Apparatus and method for alignment of a bonding tool

The invention provides an apparatus and method for aligning a bonding tool. A force sensor having a plurality of force sensing sections is configured to measure a force generated by the bonding tool on the force sensor. Each sensing section is adapted to individually detect an amount of force from a part of the bonding tool acting on that sensing section, so that an alignment of the bonding tool may be determined.

FIELD OF THE INVENTION

The invention relates to an apparatus and method for determining an alignment of a bonding tool, such as a die pick and place tool found on a semiconductor die bonding machine, in particular by utilizing a force sensor.

BACKGROUND AND PRIOR ART

In a die bonding operation in a semiconductor assembly and manufacturing process, a bonding tool may pick up a die (which may be an electronic device such as a semiconductor integrated circuit device or semiconductor chip) at a first location and move the die to a second location at which the die is to be bonded. Commonly, the die is bonded to a substrate, such as a circuit board, or another die. At the bonding location, the bonding tool will move downwardly (this is known as the z-direction) until the die touches the substrate or another die. In order to apply a required bond force during the bond operation, the bonding tool may be driven downwardly further to exert the required force on the die.

Apparatus and methods for controlling this bond force need to meet a number of requirements. For example, they should preferably be able to apply a bond force over a wide range, should be susceptible to feedback and control, and should be able to respond rapidly to required changes in the bond force.

There are demanding requirements placed on die tilt and bond line thickness specifications. The bonding tool usually has a collet that is used to contact a die to be bonded. The collet is often made of metals like stainless steel or tungsten carbide, with a compliant material (such as rubber material) added to the tip of the collet to form a contact surface to hold the die. The collet includes a suction aperture through which a holding force can be provided by air suction. The contacting surface of the collet of the bonding tool should be parallel to a pick-up surface and a placement surface so that a bonding force is evenly distributed onto a die to be picked and placed, and the die can be placed correctly onto a bonding position.

If a bonding tool is not aligned properly, so that the contacting surface is not substantially parallel to the placement surface, die crack or an unacceptable placement error may occur. In order to produce good results, the bonding tool of a modern die bonder needs to be carefully set up to achieve leveling that is better than 16 microns. In other words, a vertical distance between a lowest point on a die and a highest point on the die when carried by the bonding tool must be smaller than 16 microns.

One method of aligning the bonding tool is to mount a circular stamp on the bonding tool and then to land the stamp on a perfectly flat anvil block. A carbon paper is placed on the anvil block to obtain an imprint of the circular stamp. By checking the roundness or completeness of the imprint, an operator is able to visually determine whether the bonding tool has been set up correctly to achieve an acceptable leveling. If the imprint shows an incomplete circle, the operator may correct the alignment of the bonding tool according to the tilting direction of the bonding tool as interpreted from the imprint. This method is manual and not very accurate, since it relies on subjective visual determination by the operator.

Another method of aligning a bonding tool is disclosed in U.S. Pat. No. 6,179,938 for “Method and Apparatus for Aligning the Bonding Head of a Bonder, In Particular a Die Bonder”. In this patent, an alignment plate provided with two plane parallel surfaces is placed on a supporting surface which is set plane parallel to the bonding surface upon which the semiconductor chip will be bonded to a carrier material. A measuring device is then calibrated, after which the alignment plate is held at a slight distance above the measuring device. The alignment of the bonding tool is adjusted until the signal from the measuring device is equal to the signal obtained during calibration.

A disadvantage of this method is that there has to be prior calibration each time the bonding tool is to be aligned. The inductive range-finding method used by the alignment apparatus requires prior calibration each time alignment is to be measured, since it uses separate coils that are susceptible in different ways to external influences such as temperature, humidity and so forth. Each coil also forms a magnetic field that may influence the other coils, thus affecting accuracy. Furthermore, the bonding tool has to pick up a specially-prepared alignment plate and move it over the measuring device each time alignment is to be determined. Evidently, closed-loop control to maintain alignment of the bonding tool during bonding will not be possible.

