1. Field of the Invention
The present invention relates to a pellet picking method and a pellet picking apparatus used preferably in semiconductor device fabrication and more particularly, to a pellet picking method and a pellet picking apparatus for picking up one or more pellets arranged on an adhesive sheet with a collet, where the pellet or pellets is/are usually formed by separation of a wafer through scribing or sawing.
2. Description of the Related Art
Semiconductor pellets are typically formed by cutting or separating a semiconductor wafer with a specific diameter such as 6 or 8 inches into pieces. The pellets are typically rectangular or square. Semiconductor devices and/or elements or semiconductor integrated circuits are formed in the corresponding areas of the wafer to the pellets and therefore, they are formed in each of the pellets. Semiconductor pellets will be often called simply xe2x80x9cpelletsxe2x80x9d hereinafter.
The pellet has a variety of shapes and sizes. For example, the pellet has an approximately square shape of 0.5 mmxc3x970.5 mm to 30 mmxc3x9730 mm. The pellet may have an elongated, strap-like shape. The square pellet may be formed by separating the wafer into strap-shaped parts in a step and by separating each of the strap-shaped parts into square pieces in another step.
The above-described separation process of the wafer or its strap-shaped part is typically performed in the following way.
The wafer itself or the strap-shaped part is adhered onto an adhesive sheet and then, it is cut by using an edged tool such as a dicing saw on the sheet, thereby forming the pellets. The sheet is usually stretched or spread on a circular support ring with almost no looseness. In this separation process, the cutting blade of the tool is lowered to penetrate the wafer or the strap-shaped part and then, it is lowered into the sheet to create a trench with about one-third to one-second (⅓ to xc2xd) of the thickness of the sheet. The thickness of the sheet is typically approximately 0.1 mm.
Subsequently, the wafer or the strap-shaped part thus formed is mounted on a pellet picking apparatus. At this time, the wafer or the strap-shaped part is kept adhered on the sheet while they are mounted on the apparatus along with the support ring. Then, the apparatus picks up or separates the pellets from the sheet one by one. If the apparatus is built in a die or pellet bonding apparatus, each of the pellets thus separated is bonded to the specific land of a leadframe If the apparatus is not built in a die or pellet bonding apparatus, the pellets thus separated are placed in respective pockets of a tray.
Next, a most popular pellet picking method will be explained with reference to FIGS. 1 and 2 and FIGS. 3A to 3D. In this method, the pellet on the adhesive sheet is pushed up by the needles by way of the sheet, thereby detaching the pellet from the sheet. Then, the pellet thus detached is picked up by the suction collet.
FIGS. 1 and 2 schematically show a prior-art pellet picking apparatus 100, which performs the most popular pellet picking method described above. In FIGS. 1 and 2, an adhesive sheet 101 on which a semiconductor pellet 103 is adhered is also shown. Needless to say, a lot of pellets 103 are adhered onto the sheet 101. However, only one of the pellets 103 is shown for the sake of simplification and facilitation in understanding.
The basic configuration of the prior-art pellet picking apparatus of this type has been already known and therefore, its detailed explanation is omitted here. An example of the basic configuration of the apparatus is disclosed in the Japanese Non-Examined patent publication No. 4-12549 published in 1992. This apparatus is comprised of a wafer table on which an adhesive sheet with a semiconductor wafer (which have been separated into pellets) is placed, an X-Y table for moving horizontally the table along the X and Y directions, a pushing-up needle located under the table and movable vertically by an actuator, a vacuum-suction collet located over the table and movable horizontally and vertically by an actuator, an optically recognition means (e.g., a video camera) for recognizing the position and shape of the pellets on the adhesive sheet, and a controller for controlling the operation of the wafer table, the X-Y table, the needle, the collet, and the optically recognition means. The sheet with the wafer is stretched or spread on a circular support ring. The sheet is placed on the wafer table along with this ring.
