minutes. Most preferably the temperature of the cleansing etchant is 125 degrees Celsius, and the etching process has a duration of approximately 20 minutes. 
     If peroxysulfuric acid is used, it is heated to the desired temperature by suitable heating means such as by heating the acid on a hot plate, by wrapping heating elements around the acid tank, or by placing an immersion heater in the tank. If a solution of sulfuric acid and an oxidizing agent is used, the sulfuric acid is heated to the desired temperature, and small amounts (e.g., 50 to 100 mL) of oxidizing agent are added before each cleansing step. Alternatively, a larger amount of oxidizing agent may be added to room temperature sulfuric acid, and the resultant exothermic reaction will raise the temperature of the solution to the desired 110 to 130 degree range. However, this second method requires continual recharging with fresh oxidizing agent to keep the reaction proceeding, and eventually the cleaning rate is lowered because excess water is produced as a reaction byproduct. 
     As can be seen by the embodiments described herein, the present invention encompasses processes of cleaning an anti-reflective coating prior to the formation of a photoresist layer thereon. The cleaning occurs by application of a cleansing etchant to the anti-reflective coating to remove nitrogen-containing contaminants which may react adversely with chemically-amplified photoresist. Removal of the contaminants results in CA photoresists having improved vertical profiles that are substantially free from defects such as resist footing. 
     The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.