Stone working – Splitting – shearing – and punching
Reexamination Certificate
2000-05-26
2002-07-16
Rachuba, M. (Department: 3723)
Stone working
Splitting, shearing, and punching
C125S012000, C225S096500, C225S097000, C225S103000, C225S104000
Reexamination Certificate
active
06418922
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the manufacturing of semiconductor products and, more specifically, to the cutting of semiconductor wafers to individualize the components or circuits that have been formed thereon.
2. Discussion of the Related Art
FIG. 1
very schematically shows a wafer
1
made of a semiconductor material (for example, silicon) in a handling frame
2
used, in particular, to handle the wafer in the final steps of the manufacturing methods and, especially, after wafer
1
has been thinned, for example, by rectification of its rear surface. Frame
2
generally has the shape of a ring with an internal diameter slightly greater than the diameter of wafer
1
. Wafer
1
is attached to frame
2
by means of a sheet
3
, pasted by its periphery on frame
2
and the adhesive central portion of which is intended for receiving one of the surfaces of wafer
1
. Generally, the surface of wafer
1
that is pasted on sheet
3
is the front surface (that which generally includes the contacts) to protect it during the steps of cutting and, if present, of rectification of the rear surface.
Once the wafer has been thinned, since it is very fragile, it is desirable to leave it in the same frame until the chips included therein are individualized.
The present invention more specifically relates to a cutting technology that consists of forming, generally, by sawing, a preliminary partial cutting from a first surface of the wafer, then to break the remaining thickness by means of a knife or blade that hits the other surface.
Thus, the present invention more specifically relates to the cutting of silicon wafers according to a method essentially including two steps, respectively of sawing and of breaking. This conventional method is illustrated in
FIGS. 2A and 2B
that are partial cross-section views of a wafer
1
in a cutting region.
According to this method, in a first step (FIG.
2
A), preliminary cuttings
4
are formed between the chips to be separated. The preliminary cuttings are generally formed down to a depth ranging from one third to two thirds of the thickness of wafer
1
.
Then, in a second step, the remaining silicon portion in the cutting regions is broken by means of a knife
5
that is vertically displaced (perpendicularly to the wafer plane) to hit adhesive sheet
3
right behind cutting blank
4
. Fractures
6
are thus obtained along all cutting paths. The last step of the method consists of removing, from adhesive sheet
3
, the individualized chips.
This cutting technique in two steps has many advantages, in particular, that of avoiding that the chips be notched on their rear surface.
However, such a cutting method raises implementation problems. One of these problems is the circular shape of the semiconductor wafers. Indeed, since the wafers are circular, the lengths of the cutting paths are different at the wafer periphery and towards its center. As a result, the length of knife
5
must be at least equal to the wafer diameter to enable breaking it in its central portion. The wafer is indeed first displaced (by the displacement of the frame) step by step (according to the interval between two parallel cutting paths) in a first horizontal direction (arrow f) to successively place the cutting paths perpendicular to this first direction in front of the knife that is moved vertically (arrow f
2
). Then, the wafer is turned by ninety degrees so that, with a same horizontal step by step motion (arrow f), the preliminary cuttings that are parallel to the first direction are broken, the chips being generally square-shaped.
FIG. 3
illustrates, in a simplified perspective view, the implementation of the conventional cutting method by means of a knife
5
, the length of which is at least equal to the diameter of wafer
1
.
Since wafer
1
is moved laterally (arrow f) to move on from cutting path to cutting path, it is necessary for the wafer support (frame
2
′) to have a sufficient diameter to enable the knife
5
to pass even at the edge of wafer
1
. Accordingly, the diameter of frame
2
′ is much greater than the frame diameter (
2
,
FIG. 1
) that is required for the other steps of the process.
The use of a frame
2
′ of greater diameter has several disadvantages.
A first disadvantage is that it is necessary to use larger stands and different new machines to handle frames of greater diameter. This results in the need to perform significant investments.
Another disadvantage is that this causes an excess consumption of adhesive sheet
3
, which is unjustified considering the wafer diameter.
For example, to process wafers having a diameter of approximately 15 centimeters (6 inches), a knife
5
having a length greater than 15 centimeters must be used. This requires the use of frames
2
′ corresponding, for example, to those otherwise used for wafers having a diameter of approximately 20 centimeters (8 inches).
SUMMARY OF THE INVENTION
The present invention aims at providing a novel cutting method and adapted tools that overcome at least the above-mentioned disadvantages.
The present invention more specifically aims at enabling the cutting of semiconductor wafers by a sawing and breaking method that requires no increase of the diameter of the frames supporting the wafers.
The present invention also aims at providing a solution that is compatible with current cutting equipment.
To achieve these and other objects, the present invention provides a method of cutting a wafer of a semiconductor material, including breaking the wafer along cutting paths by means of a knife applied by hitting a sheet supporting the wafer in a frame, by using knives of different lengths according to the wafer region in which the cutting path is located.
According to an embodiment of the present invention, the wafer is moved step by step above a tool block according to a direction perpendicular to the direction of the cutting paths, and the length of the different knives is adapted to the available free space inside the frame in the corresponding region.
According to an embodiment of the present invention, the wafer is first submitted to a cutting by partial sawing from the surface opposite to the supporting sheet, and the same frame is used for the sawing and the breaking.
The present invention also provides a tool block for cutting a semiconductor wafer by breaking this wafer along rectilinear cutting paths by means of a knife, including means for receiving at least two knives of different lengths and for being rotated step-by-step around an axis to change the knife that is active in the wafer cutting.
According to an embodiment of the present invention, the tool block includes three knives of different lengths around a cylindrical core, the knives being regularly distributed around the core by forming equal angles with one another.
According to an embodiment of the present invention, each knife is removably attached on a plate moving along with a core, adapted to be rotated around its axis.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.
REFERENCES:
patent: 4044937 (1977-08-01), Hill et al.
patent: 5601128 (1997-02-01), Furphy
patent: 6205994 (2001-03-01), Freund et al.
Dubois André
Levivier Jean-Pierre
Morris James H.
Rachuba M.
STMicroelectronics S.A.
Wolf Greenfield & Sacks P.C.
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