Abstract:
A novel cleaning method for preventing defects and particles resulting from post tungsten etch back or tungsten chemical mechanical polish is provided. The cleaning method comprises providing a stack structure of a semiconductor device including a tungsten plug in a dielectric layer. The tungsten plug has a top excess portion. A surface of the stack structure is then contacted with a cleaning solution comprising hydrogen peroxide. Next, the surface of the stack structure is contacted with dilute hydrofluoric acid. The cleaning solution and hydrofluoric acid are capable of removing the top excess portion and particles on the surface of the stack structure.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a cleaning method for use in semiconductor device fabrication, and, more particularly, to a cleaning method for preventing or reducing defects and particles resulting from post-tungsten etch back or tungsten chemical mechanical polish.  
         [0003]     2. Description of the Related Art  
         [0004]     In semiconductor chip fabrication processing, it is well known that there is a need to clean the surface of the wafer after certain fabrication operations that leave unwanted residues or organic contaminants on the wafer surface. Examples of such fabrication operations include plasma etching (e.g., tungsten etch back (WEB)) and chemical mechanical polishing (CMP).  
         [0005]     If unwanted residual materials or organic contaminants are left on the wafer surface during subsequent fabrication operations, this may cause inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is desirable to clean the wafer adequately, yet efficiently, after fabrication operations that leaves unwanted residue or contaminants on the wafer surface.  
         [0006]      FIG. 1  is a schematic diagram illustrating unwanted residues or organic contaminants that remain on a wafer surface after a tungsten etch back or a tungsten chemical mechanical polish is applied in the conventional fabrication method to form a metal via.  
         [0007]     A first layer  100 , which may be a semiconductor substrate or a stack layer with a metal layer formed on a semiconductor substrate, is provided. A dielectric layer  105  is formed and patterned to form vias  110 , which expose a portion of the first layer  100 . A conforming metal barrier layer  115  is then formed on the patterned dielectric layer  105  and covers the profile of the vias  110 , wherein the metal barrier layer  115  may include a Titanium nitride layer. A blanket metal layer  120 , preferably a tungsten layer, is formed on the metal barrier layer  115  and into the vias  110 , wherein the method for forming the tungsten layer  120  includes chemical vapor deposition (CVD). A tungsten etch back or tungsten chemical mechanical polish is performed to remove a portion of the tungsten layer  120  outside the vias  110  until the top surface of the tungsten layer  120  in the vias is close to the level of the top surface of the metal barrier layer  115 , thus forming tungsten plugs  120   a.  Conventionally, the top portions of the tungsten plugs  120   a  are higher than the level of the top surface of the metal barrier layer  115  so as to avoid over polish or dishing of chemical mechanical polish. However, as the size of semiconductor devices shrinks, distances between adjacent tungsten plugs  120   a  also shorten. Thus, the top excess portions (dotted areas) of adjacent tungsten plugs  120   a  need to be removed to prevent the tungsten plugs  120   a  from contacting with each other and resulting in electrical short circuits.  
         [0008]     Unwanted residues or organic containments (i.e. defects and particles)  125  produced during tungsten etch back or tungsten chemical mechanical polish may be left on the surface of the tungsten plugs  120   a  or the metal barrier layer  115 . The defects and particles  125  remaining on the metal barrier layer  115  or the tungsten plugs  120   a  may provide a large resistance which affects the performance and the reliability of the device. Thus, the overall yield from the metal via fabrication process is reduced.  
         [0009]     Traditionally, defects and particles  125  on the surface of the tungsten plugs  120   a  and the metal barrier layer  115  are removed by utilizing a DNS (Dai Nippon Screen brush) cleaner, which is used to brush clean the surface of the tungsten plugs  120   a  and the metal barrier layer  115 . However, a problem with using the DNS cleaner is that it can scratch the surface of the tungsten plugs  120   a  and the metal barrier layer  115  while attempting to remove the residual particles or etching residue.  
         [0010]     U.S. Pat. No. 5,804,091, granted Sep. 8, 1998, Lo et al., discloses a method of preventing defects and particles produced after tungsten etch back. The method includes utilizing an Ar plasma process, baking process, and D.I. water flushing with mega-sonic shaking to reduce defects and particles on the wafer surface. However, the method fails to reduce small-sized defects and particles  125  and fails to remove the top excess portion of the tungsten plugs  120   a.    
