1. Field of the Invention
The present invention relates to a thinner composition for removing photoresist, a method for reworking wafers for manufacturing semiconductor devices using the thinner composition, and to a method of manufacturing semiconductor devices that includes rinsing selected areas of the wafer to remove unwanted photoresist. U.S. application Ser. Nos. 08/771,773 and 08/771,774, both filed on Dec. 20, 1996, are both hereby incorporated by reference in their entirety.
More particularly, the present invention relates to a thinner composition for selectively removing photoresist which is coated unnecessarily on the edges or the backside of the wafer, or for removing an entire photoresist layer where an etching failure occurred during photoetching of the semiconductor device. The thinner composition is either a mixture of Ethyl-3-Ethoxy Propionate (EEP), Ethyl Lactate (EL), and .tau.-Butyro Lactone (GBL), or a mixture of EEP and EL, or a mixture of EL and GBL. The present invention also includes a rework method for removing all of the unwanted photoresist coated on the wafer surface during manufacturing, and a method for manufacturing semiconductor devices that includes selectively rinsing photoresist that was unnecessarily coated on the edges or the backside of the wafer with the thinners of the present invention to remove the unwanted photoresist.
2. Description of the Related Art
A photoetching process is generally used to form a predetermined circuit pattern on a semiconductor substrate or on a specific layer that has already been formed over the semiconductor substrate during the fabrication processes. The photoetching process is typically carried out by coating photoresist onto the substrate which causes a photo-chemical reaction on the semiconductor substrate.
First, a primer is coated over the semiconductor substrate in order to improve the adhesiveness of the photoresist to the semiconductor substrate or to the specific layer that is being coated. Next, the photoresist is coated onto the primer.
After coating the photoresist, a rinsing step is carried out to remove unwanted photoresist that was deposited unnecessarily on the edges or backside of the semiconductor substrate.
Subsequently, a soft baking step is carried out to remove the solvent inside the photoresist coated on the semiconductor substrate, and to improve the adhesiveness of the photoresist to the semiconductor substrate. Thereafter, the photoresist is exposed and developed to create a predetermined photoresist pattern on the substrate.
The photoresist pattern formed as outlined above serves either as an etching mask for selectively etching the semiconductor substrate or a specific sub-layer, or as an ion-injection mask for injecting ions into the semiconductor substrate or the sub-layer in subsequent fabrication processes.
If the photoetching process results in an etching failure, all of the photoresist coated on the semiconductor substrate should be removed, and new photoresist should be recoated onto the substrate in order to reexecute the photoetching process. This is typically referred to as reworking the substrate.
The kinds of conventional thinners that are used to remove the photoresist in the rework process differ depending on the chemistry of the specific layer on which the pattern is to be formed. For example, if the layer to be patterned is a metallic layer, then sulfuric acid, or a mixture of sulfuric acid and hydrogen peroxide must be used in the rework process to strip off any unwanted photoresist before the metallic layer is formed, and N-butyl acetate must be used to remove the photoresist if the rework process is necessary due to an etching failure that occurs after the metallic layer has been formed.
In summary, sulfuric acid or a mixture of sulfuric acid and hydrogen peroxide cannot be used to remove unwanted photoresist from an existing metallic layer, and N-butyl acetate cannot be used to remove the photoresist before forming a metallic layer. As a result, a separate rework apparatus for each chemical is needed if these conventional thinners are used. Another complication can be encountered if the specific kind of photoresist being used is incompatible with the thinners, such as the N-butyl acetate, that were used in the rework process. In the conventional process, this incompatibility typically requires a different kind of chemical or rework apparatus depending on the kind of the photoresist employed. Using multiple apparatus decreases productivity. Therefore, there is a great need for new chemical thinners which are compatible with the rework process and the chosen photoresist, and which can be used to rework either a chemical or a metallic layer.