The pellets located on the sheet in the form of wafer are placed on the wafer table in such a way that the needle can touch the pellets by way of the opening of the wafer table. Then, the pellets are successively pushed up by the needle while they are held by the collet, thereby detaching successively the pellets from the sheet. Subsequently, the pellets thus detached are successively transported to their specific positions with the collet.
As shown in FIGS. 1 and 2, the prior-art pellet picking apparatus 100 comprises a table 105, one needle 102a and four needles 102b which are arranged to be movable upward and downward in the table 105, and a cylindrical suction collet 104 provided over the table 105.
The table 105 has a hole 106a for accommodating vertically the needle 102a and four holes 106b for individually accommodating vertically the needles 102b. The hole 106a is located at the center of the upper surface 105a of the table 105. The four holes 106b are regularly arranged in the vicinity of the central hole 105a so as to surround the same. The needles 102a and 102b are fixed to a driving mechanism (not shown) for making their vertical motion (which is along the Z-axis).
The collet 104 is fixed to a driving mechanism (not shown) for making its vertical motion (which is along the Z-axis) and its horizontal motion (which is along the X-Y plane). The collet 104 has a suction or absorption end 104a. The collet 104 is located over the table 105 in such a way that the suction or absorption end 104a is opposed to the upper surface 105a of the table 105. The collet 104 is connected to a vacuum generator (not shown), thereby enabling the collet 104 to generate a suction or absorption force at the end 104a . 
The prior-art pellet picking apparatus 100 operates in the following way.
First, as shown in FIG. 2, the adhesive sheet 101, on which the pellet 103 has been adhered, is moved horizontally (i.e., along the X-Y plane) while the collet 104 is kept apart from the pellet 103, thereby making positional alignment of the pellet 103 with respect to the table 105. The sheet 101 has been stretched or spread on a circular support ring and is placed on the X-Y stage (not shown) Thus, the sheet 101 is optionally moved along the X-Y plane by driving the X-Y stage for the positional alignment.
If the pellet picking apparatus 100 is applied to the die-or pellet bonding apparatus, the positional alignment of the sheet 101 is realized at high accuracy by visual recognition with a camera or cameras provided over the pellet 103. This is to place the pellet 103 onto the land of a leadframe at high accuracy in the pellet bonding apparatus.
Next, as shown in FIG. 3A, the collet 104 is lowered until the suction end 104a is contacted with the surface of the pellet 103, in which the air in the collet 104 is being exhausted by the vacuum generator to generate a suction force at the end 104a. Thus, the pellet 103 is held by the collet 104 due to the suction force. The state at this stage is shown in FIG. 3A.
Subsequently, as shown in FIG. 3B, the four peripheral needles 102a are moved upward simultaneously to push the peripheral area of the pellet 103 up. At this time, the collet 104 is moved upward to be synchronous with the motion of the pellet 103 without separating the pellet 103. Thus, the adhered area of the periphery of the pellet 103 begins to be detached from the adhesive sheet 101, because the sheet 101 is fixed.
Following this, as shown in FIG. 3C, the peripheral needles 102b are lowered into the holes 106b while the central needle 1012a is elevated. Thus, almost all the adhered area of the pellet 103 is detached from the sheet 101 except for the adhered area near the contact point of the needle 102a. 
Finally, as shown in FIG. 3D, the collet 104 is elevated further along with the sucked pellet 103. Thus, the pellet 103 is entirely separated from the sheet 101. The pellet 103 thus separated and held by the collet 104 is moved to a specific position and then, placed on the position by releasing the suction force of the collet 104.
In the above explanation, the pellet 103 is pushed up by the needles 102a and 102b. However, the pellet 103 may be pushed up by only the center needle 102a or by only the peripheral needles 102b. 
The above-described method of pushing the pellet 103 up with the needles 102a and/or 102b is referred as the xe2x80x9cneedle pushing methodxe2x80x9d in the following explanation.
The above-described needle pushing method has the following problems.
