Abstract:
Circuit elements, such as aluminum interconnects, and a protective film for protecting these circuit elements are formed on a surface of a semiconductor substrate. Resist is formed covering the protective film. The semiconductor substrate on which the resist covering the protective film is formed is dipped into pure water so as to allow the water to filter into a gap between the resist and semiconductor substrate. Then the semiconductor substrate having the resist thereon is dried in high temperature air, and the resist is adhered to the semiconductor substrate by a sticking function due to the surface tension generated when the water is decreasing. The semiconductor substrate to which the resist is adhered is cleaned by a hydrogen fluoride aqueous solution.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device fabrication method, and more particularly to an HF (hydrogen fluoride) cleaning technology used in the fabrication method. 
     2. Description of the Related Art 
       FIG. 1A  and  FIG. 1B  of the accompanying drawings depict an acceleration sensor  20  which is formed by processing a silicon substrate.  FIG. 1A  is a perspective view and  FIG. 1B  is a partial cross-sectional view taken along the  1 B- 1 B line in  FIG. 1A . 
     This acceleration sensor  20  has an outer frame section  1   a , a weight section  1   b  and beam sections  1   c . These sections are integrated by processing a silicon substrate  1 . The outer frame section  1   a  supports the weight section  1   b  formed at the center via the flexible four beams  1   c.    
     On the surface of each beam section  1   c , an insulation film  2  made of SiO 2  is formed, and piezo resistors  3  are provided on the insulation film  2 . The resistance values of the piezo resistors  3  are changed according to the deflection of the beam sections  1   c  when displacement of the weight section  1   b  is caused by an acceleration. The piezo resistors  3  are connected to the pads  5  provided on the surface of the outer frame  1   a  by aluminum wires  4 . The pads  5  are provided for connection to external devices. 
     The surfaces of the insulation film  2 , piezo resistors  3  and aluminum wires  4  are protected by a protective film  6 . The pads  5  are not protected (covered) by the protective film  6 . 
     The acceleration sensors are fabricated by forming an insulation film  2 , piezo resistors  3 , aluminum wires  4  and pad  5  for each of many acceleration sensors on the silicon substrate  1 , covering each insulation film  2  and other areas by a protective film  6 , and removing the silicon between the outer frame section  1   a  and weight section  1   b  by etching for each acceleration sensor, so as to from a space between the outer frame section  1   a  and weight section  1   b . The substrate is cut into individual sensors to have the acceleration sensor shown in  FIG. 1A . 
     After forming the space section by etching, cleaning is performed to remove foreign substance attached to the surface of the outer frame section  1   a  and weight section  1   b . This cleaning is performed using HF aqueous solution to remove SiO 2  residue. In order to protect the insulation film  2 , piezo resistors  3 , aluminum wires  4  and pads  5  formed on the silicon substrate  1  from the HF aqueous solution (cleaning solution), the silicon substrate is covered with resist  11 , as shown in  FIG. 1B , and then the silicon substrate having the resist  11  thereon is dipped into the HF aqueous solution for cleaning. 
     This acceleration sensor is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2003-139791. 
     If the cleaning is carried out with the HF aqueous solution, the following problem arises. Because of insufficient adhesion between the silicon substrate  1  and resist  11 , HF aqueous solution enters (filters into) the gap between the silicon substrate  1  and the resist  11 , etches the insulation film  2 , and corrodes the aluminum wires  4 . As a result, the HF aqueous solution deteriorates qualities of the product. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to prevent HF aqueous solution from filtering into the gap between the silicon substrate and the resist during cleaning. 
     According to one aspect of the present invention, there is provided a semiconductor device fabrication method that includes the step of preparing a semiconductor substrate on which one or more circuit elements and a protective film for protecting the circuit element(s) are formed, and the step of forming a resist that covers the protective film. The fabrication method also includes a pure water dipping step for dipping the semiconductor substrate, on which the resist covering the protective film is formed, into pure water so as to allow water to filter into a gap between the resist and semiconductor substrate. The fabrication method also includes a drying step for adhering the resist to the semiconductor substrate by drying the semiconductor substrate having the resist thereon at high temperature so as to evaporate the water that has filtered into the gap between the resist and semiconductor substrate. 
