Development of a Hybrid CMP-ELID Grinding System

 
 
Assoc Prof AS Kumar (Department of Mechanical Engineering)
 
 
emand for mirror surfaces on metals, silicon and glass is increasing due to their varied applications in the field of optical communications, lasers, etc. Such mirrors are traditionally manufactured by a polishing process which is tedious and time consuming. An alternative, efficient

and more economical process in this regard is the Electrolytic In Process Dressing (ELID) and grinding technique. A schematic representation of the ELID grinding process is shown in Figure 1. It uses a super fine diamond grid embedded metal bonded grinding wheel which is continuously dressed by means of an electrolytic process, to realize stable finish grinding. Although this technique has been successful in obtaining nanometric surface finish, the formation of oxide layers on the silicon wafer surface could not be avoided.

  Figure 1: Schematic representation of the ELID grinding technique.  

In this research, a combined chemical mechanical polishing (CMP)ELID machining system has been developed. The CMP process involves selective chemical reaction to increase the mechanical removal rate of the oxide layer on the machined surface, thus improving the efficiency of the ELID grinding process. In the combined CMP-ELID process, the correct composition of the CMP slurry and proper control of the ELID grinding process are crucial factors to ensure successful results.

The basic setup consisted of a cast iron bonded grinding wheel with diamond grits as the anode and a copper injection electrolyte type cathode. The oxide layer is formed on the wheel during the electrolysis and gets deposited on the metal surface during grinding. The electrolyte used was CG7 of Fuji Die® diluted to 50:1 volume ratio with water. Other experimental parameters needed to be optimized included electrolyte flow rate, electrode gap, spindle speed and peak voltage and duty cycle of the dressing operation. In-process sensor feedback was used to obtain the optimal condition of electrolysis and to actively monitor the process and improve the quality of the machined surface. The sensors measure the shape of the grinding wheel including information about the oxidation layer formed on the wheel surface during electrolysis.

The growth of the oxide layer on the cast iron grinding wheel under different electrolytic conditions was carefully studied. The wheel was dressed under the specific conditions for 5 minutes, after which the electrolyte flow was stopped. The wheel was dried and its profile was measured by a laser sensor to 100 nm accuracy. The measurements were compared with the initial wheel profile without oxide layer to determine the amount of wheel growth. The process was repeated to extend the total dressing time to 45 minutes. Figure 2 shows the oxide layer growth behaviour with electrolyte flow rate obtained in the above test.

  Figure 2: Variation of oxide layer growth with electrolyte flow rate.  

 

  Figure 3: (a) Machined silicon sample using ELID grinding. (b) and (c) show the details of the machined surfaces with and without ELID grinding, respectively.  

After desired oxide growth, the wheel was used to machine silicon wafer mounted horizontally on the machine table. Using the said set-up, surface roughness of 2 nm was achieved on silicon using a #400 wheel. Figure 3(a) shows a silicon sample machined using the developed process, while Figure 3(b) and 3(c) give the details of the machined surfaces with and without ELID grinding, respectively. The improvement in surface quality in the former is evident in these figures.

A. Senthil Kumar is an Associate Professor of Mechanical Engineering at NUS. His expertise is manufacturing with focus on hybrid machining of micro/nano structures and fixture design. He is the recipient of several awards including Serope Kalpakjian’s Outstanding Young Manufacturing Engineer Award from the Society of Manufacturing Engineering, USA.

Email: mpeask@nus.edu.sg
 
 


Engineering Research · Research Developments
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