         [0011]     In view of the foregoing, there is a need for a cleaning method for effectively removing particles, defects and top excess portions of the tungsten plug in post tungsten etch back or tungsten chemical mechanical polish operations.  
       SUMMARY OF THE INVENTION  
       [0012]     To achieve the foregoing and other objects, and in accordance with purposes of the present invention, as described herein, embodiments of the present invention provide a cleaning method for preventing defects and particles for post tungsten etch back or tungsten chemical mechanical polish.  
         [0013]     One aspect of the present invention is to provide a cleaning method used in semiconductor device fabrication. The cleaning method comprises providing a stack structure of a semiconductor device including a tungsten plug in a dielectric layer. The tungsten plug has a top excess portion. A surface of the stack structure is contacted with a cleaning solution comprising hydrogen peroxide (H 2 O 2 ). The surface of the stack structure is then contacted with dilute hydrofluoric acid (DHF). The cleaning solution and hydrofluoric acid are capable of removing the top excess portion and particles on the surface of the stack structure.  
         [0014]     Another aspect of the present invention is to provide a method of forming a semiconductor device. The method comprises: providing a dielectric layer with a via. Tungsten is deposited over the via of the dielectric layer. Next, part of the tungsten is removed, leaving a top excess portion of tungsten. The semiconductor device is then contacted with a cleaning solution comprising hydrogen peroxide. The semiconductor device is then contacted with dilute hydrofluoric acid. These contacting steps are capable of removing the top excess portion of tungsten and cleaning the semiconductor device.  
         [0015]     The cleaning solution may further comprise ammonium hydroxide (NH 4 OH), in which a volume ratio of hydrogen peroxide, ammonium hydroxide and deionized water is 1:4:110. The step of contacting a surface of the stack structure with a cleaning solution may be conducted at a temperature within a range of 20° C. to 70° C. and for a period within a range of 1-10 minutes, and optionally may be conducted with mega-sonic shaking.  
         [0016]     Dilute hydrofluoric acid may include a volume ratio of 500:1 deionized water: hydrofluoric acid (HF). The step of contacting a surface of the stack structure with dilute hydrofluoric acid may be conducted at a temperature of 30° C. and for 4 minutes.  
         [0017]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain features, advantages, and principles of the invention.  
         [0019]      FIGS. 1A-1B  are schematic cross-sectional views illustrating a conventional method of forming tungsten plugs.  
         [0020]      FIGS. 2A-2C  are schematic cross-sectional views of forming tungsten plugs in accordance with certain embodiments of the present invention.  
         [0021]      FIG. 3  is a process flow chart of one embodiment of the present invention.  
         [0022]      FIG. 4  is a plot showing the selectivity for etching tungsten relative to borophosphosilicate glass or Titanium and Titanium nitride (Ti/TiN) under various volume ratios of hydrogen peroxide to ammonium hydroxide. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]      FIGS. 2A-2C  are schematic diagrams illustrating the formation of contact plugs in a dielectric layer in accordance with certain embodiments of the present invention. The process sequence is performed with a first layer  200 , having a dielectric layer  205  with a low dielectric constant, such as silicon oxide or doped silicon oxide (e.g. borophosphosilicate glass, BPSG) formed thereon. The first layer  200  may be a semiconductor substrate or a stack layer with a metal layer formed on a semiconductor substrate. Vias  210  are patterned and etched into the dielectric layer  205  to expose the first layer  200 . A conforming metal barrier layer  215 , such as a single layer of titanium nitride (TiN) or a bilayer of titanium and titanium nitride (Ti/TiN), is deposited in the vias  210  and on the field of the dielectric layer  205  to provide an adhesive layer on which contact vias  220 , such as tungsten can be deposited using chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), high-density plasma chemical vapor deposition (HDP CVD) or other deposition techniques to fill the vias  210 . Tungsten is deposited as a blanket layer to fill the vias  210  and form the blanket layer  220 , as shown in  FIG. 2A .  