     In the pure water dipping step, water is forced to filter into the gap between the resist and semiconductor substrate. Then, this water is evaporated at high temperature by the drying step. Strong surface tension is generated when water is decreased by evaporation during the drying step. In other words, a sticking function for the resist to adhere to the semiconductor substrate is generated. Because of the sticking function, the hydrogen fluoride aqueous solution is prevented from filtering into the gap between the semiconductor substrate and resist during a cleaning step. Therefore, corrosion of the circuit elements can be prevented. 
     Between the pure water dipping step and drying step, a surface active agent dipping step may be carried out for dipping the semiconductor device into a bath of a surface active agent, so as to substitute the water, which is filtered in the gap between the semiconductor substrate and the resist, with the surface active agent. A second pure water dipping step may be carried out after the surface active agent dipping step for substituting the surface active agent with the pure water. By performing the surface active agent dipping step in this way, airing, which would be generated in the first pure water dipping step, can be removed. 
     Between the surface active agent dipping step and the second pure water dipping step, an etchant dipping step may be performed for dipping the semiconductor device into a bath of hydrogen fluoride aqueous solution, so as to remove a natural oxide film from the surface of the silicon substrate. The second pure water dipping step removes the hydrogen fluoride aqueous solution filtered into the gap between the semiconductor substrate and the resist, and substitutes the hydrogen fluoride aqueous solution with the pure water. By performing the etchant dipping step, the semiconductor substrate has bumps (concave and convex) in its surface. The resist are more strongly adhered to the semiconductor substrate during the subsequent drying step because of the bumps formed on the semiconductor substrate surface. 
     The above mentioned and other objects, aspects and advantages of the present invention will be more completely understood by reading the following detailed description and appended claims when read in conjunction with the accompanying drawings. The drawings are, however, merely for description, and do not limit the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  depicts a perspective view of an acceleration sensor formed by processing a silicon substrate. 
         FIG. 1B  is a cross-sectional view taken along the line  1 B- 1 B in  FIG. 1A . 
         FIG. 2A  to  FIG. 2G  are diagrams of a semiconductor device fabrication method according to Embodiment 1 of the present invention.  FIG. 2A  to  FIG. 2C  are cross-sectional views.  FIG. 2D  is an enlarged view of a portion A in  FIG. 2B .  FIG. 2E  to  FIG. 2G  are similar enlarged views. 
         FIG. 3A  to  FIG. 3F  are diagrams of the semiconductor device fabrication method according to Embodiment 2 of the present invention.  FIG. 3A  to  FIG. 3C  are cross-sectional views.  FIG. 3D  is an enlarged view of a circled portion in  FIG. 3A .  FIG. 3E  and  FIG. 3F  are similar enlarged views. 
         FIG. 4A  to  FIG. 4F  are diagrams of the semiconductor device fabrication method according to Embodiment 3 of the present invention.  FIG. 4A  is a cross-sectional view.  FIG. 4B  is an enlarged view of a circled portion in  FIG. 4A .  FIG. 4C  to  FIG. 4F  are similar enlarged views. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1 
     Referring to  FIG. 2A  to  FIG. 2G , a semiconductor device fabrication method according to Embodiment 1 of the present invention will be described. In this embodiment, the semiconductor device is the acceleration sensor, and the semiconductor device fabrication method will be described using the acceleration sensor shown in  FIG. 1A  and  FIG. 1B . 
     (1) Step 1 (Resist Forming Processing) 
     As  FIG. 2A  and  FIG. 1A  show, the piezo resistors  3  and aluminum interconnects  4  are formed on the insulation film  2  made of SiO 2 . The insulation film  2  is formed on the surface of the silicon substrate (beam sections)  1 . The insulation film  2  and aluminum interconnects  4  are covered by protective film  6  for protecting them from corrosion. Then the silicon between the outer frame section  1   a  and weight section  1   b  is removed by etching leaving the beam sections  1   c , so as to create a space where the weight section  1   b  can displace according to acceleration. In order to perform cleaning to remove foreign substances attached to the surface of the outer frame section  1   a  and weight section  1   b , the surface of the protective film  6  is covered with resist  11  to protect the protective film from the HF aqueous solution (cleaning liquid). The resist  11  is thermo-setting resist, for example. 
     To form the resist  11 , photosensitive resist material is coated on the entire surface of the silicon substrate  1  for about 2 μm (thickness), and this is patterned to a predetermined pattern using an ordinary photolithography technology. 