SUMMARY OF THE INVENTION

It is thus an object of the invention to seek to provide a sensor for measuring an alignment of a bonding tool quickly and accurately while avoiding some of the drawbacks of the abovementioned prior art.

According to a first aspect of the invention, there is provided an apparatus for aligning a bonding tool, comprising a force sensor configured to measure a force generated by the bonding tool on the force sensor, wherein the force sensor comprises a plurality of force sensing sections, each sensing section being adapted to individually detect an amount of force from a part of the bonding tool acting on that sensing section.

According to a second aspect of the invention, there is provided a method for aligning a bonding tool, comprising the steps of: providing a force sensor comprising a plurality of force sensing sections, each sensing section being adapted to individually detect an amount of force from a part of the bonding tool acting on that sensing section; causing the bonding tool to generate a force onto the force sensor; measuring the force generated by the bonding tool onto the force sensor; and adjusting an alignment of the bonding tool based upon the amount of force measured by each sensing section.

It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1is an isometric view of a piezoelectric force sensor10that is usable to detect an alignment of a bonding tool according to the preferred embodiment of the invention. The force sensor10comprises a plurality of ceramic fibers12with piezoelectric properties that are embedded in hardened epoxy material16and extend through to opposite flat surfaces of the force sensor10. In this embodiment, the force sensor10is fabricated in a ring form with a hollow center15. This ring form is suitable for the specific mounting methods for the apparatus mentioned herein, but it should be appreciated that the force sensor10may also take other forms.

InFIG. 1, the ring is nominally divisible into four equal sections or quadrants with a collection of ceramic fibers12in each section of the ring. A bundle of four ceramic fibers12is grouped into each section. Therefore, it is possible to determine a force exerted on each sensing section of the force sensor10, relative to the other three sections.

FIGS. 2aand2billustrate schematically one method by which the force sensor10ofFIG. 1may be fabricated. Referring first toFIG. 2a, a mold18is prepared, which has a central shaft14which is used to fashion the hollow center15of the force sensor10. Strips of ceramic fibers12are then inserted into the mold18. As shown inFIG. 2b, epoxy16is then added into the mold18and allowed to harden. The cylindrically-shaped hardened epoxy16with embedded ceramic fibers12is removed from the mold18and central shaft14, then diced and polished to form a plurality of the ceramic ring illustrated inFIG. 1. Chromium-gold electrodes are then applied to cover the four sensing sections containing the fibers12. Thereafter, an electric field is applied to the ceramic fibers12at an elevated temperature to elicit the piezoelectric properties in the fibers12.

FIGS. 3aand3billustrate cross-sectional and side elevation views respectively of a molding apparatus20that can be implemented to form the piezoelectric force sensor10ofFIG. 1. A base22of the molding apparatus20is made of stainless steel. Two pieces of Teflon24are assembled to form a cube with a shape of a hollow mold18at its center. After assembly, the two pieces of Teflon24are secured together with screws26.

A round disc28with an outer diameter that is the same as the inner diameter of the mold18is inserted into the bottom of the mold18. A central shaft14made of Teflon is inserted into the middle of the mold18. The central shaft14has the same diameter as the hollow center15of the force sensor10to be formed, and is preferably embedded with steel. The round disc28has an inner insert that has a diameter that is the same as the diameter of the central shaft14. It also contains four groups of smaller holes that have diameters that are the same as the diameter of each of the ceramic fibers12. Thus, the round disc28has inserts that are suitably sized for inserting and positioning both the central shaft14and groups of ceramic fibers12.

After insertion of the central shaft14and ceramic fibers12into the round disc28, a top plate32with holes corresponding to the positions of the central shaft14and ceramic fibers12is used to cover the mold18and align the ceramic fibers12. The mold18is then filled with epoxy material16to form a composite rod. After the epoxy material16has been totally cured, the top plate32is removed. The screws26are unscrewed and the two pieces of Teflon24are separated. The cylindrically-shaped composite rod can then be removed from the molding apparatus20and the central shaft24can be pulled out from its center. Thereafter, the composite rod can be diced to form force sensors10in the form of rings as shown inFIG. 1.

