Method for cleaning a polymer

A method for cleaning a polymer in a trench produced by a dry anisotropic etching process is disclosed. The method includes steps of introducing a gas into a trench for reacting with the polymer to form a volatile substance and removing the remaining polymer in the trench by a wet etching process. The gas is preferably a gaseous hydrofluoric acid to clean the polymer. Because the gas can enter the small hole of the deep trench easily, this method can completely remove the polymer in the trench.

FIELD OF THE INVENTION
 The present invention relates to a semiconductor manufacturing process, and
 more particularly to a method for cleaning a polymer that is produced in
 an etching process.
 BACKGROUND OF THE INVENTION
 In the semiconductor manufacturing process, a deep trench has been widely
 used in devices, such as when manufacturing a capacitor of a dynamic
 random access memory (DRAM) unit. FIG. 1 includes steps of manufacturing
 the deep trenches applied to a capacitor of a DRAM unit. In FIG. 1(a), a
 dielectric layer 11 is formed on a silicon substrate 10 and is usually
 made of silicon dioxide/silicon nitride/silicon dioxide or just silicon
 dioxide. This dielectric layer is used as a hard mask for a subsequent
 etching process. The dielectric layer 11 is patterned by a
 photolithography and an etching process. In FIG. 1(b), a photoresist layer
 12 is formed on the dielectric layer 11 and a pattern is defined on
 photoresist layer 12 by a photolithography process. A window of hard mask
 13 on the dielectric layer 11 is formed by a dry anisotropic etching
 process, then the pattern is transferred to the silicon substrate 10 after
 removing the photoresist layer 12 as shown in FIG. 1(c). Thereafter, the
 part of the silicon substrate 10 which is not covered by the hard mask is
 etched by another dry etching process to obtain a deep trench as shown in
 FIGS. 1(d) and 1(e).
 However, an oxide-rich-polymer 14 will remain at the sidewalls and the
 bottom of the trenches during the dry etching process. In order to remove
 this polymer, a wet etching process is commonly used. A buffer oxide
 etcher (BOE) containing a mixture of hydrofluoric acid and ammonium
 fluoride is used as an etching agent for the wet etching process. However,
 the polymer deposition may be changed due to the poor quality of the BOE
 buffer, the changed chamber condition of the dry etching process, the
 performance of the spin dryer in the wet etching machine, or a reduced
 size of the deep trench, resulting in that the BOE solution can not enter
 the bottom of the deep trench easily. Therefore, the polymer in the bottom
 of the deep trench can not be removed completely by the wet etching
 process.
 After cleaning the deep trench, another dielectric layer is formed on the
 silicon substrate and then the trenches are filled with polysilicon or
 .alpha.-silicon, as shown in FIG. 2. If the polymer in the trench is not
 removed completely, it will influence the quality of the dielectric layer
 so that the capacitance of the cell will be reduced or a current leakage
 will occur. Furthermore, the polysilicon or .alpha.-silicon will not fill
 the trench very well resulting in an occurrence of a void or a seam on the
 sidewall, poor quality of DRAM, or even a decreased yield of DRAM.
 Because the size of the semiconductor device will be minimized, the hole
 size of the trench of a capacitor in a DRAM unit becomes smaller, the
 ratio of hole size (0.35 .mu.m) to depth (7 .mu.m) is 1:20 generally, and
 it becomes harder to clean the polymer in such a small hole of the trench
 by a wet etching process. For the above reasons, it is desirable to
 develop a method to improve the defects of the prior art. It is attempted
 by the applicant to develop such a process.
 SUMMARY OF THE INVENTION
 A major object of the present invention is to provide a method for cleaning
 a polymer formed in a trench of a semiconductor.
 The process for removing a polymer formed in a trench according to the
 present invention includes the steps of introducing a gas into the trench
 for reacting with the polymer to form a volatile substance and removing
 the polymer remaining in the trench.