     (2) Step 2 (Pure Water Dipping Processing) 
     As  FIG. 2B  shows, the silicon substrate  1  which has the protective film  6  covered with the resist  11  is dipped into the pure water  12 . By this dipping, as shown in  FIG. 2D  (i.e., the enlarged view of the area A in  FIG. 2B ), water is filtered into the gap between the silicon substrate  1  and resist  11 . In this particular embodiment, the water  12  is still in the water tank. 
     (3) Step 3 (Drying Processing) 
     The silicon substrate  1  that contains the water  12  in the gap between the resist  11  and substrate  1  is lifted out from the pure water tank  12 , and is placed into a high temperature chamber, as shown in  FIG. 2C , so as to perform drying processing in the high temperature (130 to 150° C.) air  13 . By this drying processing, the water  12  filtered into the gap between the silicon substrate  1  and resist  11  gradually evaporates. 
     When the water  12  decreases by evaporation, a sticking function is generated. In other words, the resist  11  is attracted by (pulled toward) the silicon substrate  1  because of the surface tension. By this sticking function, as the enlarged views ( FIG. 2E  to  FIG. 2G ) show, the distance between the silicon substrate  1  and resist  11  decreases as the water  12  decreases, and finally the resist  11  firmly adheres to the silicon substrate  1 . 
     (4) Step 4 (HF Cleaning Processing) 
     Then the entire silicon substrate  1 , which has the resist  11  to protect the circuit area, is cleaned using the HF aqueous solution of which concentration is about 5%, so as to remove contaminants and residual SiO 2  attached on the surface. The contacting faces of the silicon substrate  1  and resist  11  are adhered to each other by the sticking function generated by the drying processing, so that the entry of HF aqueous solution is prevented. 
     In this way, according to the semiconductor fabrication method in Embodiment 1, the semiconductor device of which circuit area is protected by the resist  11  is dipped into the pure water  12 , allowing water to filter into the gap between the semiconductor substrate  1  and resist  11 , and the water is evaporated by the drying processing, so that the resist  11  is adhered to the semiconductor substrate  1  by the sticking function. Therefore even if the semiconductor device is dipped into the HF aqueous solution (cleaning liquid), the HF aqueous solution does not filter into the inside of the semiconductor device through the gap between the resist  11  and semiconductor substrate  1 . Consequently, corrosion of the aluminum wires  4  and other elements inside the semiconductor device can be prevented. 
     Embodiment 2 
       FIG. 3A  to  FIG. 3F  are diagrams of the semiconductor device fabrication method according to Embodiment 2 of the present invention. The second embodiment has Steps 2a and 2b. Step 2a and Step 2b are additional steps executed between Step 2 and Step 3 of Embodiment 1. Alternatively, it can be said that Step 2 of Embodiment 1 is performed by three steps: the first pure water dipping processing (Step 2), surface active agent dipping processing (Step 2a) and the second pure water dipping processing (Step 2b). 
     The following description deals with only the differences between the first embodiment and second embodiment to avoid redundant explanation. 
     When Step 2 (pure water dipping process) of Embodiment 1 is performed, that is, when the semiconductor device of which circuit area (e.g., protective film  6 ) on the substrate  1  is covered with the resist  11  is dipped into the pure water  12  as shown in  FIG. 3A , the pure water  12  may not filter into the gap between the silicon substrate  1  and resist  11  evenly or sufficiently, and air bubbles may remain in the gap as shown in  FIG. 3D . Trapping of the air bubbles in the gap is so called “airing.” If airing occurs, surface tension is not generated in that portion, so that the sticking function becomes insufficient even if drying processing is performed. Steps 2a and 2b are processings to remove airing. 
     (1) Step 2a (Surface Active Agent Dipping Processing) 
     After the first pure water dipping processing in Step 2 of Embodiment 1, the semiconductor device is dipped into a bath filled with the surface active agent  14  as shown in  FIG. 3B . As a result, as  FIG. 3E  shows, the surface active agent  14  filters into the gap between the silicon substrate  1  and resist  11 , and air is released while changing the surface status from hydrophobic to hydrophilic. By this air releasing, the air bubbles between the silicon substrate  1  and resist  11  disappear, and the pure water  12  in the gap is substituted with the surface active agent  14 . 
