Method and apparatus for machining semiconductor material

A method and an apparatus are for machining semiconductor material with a inding tool while feeding a liquid cleaning agent to the working surface of the grinding tool, has the cleaning agent being exposed to sound waves having a specific frequency and having a specific intensity. In one embodiment of the method, the cleaning agent is exposed to sound waves in at least one nozzle and then the cleaning agent is directed against the working surface of the grinding tool. In another embodiment, the cleaning agent is guided through at least two cleaning agent jets against the working surface of the grinding tool, which cleaning agent jets differ from each other in that they are exposed to sound waves of different frequencies.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for machining semiconductor 
material with a grinding tool while feeding a liquid cleaning agent to the 
working surface of the grinding tool, with the cleaning agent being 
exposed to sound waves having a specific frequency and a specific 
intensity. The invention also relates to an apparatus for carrying out the 
method. 
2. The Prior Art 
The machining of semiconductor material with grinding tools often requires 
extreme precision. This applies particularly to the production of 
semiconductor wafers from an ingot-type crystal with the aid of abrasive 
cutting machines, so-called annular saws. In this operation, it is 
important that the semiconductor wafers produced have as flat and as 
parallel side faces as possible. Geometrical errors manifest themselves, 
in particular, in sagging of the wafers, which is referred to as warp in 
specialist jargon. Efforts are continually being made to keep the warp of 
the semiconductor wafers as small as possible in their production from a 
crystal ingot. 
U.S. Pat. No. 5,313,741 discloses an abrasive cutting method in which the 
cutting force which the saw blade exerts on the workpiece is continuously 
measured in the cutting direction and in two directions orthogonal 
thereto. In conjunction with this operation, a cleaning liquid is sprayed 
against the cutting edge of the annular saw from a nozzle and the liquid 
jet is exposed to ultrasound from an external source. If the measured 
cutting force exceeds a specific limit value, the speed of advance is 
reduced and the speed of rotation of the saw blade is increased 
simultaneously. Although this method makes lower warp values probable in 
the semiconductor wafers produced, an economical disadvantage has also to 
be accepted as a result of the reduction in the speed of advance because 
the output of semiconductor wafers per machine and unit time is reduced. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the disadvantages 
mentioned above. 
The present invention achieves the above object by providing a method for 
machining semiconductor material with a grinding tool while feeding a 
liquid cleaning agent to the working surface of the grinding tool. The 
cleaning agent is exposed to sound waves having a specific frequency and a 
specific intensity. The improved method further comprises exposing the 
cleaning agent to sound waves in at least one nozzle and then directing 
the liquid cleaning agent which flows through the nozzle against the 
working surface of the grinding tool after the liquid cleaning agent exits 
the nozzle. 
The present invention also relates to a method which comprises directing 
the liquid cleaning agent in the form of at least two cleaning agent jets 
from two different nozzles against the working surface of the grinding 
tool, which jets differ from each other in that they are exposed to sound 
waves of different frequencies. 
If the present invention is used to produce semiconductor wafers by 
abrasive cutting of a crystal ingot by means of an annular saw, it can not 
only be established that the semiconductor wafers produced have lower warp 
values, but also that an increase in the speed of advance of the grinding 
tool and therefore an increase in the output of semiconductor wafers per 
machine and unit time is possible. The reason for this is attributable to 
the considerably improved action of the cleaning agent which prevents the 
cutting space behind the abrasive particles (for example, diamond 
particles) from being obstructed. 
As a result of the increase in output to be noted, the present invention is 
essentially suitable for all types of grinding treatment of semiconductor 
material and is not restricted to the treatment of semiconductor wafers. 
In addition to the cutting of a crystal ingot into semiconductor wafers, 
preferred fields of application include the rounding of the edges of 
semiconductor wafers in the thickness direction and along their 
circumferential lines, the cylindrical grinding of crystal ingots and the 
grinding of flat surfaces with cup wheels, such as, for example, the 
grinding treatment of the side faces of semiconductor wafers or of the 
ingot end face of a crystal ingot. 
According to one embodiment of the invention a liquid cleaning agent is, 
during the grinding process, directed from at least one nozzle against the 
working surface of the grinding tool, for example against the cutting edge 
of an annular saw. In this process, the cleaning agent is exposed to sound 
waves having a specific frequency and intensity while still in the nozzle. 
A further embodiment of the invention includes directing at least two 
cleaning agent jets against the working surface of the grinding tool, 
which liquid cleaning agent jets differ from each other in that they are 
exposed to sound waves of different frequencies. This method is based on 
the discovery that there is a relationship between the cleaning action of 
the cleaning agent, the frequency of the sound waves and the size of the 
particles of the semiconductor material cut during grinding. Under the 
application of sound waves having relatively high frequencies, relatively 
small particles are preferentially removed and under the application of 
sound waves having relatively low frequencies, relatively large particles 
are preferentially removed. Size and size distribution of the particles 
depend, in particular, on the particle size of the grinding particle 
bonded to and bound up into the working surface of the grinding tool. 