 In accordance with the present invention, the deep trench is produced by
 steps as follows. First, providing a silicon substrate, then forming a
 dielectric layer on the silicon substrate to serve as a hard mask, and
 forming a photoresist layer on the dielectric layer. Etching a portion of
 the photoresist layer to expose a portion of the hard mask and removing
 the exposed hard mask to expose a corresponding portion of the silicon
 substrate. Finally, removing the photoresist layer remaining on said
 dielectric layer and removing the exposed portion of the silicon substrate
 to form the trench. The exposed portion of the silicon substrate is
 removed by a dry anisotropic etching process.
 The gas for cleaning the polymer is a gaseous hydrofluoric acid and is
 applied to the trench to react with the polymer for 15 seconds at room
 temperature and 1 atm. The remaining polymer is removed by a wet etching
 process using a buffer oxide etcher (BOE) as an etching agent which
 containing hydrofluoric acid and ammonium fluoride.
 In the prior art, the method for cleaning the polymer is just a wet etching
 process. The buffer oxide etcher (BOE) of the wet etching can not enter or
 be removed from the trench easily so that it becomes more difficult to
 completely remove the polymer at the bottom of the deep trench. In the
 present invention, a gaseous hydrofluoric acid is applied to the trench to
 clean the polymer and then a wet etching process is performed to remove
 the remaining polymer in the trench. Because the gas can enter the small
 hole of the deep trench easily, this method can solve the problem
 encountered in the prior art, and can effectively improve the quality of a
 DRAM and increase the yield of the product.
 The present invention will be illustrated in details with reference to the
 accompany drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Please refer to FIG. 3 which are schematic diagrams showing a preferred
 embodiment of a process for manufacturing a trench of a capacitor in a
 DRAM unit according to the present invention.
 In FIG. 3(a), a dielectric layer 31 is formed on a silicon substrate 30,
 wherein the dielectric layer is made of silicon dioxide/silicon
 nitride/silicon dioxide or just silicon dioxide. This dielectric layer 31
 is used as a hard mask for an etching process. In FIG. 3(b), a photoresist
 layer 32 is formed on the dielectric layer 31 and a pattern is defined on
 photoresist layer 32 by a photolithography process. A window of hard mask
 33 on the dielectric layer 31 is formed by a dry anisotropic etching
 process, then the pattern is transferred to the silicon substrate 30 after
 removing the photoresist layer 32 as shown in FIG. 3(c). Thereafter, the
 part of the silicon substrate 30 which is not covered by the hard mask is
 etched by another dry etching process to obtain a deep trench as shown in
 FIGS. 3(d) and 3(e).
 An oxide-rich-polymer 34 will remains at the sidewalls and the bottom of
 the trench after the dry anisotropic etching process. In order to clean
 this polymer, a gas is applied on the silicon substrate 30. In a preferred
 embodiment, the gas is a gaseous hydrofluoric acid applied to the trench
 for 15 seconds at room temperature and 1 atm.
 As shown in FIGS. 3(f) and 3(g), most of the polymer at the bottom of the
 trench are removed but some polymer 341 are still remained at the
 sidewalls of the trench. In order to completely remove the polymer
 remained in the trench, a buffer oxide etcher (BOE) containing a mixture
 of hydrofluoric acid and ammonium fluoride is applied to the trench to
 react with the remained polymer 341.
 In the prior art, the method for cleaning the polymer is just a wet etching
 process. The buffer oxide etcher (BOE) of the wet etching process must
 enter the bottom of the trench to react with the polymer and is then
 removed by a spin dryer. However, when the hole size of the trench is
 smaller, the buffer oxide etcher can not enter or be removed from the
 trench easily so that it becomes more difficult to completely remove the
 polymer at the bottom of the deep trench.
 In the present invention, a gaseous hydrofluoric acid is applied to the
 trench to clean the polymer and then a wet etching process is performed to
 remove the remaining polymer in the trench.
 Because the gas can enter the small hole of the deep trench easily, this
 method can solve the problem encountered in the prior art. Therefore, this
 present invention can effectively improve the quality of a DRAM and
 increase the yield of the product.
 It should be understood that the scope of the present invention is not
 limited to the particular embodiment disclosed herein as the best mode
 contemplated for carrying the present invention. On the contrary, it is
 intended to cover various modifications and similar arrangements included
 within the spirit and scope of the appended claims which are to be
 accorded with the broadest interpretation so as to encompass all such
 modifications and similar structures.