Patent Application
20080076689
The FIGURE shows a pictorial cross section of a typical metal Via stack found in a silicon wafer.
(a) The Present Method:
The present invention provides a method of removing photoresist materials from a silicon wafer, by: providing a silicon wafer having a photoresist thereon; exposing the silicon wafer having the photoresist thereon to a solute of ozonated acetic anhydride, thereby removing the photoresist from the silicon wafer.
In alternate optional aspects of the method, further chemicals may be added to the Acetic Anhydride:Ozone mixture. Such optional chemicals may assist in removing plasma-damaged resists or other films like BARC (backside-anti-reflective-coatings). Such optional chemicals may include, but are not limited to: Ethylene Glycol Diacetate and Ethylene Carbonate. Such chemicals may optionally be spiked with Hydrofluoric Acid at temperatures in the optional range of 50-70 C. The advantage of this approach is that the acetic anhydride has a low flash point and adding these other chemicals raises the flash point. This is advantageous when the solution is heated.
In another alternate optional aspect of the invention, acetic acid is added to the acetic anhydride. This can be beneficial when removing a thick layer of photoresist around a Tin:Lead (Sn:Pb) solder bump over a thick copper (Cu) stud with a very thin copper seed layer. In this aspect of the invention, both the resist and the very thin copper seed layer can be removed. In optional aspects, a mixture of 50%-50% solution of acetic anhydride and acetic acid can be used. Ozone can be bubbled through the mixture to remove the resist and the copper seed layer while not affecting the Sn:PB:Cu stud. This effect may be due to the small copper etch rate in the acetic acid.
In one approach, the acetic anhydride:acetic acid:ozone solution can be used in one step. Alternately, it can be separated into two steps with the acetic anhydride:ozone solution used to remove the photoresist followed by a separate treatment in acetic acid or acetic acid:ozone used to etch a copper seed layer. This approach is particularly useful when dealing with SnPbCu or SnPbAg studs. Moreover, the process applies even if the stud is another metal alloy combination.
In further optional aspects, the photoresist may be removed with the acetic anhydride:ozone mixture. Then, the thin copper seed layer can be removed in a 40-50 C heated ammonium hydroxide:hydrogen peroxide:water mixture. This second step may possibly be accomplished in as little as 5 seconds. Alternately, the process can be done in one step, thereby reducing the size of the wetstation and the robotics needed to transfer wafers tank to tank and eliminates the additional chemicals and heaters.
It is to be understood that although the present invention is described herein as useful from a single wafer perspective, it is just as effective in batch mode with multiple substrates. As such, the present invention is a scalable process.
(b) Experimental Results:
In an attempt to remove all water molecules and acid functional groups to eliminate galvanic activity and copper etching, ozone was dissolved in Acetic Anhydride. It was unexpectly discovered that organic Photoresist films at 25° C. could be removed very fast. In experiments conducted by the inventor, a 3,300 Å film of 193 nm DUV photoresist was removed in less than 2 seconds! This photoresist strip rate exceeds 90,000 Å/min. This is approximately two orders of magnitude faster than the results achieved with the above described ozonated:water (Coldstrip) solution.
When testing the etch rate of copper films in the acetic anhydride:ozone solution, it was determined Copper had an etch rate of 0.002 um/min or 20 Å/min at 25° C. Etch rates of many other metals (Aluminum, Titanium Nitride, Tungsten, and Chromium) typically used in the semiconductor industry were not affected by the ozonated acetic anhydride solution and metal removal was measured at 0 Å/min at 25° C. Ozonating acetic anhydride at room temperature exceeded the requirements for a single wafer photoresist stripper.
