Abstract:
A method and apparatus for calibrating a marking position in a chip scale marker are provided. The method includes: (a) placing a screen which is equivalent in shape to the wafer on a wafer holder for holding the wafer; (b) irradiating a laser beam at a predetermined target point on the screen, and measuring the position of the laser beam by a camera being moved above the target point; (c) transmitting the measured position information to a controller; (d) repeating steps (b) and (c) at a plurality of predetermined points; (e) comparing the transmitted position information with the target point; and (f) calibrating the position of the laser beam irradiated on the wafer by adjusting mirrors of the galvano scanner in the event that a deviation between the position information and the target point falls beyond a predetermined value.

Description:
is application claims priority under 35 U.S.C. §§119 and/or 365 to  2001-75668  filed in Korea on Dec. 1, 2001; the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for calibrating a marking position in a chip scale marker, and more particularly, to a method and apparatus for calibrating a marking position in a chip scale marker that marks characters on a wafer-chip with a laser. 
     2. Description of the Related Art 
     In general, each wafer used in a semiconductor manufacturing process is composed of several thousand to several ten thousand chips. Once chips are completed, a marking process of marking characters and/or numbers is performed on the completed chips so as to classify the chips according to their product lot numbers. At this time, the marking process is performed by a chip scale marker using a laser beam. 
     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  30  is positioned below the wafer holder  20 . A laser beam is oscillated from a laser source of the laser  30 , irradiated on chips on the wafer w via a plurality of mirrors (not shown) of a galvano scanner  32  and an f-theta lens  34 , and finally marks as 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 . Here, reference numeral ‘ 60 ’ denotes a table on which the X-Y stage  50  and the wafer holder  20  are placed. 
     To perform a marking process precisely on the chips on a wafer, wafer alignment has to be precisely accomplished. Here, wafer alignment means positioning a wafer at a marking position according to geometrical characteristics of the wafer or a recognition part. A marking process is performed by recognizing the recognition characteristics of a wafer, such as a ball array or a recognition mark, by optical methods, and irradiating a laser beam at the marking position using a suitable optical system. At this time, recognition of the exact chip position and precise laser beam irradiation are required to perform the marking process on a chip of less than 1 mm 2 . However, even if the aforementioned conditions are satisfied, the irradiated position of a laser beam may gradually change due to external conditions such as vibration and heat. Therefore, there is a need to periodically check this change and calibrate the laser beam irradiation position if necessary. Although the period of measuring and calibrating the marking position varies according to the type of tools used and working conditions, it is necessary to periodically and conveniently check if a laser beam is irradiated at a desired position. 
     FIG. 2 is a diagram explaining a conventional method of measuring marking errors. Conventionally, a laser beam is irradiated on a wafer-type plate  70  in which a plurality of holes  70   a  having a diameter of 0.3 mm are formed, and then the position of the laser beam, which passed through these holes  70   a , is detected by a camera  40  so as to compare the position of the laser beam with a desired position thereof. Next, the irradiation path of the laser beam is calibrated based on the detected difference between the detected position and the desired position. 
     However, this conventional method is disadvantageous in that a laser beam passing through the holes  70   a  is monitored via a front glass  42  of the camera  40 . The laser beam is irradiated at an inclination with respect to the hole  70   a  as indicated with the dotted line in FIG. 2 and, thus, refracted at the front glass  42  of the camera  40 . For this reason, it is difficult to detect the exact irradiation position of the laser beam at the plate  70  that is placed at the same position as a wafer. 
     SUMMARY OF THE INVENTION 
     To solve the above-described problems, it is a first object of the present invention to provide a method of calibrating a marking position in a chip scale marker, by irradiating a laser beam on a semi-transparent screen rather than a wafer, detecting the irradiated position and calibrating the marking position. 
     It is a second object of the present invention to provide an apparatus for calibrating a marking position in a chip scale marker for the above-mentioned method. 
     To achieve an aspect of the first object, there is provided a method of calibrating a marking position in a chip scale marker which performs a marking process by irradiating a laser beam from a laser source on a wafer via a galvano scanner and an f-theta lens, including: (a) placing a screen which is equivalent in shape to the wafer on a wafer holder for holding the wafer; (b) irradiating a laser beam at a predetermined target point on the screen, and measuring the position of the laser beam by a camera being moved above the target point; (c) transmitting the measured position information to a controller; (d) repeating steps (b) and (c) at a plurality of predetermined points; (e) comparing the transmitted position information with the target point; and (f) calibrating the position of the laser beam irradiated on the wafer by adjusting mirrors of the galvano scanner in the event that a deviation between the position information and the target point falls beyond a predetermined value. 
