Electric heating – Metal heating – By arc
Reexamination Certificate
2002-09-03
2004-03-23
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121690, C219S121810
Reexamination Certificate
active
06710286
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chip scale marker and a marking method, and more particularly, to a method for marking characters on a wafer chip after calibrating a marking distance from a laser to the wafer, using a laser of a chip scale marker and a device thereof.
2. Description of the Related Art
In general, wafers used in a semiconductor manufacturing process are composed of several thousands to several tens of thousands of chips. It is required that characters or/and numbers are marked on a surface of each chip in order to classify the chips according to their production lot numbers. Presently, a chip scale marker using a laser beam is used as a tool for marking.
FIG. 1
is a schematic view of a general chip scale marker
10
, illustrated with a wafer w. Referring to
FIG. 1
, the wafer w is placed on a wafer holder
20
and a laser system
30
is positioned below the wafer holder
20
. A laser beam oscillated from a laser source of the laser system
30
, is irradiated on chips of the wafer w via a galvano scanner (not shown) and an f-theta lens (not shown), and finally marks characters on the chips.
Above the wafer holder
20
, a camera
40
is positioned for monitoring an object held by the wafer holder
20
. The camera
40
is connected to an X-Y stage
50
and moves with the X-Y stage
50
. The reference numeral
60
denotes a table, on which the X-Y stage
50
and the wafer holder
20
are placed.
FIG. 2
is a view showing a depth of focus D.O.F of a laser beam irradiated on the wafer.
FIG. 3
is a view showing a warpage of wafer on a wafer holder. Referring to
FIG. 2
, a laser beam to be irradiated on a horizontal wafer chip from the f-theta lens
34
via the galvano scanner (not shown). Here, it is possible to obtain good quality of marking only if a marking surface is placed in the range of the depth of focus D.O.F. Here, the depth of focus is calculated as follows.
D.O.F=±2&lgr;(ƒ/D)
2
Here, D denotes the diameter of an incident beam, f denotes the focus distance and &lgr; is the wavelength of a laser beam.
However, in a wafer including a plurality of chips, there is a warpage in a certain direction due to weight of the wafer, coating on the wafer surface and other processes (refer to FIG.
3
). This warpage becomes severer as the wafer is bigger, the wafer is thinner, and the wafer shrinks more, when the coatings on the wafer harden. If a height deviation h of wafer surface to be marked due to warpage is bigger than the depth of focus, the marking quality decreases because a beam laser density changes depending on the position of a chip on a wafer surface.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for measuring the wafer warpage, calibrating a marking distance, and marking a wafer.
It is another object of the present invention to provide a chip scale marker for the above marking method.
To accomplish the first object of the present invention, there is provided a method for marking, using a chip scale marker, wherein a laser beam is irradiated from a laser source on the wafer chips via a galvano scanner and an f-theta lens, the method comprising: (a) measuring position information of a plurality of points on the wafer; (b) transmitting the measured position information to a controller; (c) calculating a deviation between a marking distance between the f-theta lens and the point on the wafer surface and a focus distance of the f-theta lens from the transmitted position information; and (d) if the deviation is greater than a predetermined value in the step (c), calibrating the wafer chip to be positioned at the focus distance of the f-theta lens.
It is preferable that the step (a) is measured using a non-contact sensor, a laser sensor. Preferably, the step (a) is sequentially performed at each chip of the wafer or performed on a predetermined plurality of wafer chips positioned on at least one straight line crossing an axis of the wafer. It is preferable that the step (c) further comprises calculating a marking height deviation between the maximum and the minimum of the deviations. It is preferable that the step (d) comprises: (d
1
) equally dividing the marking height deviation into a predetermined number n, and forming the divided n regions at a marking surface of the wafer by forming contour lines with the equally divided height deviation; (d
2
) adjusting the wafer chips of a selected region at a predetermined distance from the f-theta lens; (d
3
) marking wafer chips of the selected region; and (d
4
) repeating steps (d
2
) and (d
3
).
The length of the equally divided height deviation may be smaller than a depth of focus of the f-theta lens, preferably is smaller than ½ of the depth of focus of the f-theta lens.
To accomplish another object of the present invention, there is provided a chip scale marker that includes a laser system for marking a wafer, a wafer holder for supporting a wafer to be processed, and a camera which moves while being connected to an X-Y stage over the wafer holder and monitors an object held by the wafer holder, the chip scale marker further comprising: a sensor for measuring a vertical position of each chip of the wafer; and a means for moving the wafer holder in a vertical direction. It is preferable that the sensor is connected to the X-Y stage.
To accomplish the second object of the present invention, there is provided a chip scale marker, further comprising a means for moving the laser system for wafer marking in a vertical direction, instead of a means for vertical moving of the wafer holder.
In addition, to achieve the second object of the present invention, there is provided a chip scale marker, further comprising; a focus distance correction lens located between the laser oscillator and the galvano scanner; and a means for moving the focus distance correction lens in a horizontal direction.
REFERENCES:
patent: 4598242 (1986-07-01), Hayashi et al.
patent: 5524131 (1996-06-01), Uzawa et al.
patent: 5719372 (1998-02-01), Togari et al.
Burns Doane , Swecker, Mathis LLP
EO Technics Co., Ltd.
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