     (2) Step 2b (Pure Water Dipping Processing) 
     As shown in  FIG. 3C , the semiconductor device having the surface active agent  14  filtered into the gap between the substrate  1  and resist  11  is dipped into the pure water  12  again. By this water dipping, as  FIG. 3F  shows, the surface active agent  14  in the gap between the silicon substrate  1  and resist  11  is substituted with the pure water  12 , and the water  12  is evenly filtered into the gap. 
     The subsequent steps are the same as Steps 3 and 4 in Embodiment 1. 
     As described above, in the semiconductor device fabrication method in Embodiment 2, the semiconductor device, after the first pure water dipping processing is performed, is dipped into the surface active agent bath, and then the second pure water dipping processing is performed. Therefore, even if airing is generated in the first pure water dipping processing, the air is pushed out by the surface active agent dipping processing, and water  12  can be evenly filtered into the gap in the second pure water dipping processing. Accordingly, the silicon substrate  1  and resist  11  can be adhered to each other more reliably in the subsequent drying processing, and problems in HF cleaning processing can be prevented. 
     Embodiment 3 
       FIG. 4A  to  FIG. 4F  are diagrams of the semiconductor device fabrication method according to Embodiment 3 of the present invention. The third embodiment has an etchant dipping processing Step 2ax. Step 2ax is a step performed between step 2a and step 2b of Embodiment 2. It can be said that step 2 of Embodiment 1 is performed by four steps: the first pure water dipping processing (Step 2), surface active agent dipping processing (Step 2a), etchant dipping processing (Step 2ax), and second pure water dipping processing (Step 2b). The following description deals with only the difference between the foregoing embodiments and third embodiment. 
     Step 2ax (Etchant Dipping Processing) 
     After Step 2a of Embodiment 2, i.e., after the surface active agent  14  is filtered into the gap between the semiconductor substrate  1  and the resist  11 , the semiconductor device is dipped in an HF aqueous solution  15 , of which concentration is about 5%, for a short time (e.g., 120 seconds or about 2 minutes), so as to remove the natural oxide film (SiO 2 ) on the surface of the silicon substrate  1  in a range that does not reach the protective film  6  as shown in  FIG. 4A . By this dipping, the natural oxide film on the surface of the silicon substrate  1  is etched, as  FIG. 4B  shows, and the surface of the silicon substrate  1  becomes rough with micro bumps. 
     Then the second pure water dipping processing (i.e., Step 2b of Embodiment 2) is performed so that the HF aqueous solution  15  is substituted with the pure water  12 , as  FIG. 4C  shows. Then the semiconductor device is placed in a high temperature chamber, so as to perform drying processing in high temperature air  13 . By this drying processing, the water  12  which has entered between the silicon substrate  1  and resist  11  is evaporated. Because of the surface tension generated upon the evaporation of the water  12 , the resist  11  is attracted toward the silicon substrate  1 , and the resist  11  is adhered to the silicon substrate  1 , as  FIG. 4D  to  FIG. 4F . The resist  11  enters (engages with or bites) the bumps on the surface of the silicon substrate  1 , so that strong adhesion is obtained. 
     As described above, according to the semiconductor device fabrication method of Embodiment 3, a short period of etchant dipping processing is performed on the semiconductor device, after the surface active agent dipping processing. By this etchant dipping, the silicon substrate  1  has small concave and convex in the surface, while the natural oxide film is removed. Therefore the silicon substrate  1  and resist  11  can be strongly adhered by the subsequent pure water dipping processing and drying processing. Accordingly, problems with the HF cleaning processing can be prevented. 
     The present invention is not limited to the above described embodiments, and various modifications and changes can be made. For example: 
     (a) The semiconductor device is the acceleration sensor in the illustrated embodiments, but the present invention can be applied to semiconductor devices in general, such as integrated circuits including an ordinary LSI. 
     (b) In the pure water dipping processing, the semiconductor device is dipped into the still pure water  12 , but may be dipped in running water. 
     (c) The semiconductor device in the pure water bath may be cleaned with ultrasonic by applying ultrasonic waves to the pure water  12 . 
     This application is based on Japanese Patent Application No. 2005-284286 filed on Sep. 29, 2005 and the entire disclosure thereof is incorporated herein by reference.