The first problem is that the pellet tends to be damaged. Specifically, since the load is concentrated to the point of the needle, the pellet tends to have flaws or damages such as cracks and chipping. Sometimes, the pellet is cracked or broken. In particular, recently, the thickness of the pellet has been decreasing rapidly. The pellet may be as thin as approximately 0.05 mm. If the pellet is made of a compound semiconductor such as gallium arsenide (GaAs) and indium phosphide (InP) which are more fragile than silicon (Si), the pellet tends to be cracked or broken more easily.
To prevent the flaws or damages of the pellet, the following measures have been developed and taken.
(a) To select an appropriate shape of the point of the needle so as to accord with the size and thickness of the pellet.
(b) To adjust the moving speed and stroke of the needle (e.g., to lower the speed and decrease the stroke).
However, it is difficult for these measures (a) and (b) to solve the above-identified problem that the pellet tends to be damaged.
Additionally, the measure (a) has a disadvantage that the design and formation themselves of the needle having an optimized shape is very difficult and that the needle needs to be replaced with another responsive to the change of the pellet.
Similarly, the measure (b) also has a disadvantage that an optimized condition or adjustment of the needle is very difficult to be realized responsive to the change of the pellet. If the moving speed of the needle is lowered, the damage of the pellet can be avoided. In this case, however, the lowered speed will degrade the productivity.
As explained above, as long as the conventional needle pushing method is used, a lot of replaceable parts are necessary and/or the control of the vertical motion of the needle is complicated in order to avoid the damages of the pellet. This not only raises the fabrication cost of the pellet picking apparatus but also degrades the productivity. Moreover, it is basically difficult to picking the pellet up without any damages.
On the other hand, an improved adhesive sheet, which is termed the xe2x80x9cUV adhesive sheetxe2x80x9d, has ever been developed and used practically. The UV adhesive sheet is a sheet that the adhesive agent coated on the sheet is cured due to irradiation of ultraviolet (UV) light, resulting in the loss of its adhesion.
If the UV adhesive sheet is used for this purpose, UV light is irradiated to the lower face of the sheet prior to detachment of the pellet from the sheet. Since the sheet is translucent for UV light, the irradiated UV light penetrates the sheet, curing the adhesive agent on the upper face of the sheet. Thus, the adhesive agent loses its adhesion and as a result, the pellet is easily separated from the sheet.
However, even if the UV adhesive sheet is used in the conventional needle pushing method, the concentrated load or force applicable to the pellet by the needle is not lowered. Thus, the damage of the pellet cannot be prevented with the use of the UV adhesive sheet.
The second problem of the conventional needle pushing method is that there is a possibility that the pellet fails to be separated from the adhesive sheet, in other words, picking failure occurs. This problem is caused by the following reason.
When the pellet on the adhesive sheet is separated from the sheet by the pushing-up motion of the central needle, the adhering force of the sheet is left at the central area of the pellet with which the sheet is contacted by the point of the needle. Thus, the pellet is unable to be separated from the sheet by the suction force of the collet.
The second problem may occur even if the UV adhesive sheet is used.
Furthermore, there is a method of picking the pellet from the adhesive sheet without using the needle. For example, the adhesive sheet is placed on the stage and sucked downward by way of a hole or holes of the stage, thereby separating the sheet from the pellet. In this case, however, the adhered area of the sheet which is not sucked is unable to be separated from the pellet by the sucking force. As a result, the pellet picking is not ensured.
There is another method of picking the pellet from the adhesive sheet using the needle and the sucking force. For example, the adhesive sheet is placed on the stage and the area corresponding to the periphery of the pellet is sucked downward by way of a hole or holes of the stage and at the same time, the center of the pellet is pushed up by the point of the needle. Thus, the sheet is separated from the pellet. In this case, however, the load by the needle is concentrated to the center of the pellet. Thus, the pellet is not prevented from being damaged.
Accordingly, an object of the present invention is to provide a pellet picking method and a pellet picking apparatus that prevent the pellet from being damaged during the pellet picking process.
Another object of the present invention is to provide a pellet picking method and a pellet picking apparatus that ensure the picking operation of the pellet without damages.