A rinsing step is typically carried out during manufacturing to remove any photoresist that is unnecessarily coated on the edges or the backside of the semiconductor substrate during the photoetching process. Unwanted photoresist remaining on the edges or the backside of the wafer can cause an etching failure, or can generate particle contamination during subsequent processing, such as during etching or ion-injection, thereby decreasing the production yield.
According to the conventional method, in order to remove the photoresist from the edges or backside of the wafer, spray nozzles are provided above and below the wafer edge so that a thinner comprising an organic solvent can be sprayed onto the edges or backside of the wafer. The performance of the thinner to selectively remove unwanted photoresist is determined by its solubility rate, volatility and viscosity.
The solubility rate of a thinner determines how effectively the thinner can dissolve and remove the photoresist. With respect to volatility, the thinner should be easily evaporated after removing the photoresist so that the thinner does not remain on the surface of the wafer as a potential source of pollution in subsequent processing steps. However, if the volatility is too high, the thinner will evaporate during handling before completely removing the photoresist, whereby the cleaning efficiency of the photoresist is lowered and contamination problems arise in the clean room. If the volatility of the thinner is too low, the thinner may remain on the edge portion of the wafers, particularly at the flat zone used in aligning the wafers after spin drying. If the thinner does not evaporate it will build-up on the photoresist layer after development causing a lump of photoresist to form in the flat zone portion which reduces the yield of semiconductor devices produced from the wafers.
Proper viscosity is essential to facilitate spraying the thinner through the nozzles. If the viscosity is too high, an excessively high spraying pressure is required. If viscosity is too low, the spray focus will deteriorate because the thinner cannot be concentrated or focused on the contact position of the wafer which interferes with the ability of the thinner to selectively remove unwanted photoresist during rinsing. Particularly, with respect to edge rinsing, the thinner must have the proper solubility rate in order to ensure a smooth wafer cross section after treatment. As shown in FIGS. 1 and 2, if the solubility rate is too low, a so-called tailing condition occurs. If the thinner has a low solubility, the flow of a partially solubilized photoresist tail 3 can occur while rinsing the photoresist 2 that was previously coated on the wafer 1. Alternatively, if the solubility rate is too high, erosion of the photoresist layer occurs. FIGS. 3 and 4 show an erosion portion 4 of the photoresist 2, a so-called photoresist attack, which may occur while rinsing the photoresist 2 coated onto the wafer 1. Both tailing and photoresist attack cause wafer defects, which reduce the yield of the semiconductor devices.
Conventional thinners, include ethyleneglycol monoethylether acetate (ECA), N-butyl acetate (N-BA), propyleneglycol monomethylether acetate (PGMEA) and ethyl lactate (EL). Both ethyleneglycol monoethylether acetate and N-butyl acetate have good solubility rates, however, their volatility and flammability are unacceptably high, further, these compounds are toxic to humans if inhaled or contacted on the skin. The toxic effects of ECA include leukopenia and miscarriage of an embryo. The solubility rate of propyleneglycol monomethylether acetate or ethyl lactate is too low to selectively rinse photoresist from the edges or backside of the wafer. A rinse method using a solvent comprising an ether compound represented by the general formula of R.sup.1 --O--(--CHR.sup.2 --CH.sub.2 --O--).sub.n --H was disclosed in Korean Patent Publication No. 90-5345, which is incorporated herein by reference. According to the publication, the rinse solvent is nontoxic. This solvent may be used to remove or peel off a photoresist layer by completely immersing a wafer coated with photoresist in the solvent. However, this solvent cannot be sprayed onto a wafer to selectively remove the unwanted photoresist (i.e., remove the photoresist from the edge and backside portions of the wafer) because the viscosity is too high.
Accordingly, there is a need for a nontoxic thinner that has the proper solubility, volatility, and viscosity rates for certain photoresists in a rework process or a rinse process for semiconductor manufacturing. There is also a need for a reliable rework method and a method of manufacturing semiconductor devices using the thinner.