     To achieve another aspect of the first object, there is provided a method of calibrating a marking position in a chip scale marker which performs a marking process by irradiating a laser beam from a laser source on a wafer via a galvano scanner and an f-theta lens, including: (a) unloading the wafer from a wafer holder; (b) positioning a camera screen at the front of a camera for measuring a beam position; (c) moving the camera and the camera screen to a predetermined target point; (d) irradiating a laser beam at the target point and measuring the position of the laser beam irradiated on the camera screen; (e) transmitting the measured position information to a controller; (f) repeating steps (c) to (e) at several predetermined points: (g) comparing the transmitted position with the target point; and (h) calibrating the position of the laser beam irradiated on the wafer by adjusting the position of mirrors of the galvano scanner in the event that a deviation between the position and the target point is beyond a predetermined value. 
     To achieve the second object, there is provided an apparatus for calibrating a marking point in a chip scale marker having wafer marking laser, a wafer holder for holding a wafer, and a camera moving while connected to an X-Y stage above the wafer holder and measuring an object held by the wafer holder. The apparatus includes a screen; and a controller for receiving information regarding the position of a laser beam irradiated on the screen and for calibrating the position of mirrors of a galvano scanner of the wafer marking laser in the event that a deviation between the laser beam position and a target point is beyond a predetermined value. 
     Preferably, the screen is equivalent in shape to the wafer, and the screen, on which a laser beam from the laser is irradiated and a beam point is marked, is placed on the wafer holder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
     FIG. 1 is a schematic view of a general chip scale marker; 
     FIG. 2 is a diagram explaining a conventional method of measuring marking errors, 
     FIG. 3 is a schematic view of a first embodiment of an apparatus for calibrating marking position in a chip scale marker according to the present invention; 
     FIG. 4 is a perspective view of a wafer holder and a screen of FIG. 3; 
     FIG. 5 is a diagram showing the light path from the irradiated point on a screen by a laser; 
     FIG. 6 is a schematic view of the center point of a camera and a laser beam point that deviates from the center point; 
     FIG. 7 is a schematic view of a second embodiment of an apparatus for calibrating marking position in a chip scale marker; and 
     FIG. 8 is a partially cutaway perspective view of another embodiment of a screen according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a first embodiment of an apparatus for calibrating a marking position in a chip scale marker according to the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the thickness of layers or regions is exaggerated for clarity. 
     FIG. 3 is a schematic view of a first embodiment of an apparatus for calibrating a marking position in a chip scale marker according to the present invention. Referring to FIG. 3, a screen  180  is placed on a wafer holder  120 , and a laser  130  is positioned below the wafer holder  120 . To mark a corresponding product lot number on a wafer, a laser beam is oscillated from a laser source of the laser  130  and irradiated on the wafer via a plurality of mirrors (not shown) of a galvano scanner  132  and an f-theta lens  134 . Installed above the wafer holder  120  is a camera  140  for observing the wafer or the screen  180 . The camera  140  is moved and held by an X-Y stage  150 . The position measured by the camera  140  and the X-Y stage  150  is input to a controller  170  as an electrical signal, and a signal output from the controller  170  is transmitted to the galvano scanner  132  and the X-Y stage  150 . 
     The screen  180  on which a marking process is to be performed is equivalent in shape to a wafer, and is composed of two layers as shown in FIG.  5 . In detail, a lower layer  182  is a fluorescent layer that absorbs a laser beam and emits light, and an upper layer  184  is a layer that passes the light emitted by the lower layer  182 . Preferably, the lower layer  182  is formed of a rigid material so that it does not transform even when positioned in a central hole  122  of the wafer holder  120  illustrated in FIG.  4 . 
     FIG. 4 is a perspective view of the wafer holder  120 , illustrated together with a screen  180 . The wafer holder  120  has the central hole  122  at the center and a plurality of protrusions  128  that hold the wafer within the inner circumference of the wafer holder  120 . A plurality of holes  124  are formed around the central hole  122  and are each covered with a semi-translucent layer  126  capable of partially transmitting light. Preferably, these holes  124  are aligned to form a concentric circle a predetermined distance away from the inner circumference of the wafer holder  120 . Here, the semi-translucent layer  126  functions as the screen  180 . 
     Hereinafter, the operations of an apparatus for calibrating a marking position in the chip scale marker  100  having the above composition will be described in detail with reference to the drawings. 
     FIG. 5 is a diagram showing the path of light when a laser beam is irradiated on a screen  180 , and FIG. 6 is a diagram of the center point  146  of a camera and a laser beam point  148  that deviates from the center point. 