         [0024]     A tungsten chemical mechanical polishing process or a tungsten etch back process is then performed to remove part of the tungsten outside the vias  210 , leaving the vias  210  filled with tungsten, and tungsten plugs  220   a  are therefore formed. It is should be noted that top excess portions of tungsten plugs  220   a  near the vias  210  remain (dotted area). Defects and particles  225  on the tungsten plugs  220   a  or metal barrier layer  215  may be tungsten, tungsten oxide, chemical residuals from the process of tungsten etch back or tungsten chemical mechanical polish, or contaminates, as shown in  FIG. 2B . Next, the cleaning method of one embodiment in accordance with the present invention is performed and the resulting cleaned structure is shown in  FIG. 2C .  FIG. 2C  shows that the top surface of the tungsten plugs  220   b  is approximately level with the top surface of the metal barrier layer  215  and that no defects or particles thereon are present.  
         [0025]      FIG. 3  shows a process flow chart for a cleaning method in accordance with certain embodiments of the present invention. An unclean sample having a contact plug with a top excess portion, such as the structure  20  shown in  FIG. 2B , is contacted with a cleaning solution (step  310 ), optionally accompanied by mega-sonic shaking. Next, it is contacted with dilute hydrofluoric acid (step  320 ). Optionally, the unclean sample may be rinsed or flushed with deionized water before and/or after contact with the cleaning solution or dilute hydrofluoric acid. The cleaning solution may include hydrogen peroxide (H 2 O 2 ) and ammonium hydroxide (NH 4 OH) mixed in a specific volume ratio (v/v). The specific volume ratio of hydrogen peroxide and ammonium hydroxide will be illustrated later.  
         [0026]     When the sample (e.g. structure  20  in  FIG. 2B ) is dipped in the cleaning solution and optionally shaken by a mega-sonic machine, hydrogen dioxide will oxidize the exposed portions of the tungsten plugs  220   a  (i.e. the top excess portions of the tungsten plugs  220   a ) which, in effect, may aggressively clean any defects and particles  225  on the tungsten plugs  220   a  or the metal barrier layer  215 . This process results in oxidation of the top excess portions of the tungsten plugs  220   a.  This oxidation is thereafter removed with the ammonium hydroxide. Mega-sonic shaking facilitates removing defects and particles  225 , such as tungsten, tungsten oxide, and chemical residuals or contaminates from the surface of the tungsten plugs  220   a  and the metal barrier layer  215 .  
         [0027]     The sample (e.g. structure  20  in  FIG. 2B ) is optionally rinsed with deionized water after the cleaning solution dip. Next, a dilute hydrofluoric acid dip is carried out at, preferably, a temperature of 30° C. for 4 minutes in step  320 . Dilute hydrofluoric acid is used to remove the residual tungsten oxide oxidized by hydrogen peroxide. Preferably, the volume ratio of hydrofluoric acid to deionized water is no greater than 1:500, since hydrofluoric acid with higher concentration may etch the surface of dielectric layer  205 , in  FIG. 2C . In one embodiment of the present invention, dilute hydrofluoric acid is preferably prepared by diluting hydrofluoric acid (49% by weight) with deionized water in a volume ratio of 1:500.  
         [0028]     The specific volume ratio of hydrogen peroxide and ammonium hydroxide in the cleaning solution may be selected based on the selectivity for etching tungsten (W) relative to the dielectric (e.g. borophosphosilicate glass, BPSG) or the metal barrier (e.g. Titanium and Titanium nitride, Ti/TiN).  FIG. 4  is a plot showing the selectivity for etching W relative to BPSG or Ti/TiN under various volume ratios of H 2 O 2  to NH 4 OH. The selectivity of etching tungsten relative to the dielectric is defined as a ratio of etching rate (Å/min) of tungsten to that of BPSG, and the selectivity of etching tungsten relative to the Ti/TiN is defined as a ratio of etching rate of tungsten to that of Ti/TiN. For example, hydrogen peroxide (31% by weight) and ammonium hydroxide (29% by weight) in the cleaning solution may be at a volume ratio of 1:4, and the step carried out at a temperature of 25° C. In a preferred embodiment, the cleaning solution of hydrogen peroxide (31% by weight) and ammonium hydroxide (29% by weight) in deionized water is at a volume ratio of 1:4:110 (i.e. H 2 O 2 : NH 4 OH: DIW= 1:4:110).  In these process conditions, the etching rate of W is about 50 Å/min, the etching rate of BPSG is about 20 Å/min and the etching rate of Ti/FiN is about 6 Å/min, and thus the selectivity for etching W related to BPSG and Ti/TiN are about 2.5 and about 8 respectively.  