It is therefore desirable to determine the distribution of the particle 
sizes of the semiconductor material cut and to expose the cleaning agent 
to sound waves of those frequencies which make a particularly effective 
removal of the particles probable. Preferably, 2 to 10, and particularly 
preferably 2 to 3, different frequencies are used. A further embodiment of 
the method additionally provides a weighting of the frequencies used by 
exposing the cleaning agent to sound waves of different frequencies and of 
different intensities. The action of the cleaning agent is further 
optimized in this way. 
The cleaning agent jets are produced by ejecting the liquid cleaning agent 
from nozzles. The cleaning agent jets can be acoustically irradiated by an 
external sound source, for example by a sound generator disposed outside 
the nozzles. It is again preferred, however, to expose the cleaning agent 
to sound waves in the nozzles themselves. 
To carry out the invention, ultrasonic waves having a frequency of 100 kHz 
to 10 MHz, and in particular from 0.5 to 3 MHz, are preferred. The 
intensity of the sound waves is preferably from 10 W to 500 W, with this 
numerical information relating to the electrical power output for 
producing the sound waves. 
In principle, all the liquid cleaning agents already known that are brought 
into contact with the working surface of the grinding tool during the 
grinding treatment of semiconductor material are suitable for use 
therewith. Water and liquid cooling lubricants or any mixtures thereof are 
preferred. 
The present invention also relates to an apparatus for machining 
semiconductor material with a grinding tool while feeding a cleaning agent 
to the working surface of the grinding tool, having means for exposing the 
cleaning agent to sound waves. The apparatus has at least one nozzle which 
is directed toward the working surface of the grinding tool, having a 
component which is built into the nozzle and which generates sound waves 
of a specific frequency and intensity and having an oscillator which 
excites the component to vibrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning now in detail to the drawing, there are a plurality of nozzles 2 
which are directed against the working surface 1 of the grinding tool, for 
example the cutting edge of the saw blade of an annular saw. The nozzles 
are connected via feed lines 3 to a stock container 4 which contains the 
liquid cleaning agent 5. Every nozzle may also be connected to its own 
stock container, for example if different liquid cleaning agents are 
desired. In every nozzle there is a sound generating component or 
transducer means 6 which is made, for example, of tantalum or piezoceramic 
material. These components or transducers 6 are excited by the associated 
alternating voltage sources 7, the so-called oscillators, to vibrate with 
a specific frequency and intensity. During the treatment of the 
semiconductor material, the liquid cleaning agent is sprayed under 
pressure out of the nozzles onto the working surface of the grinding tool, 
which is generally rotating. During the passage through the nozzles, sound 
energy is transferred to the liquid cleaning agent. The frequency of the 
sound waves is dependent on the component 6 built into the nozzle. Its 
intensity depends on the power output of the associated oscillator. 
Preferably, 2 to 10 nozzles are provided in which sound waves of different 
frequencies can be generated. 
A further embodiment of the apparatus has the additional feature of a 
control device 8, which can, for example be a microprocessor control 
means. Control device 8 controls the operation of each of the available 
nozzles 2 in accordance with a specified program. Suitable as controllable 
parameters are the output of cleaning agent, the sound generation in a 
nozzle and the intensity of the sound generated in the nozzle. 
Other objects and features of the present invention will become apparent 
from the following Examples, which disclose the embodiments of the present 
invention. It should be understood, however, that the Examples are 
designed for the purpose of illustration only and not as a definition of 
the limits of the invention. 
EXAMPLE 
Silicon semiconductor wafers having a wafer diameter of 200 mm were cut 
from a single crystal ingot using a conventional annular saw. During the 
cutting process water was sprayed as liquid cleaning agent out of one 
nozzle onto the cutting edge of the saw blade of the annular saw. A total 
of three test series was run, one of them as a comparison test. In the 
comparison test, the liquid cleaning agent was not exposed to sound waves. 
During the test series I), the cleaning agent was exposed to ultrasound 
with a frequency of 1 MHz in the nozzle, and during the test series II), 
it was exposed to ultrasound with a frequency of 3 MHz after a nozzle 
change with otherwise unaltered conditions. The results of the test series 
are listed below in tabular form: 
______________________________________ 
Test Test 
Comparison Series I) 
Series II) 
______________________________________ 
Advance*) 100 150 150 
Output*) 100 147 147 
Warp*)**) 100 72.5 59.4 
TTV*)**)***) 
100 89.0 72.1 
______________________________________ 
*)Numerical information in relative units 
**)Numerical value: 3 sigma (99.87%) of all wafers equal or better 
***)TTV = total thickness variation 
While several embodiments of the present invention have been shown and 
described, it is to be understood that many changes and modifications may 
be made thereunto without departing from the spirit and scope of the 
invention as defined in the appended claims.