     First, referring to FIGS. 3 and 5, the screen  180 , which is equivalent in shape to a wafer, is placed on the wafer holder  120 . Then, when light is oscillated from the laser  130 , it is irradiated on a predetermined point of the screen  180  via a plurality of mirrors of the galvano scanner  132 . The irradiated light is absorbed by the lower layer  182 , which is a fluorescent layer, and then radiated. Thereafter, the radiated light is irradiated upward via the upper layer  184 . At this time, as shown in FIG. 5, light with an inclined angle of incidence to the screen  180  as indicated by the dotted lines, is irradiated perpendicularly to the camera  140 , following the path of light indicated by the dotted lines. Preferably, the laser  130  is a Nd:YAG laser, which emits infrared light having a wavelength of 1064 nm, green light having a wavelength of 532 nm, which is a second harmonic wave, and ultraviolet light having a wavelength of 355 nm which is a third harmonic wave. Also, preferably, the camera  140  is a vision CCD camera capable of checking the wavelength of a laser beam. 
     The camera  140  is moved to be positioned above a desired point of a laser beam, i.e., the center point  146  of the camera  140 , by the X-Y stage  150 , and then reads a laser beam point  148  formed on the screen  180  below the camera  140 . At this time, the camera  140  checks how much the beam point  148  deviates from the center point  146 , and inputs X-Y coordinates of the deviation to the controller  170 . This checking is repeated at several positions. 
     Then, the controller  170  analyses the input X-Y coordinates of the deviation, and adjusts the mirrors of the galvano scanner  132  to calibrate the path of light in the event that the deviation is beyond a predetermined range. 
     Next, the screen  180  is taken away from the wafer holder  120 , and then a wafer is placed on the wafer holder  120 . At this time, the position of the wafer is the same as the position of the screen  180 . 
     The laser beam oscillated from the laser  130  is irradiated to marks characters on the wafer, following the calibrated path of light. 
     During the laser marking, it is possible to detect the vibration of the galvano scanner  132 . More specifically, a laser beam is irradiated over the semi-translucent layer  126  covering the hole  124  of the wafer holder  120 , the camera is moved above a target point on which the laser beam is to be irradiated, and then the irradiated beam point is detected and calibrated, if necessary, as described above. 
     Preferably, the screen  126  and  180  is formed of a glass or acryl that is processed to have a rough surface on which a laser beam is irradiated, and a photo attenuator, which is attached on the top of the glass or acryl. When a laser beam is irradiated at a point on the screen  180 , the laser beam is scattered by the rough surface of the screen  180 . In this case, although the laser beam is irradiated on the screen  180  at an angle, the laser beam is scattered and not transmitted to the camera  140  with an inclined angle of incidence. The irradiated laser beam forms an image on the lower layer  182 . Also, the photo attenuator can be installed on the lower layer in order to detect a laser beam point from the scattered beams on the lower layer. The light passing through the photo attenuator has only one beam point, and therefore the beam point can be easily measured using the camera  140 . 
     A two-layered screen is adopted in this embodiment, but a single-layered screen is sufficient in the case that the screen is formed of a rigid material such as semi-translucent glass. 
     FIG. 7 is a schematic view of a second embodiment of an apparatus for calibrating a marking position in a chip scale marker  200  according to the present invention. Here, the same elements as those in the first embodiment are described with the same reference numerals, and their detailed descriptions are omitted. 
     Referring to FIG. 7, a motor  292  is attached to the bottom of a support  142  for holding a camera  140  so as to install or detach the camera screen  290  at or from the front of a camera  140 . The wafer holder  220  has a central hole  222  through which a laser beam is irradiated on the camera screen  290  from a laser  130 . 
     To measure a marking position by the camera screen  290 , the camera screen  290  is positioned at the front of the camera  140  by actuating the motor  292 . Next, an X-Y stage  150  is actuated to move the camera  140  and the camera screen  290  to a predetermined position, and then a laser beam is irradiated onto the camera screen  290 . Then, the irradiated beam point is measured by the camera  140 , and the position information of the beam point is input to a controller  170 . 
     FIG. 8 is a partially cutaway perspective view of another embodiment of a screen according to the present invention. Here, the screen is made by attaching a semi-translucent layer  284  to a round frame  282 . The semi-translucent layer  284  may be formed of tracing paper, for example. Such a semi-translucent layer  284  indicates a point where the laser beam is irradiated. 
     As described above, in a method and apparatus for calibrating a marking position in a chip scale marker, according to the present invention, a marking position is measured and the direction of a laser beam is calibrated before marking characters and/or numbers on a wafer. During a marking process, the position of a laser beam can be easily adjusted by irradiating the laser beam on a semi-translucent layer formed at the edge of a wafer holder, measuring the irradiated laser beam point, and calibrating a marking position. Further, since the calibration of a marking position is performed directly on a beam point irradiated on a screen, the marking position can be precisely calibrated, thereby performing a marking process at the desired position of a wafer chip. 
     While this invention has been particularly described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.