Still another object of the present invention is to provide a pellet picking method and a pellet picking apparatus that decrease the fabrication cost of the apparatus without lowering the productivity.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a pellet picking apparatus is provided, which is an apparatus for separating a pellet adhered on an adhesive sheet from the sheet and picking the pellet thus separated.
This apparatus is comprised of:
(a) a stage having a surface with which a lower face of the adhesive sheet is contacted;
the pellet being adhered on an upper face of the sheet;
the stage having a suction hole that pulls down the sheet;
the suction hole having a suction end to be contacted with the sheet on the surface of the stage;
the stage being movable (capable of translation and rotation) in a horizontal plane with respect to a specific reference position; and
(b) a collet for holding the pellet by a suction force;
the collet having a suction end to be contacted with the pellet;
the collet being capable of holding the pellet by the suction force at the suction end;
wherein when the sheet is placed on the surface of the stage in such a way that the pellet on the sheet is located at the reference position, the collet is controlled to hold the pellet by the suction force at the suction end and at the same time, the stage is controlled to hold the sheet at the suction end of the suction hole and to be moved in the horizontal plane with respect to the reference position to thereby detach the sheet from the pellet;
and wherein the collet is controlled to pick the pellet thus detached from the sheet.
With the pellet picking apparatus according to the first aspect of the present invention, the stage has a surface with which a lower face of the adhesive sheet is contacted and a suction hole that pulls down the sheet. The suction hole has a suction end to be contacted with the sheet on the surface of the stage. The stage is movable (capable of translation and rotation) in a horizontal plane with respect to a specific reference position.
Moreover, when the adhesive sheet is placed on the surface of the stage in such a way that the pellet on the sheet is located at the reference position, the collet is controlled to hold the pellet by the suction force at the suction end and at the same time, the stage is controlled to hold the sheet at the suction end of the suction hole and to be moved in the horizontal plane with respect to the reference position to thereby detach the sheet from the pellet. The collet is controlled to pick the pellet thus detached from the sheet. Thus, the pellet is separated from the adhesive sheet without using the pushing-up operation of the needles.
As a result, the pellet can be prevented from being damaged during the pellet picking process. This means that the picking operation of the pellet without damages is ensured and that the separation failure of the pellet from the sheet is avoided.
Also, the collet can be formed by a popular, conventional structure and the suction and moving mechanism of the stage can be simplified. Thus, the complicated operation of the needles used in the conventional needle push-up method is unnecessary and at the same time, a lot of needles having various shapes and various sizes need not prepared nor replaced according to the type of the pellet to be picked. Accordingly, the fabrication cost of the apparatus can be lowered drastically, which is enabled without lowering the productivity.
According to a second aspect of the present invention, a pellet picking method is provided, which is a method for separating a pellet adhered on an adhesive sheet from the sheet and picking the pellet thus separated.
This method is comprised of the steps of:
(a) providing a stage having a surface with which a lower face of the adhesive sheet is contacted;
the pellet being adhered on an upper face of the sheet;
the stage having a suction hole that pulls down the sheet;
the suction hole having a suction end to be contacted with the sheet on the surface of the stage;
the stage being movable (capable of translation and rotation) in a horizontal plane with respect to a specific reference position;
(b) providing a collet for holding the pellet by a suction force;
the collet having a suction end to be contacted with the pellet;
the collet being capable of holding the pellet by the suction force at the suction end;
(c) placing the sheet on the surface of the stage in such a way that the pellet on the sheet is located at the reference position;
(d) holding the pellet by the suction force at the suction end of the collet;
(e) holding the sheet at the suction end of the suction hole of the stage;
(f) moving the stage in the horizontal plane with respect to the reference position to thereby detach the sheet from the pellet; and
(g) picking the pellet thus detached from the sheet by the collet.
With the pellet picking method according to the second aspect of the present invention, because of substantially the same reason as the pellet picking apparatus according to the first aspect, the pellet can be prevented from being damaged during the pellet picking process, which means that the picking operation of the pellet without damages is ensured. Also, the fabrication cost of the apparatus can be lowered drastically, which is enabled without lowering the productivity.