         [0029]     As shown in  FIG. 4 , the selectivity for etching W related to BPSG and Ti/TiN trends up as the volume ratio of the H 2 O 2  to NH 4 OH increases, which indicates that the top excess portion of the tungsten plugs  220   a  (see dotted area of  FIG. 2B ) may be removed with no or little damage to the dielectric layer  205  and the metal barrier layer  215 . However, as result of high etch selectivity, the process time of this step may be too short to control. The range of the volume ratio of H 2 O 2  to NH 4 OH is preferably from 1:1 to 1:4, and more preferably is 1:4. The volume ratio of H 2 O 2  to DIW is preferably in the range of 1:80 to 1:200, and more preferably is in the range of 1:110. Other specific process conditions of this process may be as follows: the process time is preferably in the range of 1 minute to 10 minutes, and more preferably is 4 minutes; the temperature is preferably in the range of 20° C. to 70° C., and more preferably is 25° C. In addition, it will be recognized that volume ratios of H 2 O 2 , NH 4 OH to DIW, process time and temperature may be varied to provide a balanced environment for cleaning the sample (e.g. structure  20  in  FIG. 2B ). The selectivity of etching tungsten related to BPSG or Ti/TiN is greater than 1 upon these process conditions, which indicates that the top excess portions of the tungsten plugs  220   a  can be effectively removed with no or little damage to the dielectric layer  205  and the metal barrier layer  215 . Moreover, these process conditions will be well controlled. In one embodiment of the present invention, the cleaning solution is preferably prepared by mixing a H 2 O 2  aqueous solution (31% by weight) with a NH 4 OH aqueous solution (29% by weight) in a specific volume ratio (e.g. 1:1 to 1:4) and then diluting the mixture with deionized water by a volume ratio in the range of 1:80 to 1:200.  
         [0030]     In a preferred embodiment of the present invention based on a compromise between process conditions mentioned above, the process of cleaning a unclean sample having a top excess portion of a contact plug, such as the structure  20  shown in  FIG. 2B  is as follows: the sample is first dipped in the cleaning solution, for 4 minutes at a temperature of 25° C., in which the volume ratio of the H 2 O 2 , NH 4 OH to DIW is 1:4:110, and shaken by a mega-sonic machine simultaneously. The sample is then flushed or rinsed with deionized water to remove residual chemicals (i.e. H 2 O 2  and NH 4 OH) on the sample. Next, the sample is dipped in dilute hydrofluoric acid at a temperature of 30° C. for 4 minutes. Consequently, the top excess portions of the tungsten plugs, defects and particles are removed, as in the structure shown in  FIG. 2C .  
         [0031]     In an alternative embodiment of the present invention, the cleaning solution merely includes dilute hydrogen peroxide instead of the mixture of hydrogen peroxide and ammonium hydroxide. Dilute hydrogen peroxide is preferably prepared by diluting hydrogen peroxide (31% by weight) with deionized water in a volume ratio of 1:110. Dilute hydrofluoric acid is preferably prepared by diluting hydrofluoric acid (49% by weight) with deionized water in a volume ratio of 1:500. The cleaning method in this case is similar to that mentioned in the previous embodiments except that this cleaning method is preferably performed repeatly. In particular, the sample is dipped in dilute hydrogen peroxide for 2 minutes, flushed or rinsed with deionized water, and then transferred to dilute hydrofluoric acid (e.g. 49% by weight) for 2 minutes. These processes are repeated at least three times. Consequently, defects and particles  225  can be sufficiently removed from the surface of the tungsten plugs  220   a  and the metal barrier layer  215 , and the top excess portions of the tungsten plugs  220   a  can also be removed.  
         [0032]     It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed process without departing from the scope or spirit of the present invention. For example, the technique described is suitable for use in damascene, dual damascene or other metal connect processes known in the art. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims and their equivalents.