In this embodiment, the force sensor10comprises a ring with a hollow center and each sensing section is of substantially equal size. Other embodiments of composite wafers by which a suitable sensor comprising separate sections can be fabricated are disclosed in U.S. Pat. No. 6,190,497 entitled “Ultrasonic Transducer”, in particularFIG. 4andFIG. 5therein. However, the embodiments described herein are not meant to be exhaustive, and other embodiments of force sensors that are configured to sense force distribution exerted on different sections of the sensors are possible.

FIG. 4is a plan view of a transmitting material comprising a plurality of individual electrical conductors, such as a polyimide film40. The polyimide film40is laid out on an electronic circuit, in the form of a printed circuit board42to be coupled to the force sensor10for detecting outputs from the force sensor10. The polyimide film40includes four sensing zones44,45,46,47made of electrically conductive material, whereat each group of ceramic fibers12corresponding to each sensing section of the force sensor10are positioned. When a force acts on a surface of the force sensor10, electrical currents are generated in the respective sensing zones44,45,46,47which are channeled to a signal processor on the electronic circuit or printed circuit board42connected to signal output terminals48formed on the polyimide film40. Based upon the relative strengths of the electrical currents fed through each of the four output channels of the signal output terminals48, the relative forces acting on each section of the force sensor10are determinable.

FIG. 5is a cross-sectional side view of an apparatus for aligning a bonding tool in the form of an alignment station50incorporating the force sensor10according to the preferred embodiment of the invention. The alignment station50is spaced from the bonding tool and may be a stand-alone device. The force sensor10has a plurality of force sensing sections, each sensing section being individually adapted to detect an amount of force from a part of the bonding tool acting on that sensing section.

The alignment station50comprises a base plate52, on which is mounted a printed circuit board42. A layer of polyimide film40is laid on top of the printed circuit board42. A force sensor10is positioned on top of the layer of polyimide film40such that each group of its ceramic fibers comprised in each sensing section is aligned with the respective electrodes in the sensing zones44,45,46,47of the polyimide film40. Similarly, another layer of polyimide film40is positioned on top of the force sensor10that has one common electrode for the electrical ground of all its sensor zones. A biasing member or sensing top plate54is placed on top of the top layer of polyimide film40, thereby sandwiching the force sensor10, polyimide films40and printed circuit board42between the top plate54and base plate52. Bolts56are used to secure the biasing member or top plate54to the base plate52and to provide a preload force to the top sensing surface of the force sensor10. In a conventional piezoelectric sensor, the preload force is normally necessary to obtain a substantially linear relationship between force exerted and electrical current produced.

In order to measure an alignment of a bonding tool, the alignment station50is secured onto a surface that is plane parallel to a placement surface for semiconductor dice. A collet of the bonding tool is positionable onto the alignment station50by lowering it onto the top plate54and a predetermined force is exerted onto the top plate54. Preferably, the top plate54has a contact surface area that is larger than but as close as possible to the contact surface of the collet. The force sensor10will detect the forces transmitted through the top plate54onto each sensing section of the force sensor10. If there is an equal distribution of forces through all the sensing sections, the bonding tool is properly aligned. If one or more sensing sections detect a greater force than the other sensing sections, the bonding tool is not properly aligned, and the collet needs to be adjusted by moving it towards the direction of the sensing section(s) that detect the greater force. Adjustment is made until a substantially equal distribution of forces is detected.

FIG. 6is a cross-sectional view of a bonding tool in the form of a die pick and place tool60incorporating the force sensor10according to the preferred embodiment of the invention. The force sensor10is coupled to the bonding tool. In the arrangement ofFIG. 6, a separate alignment station50is not required, and closed-loop feedback of the alignment of the pick and place tool60can be obtained.

The pick and place tool60includes a collet assembly62. The force sensor10is coupled to the collet assembly62whereby each sensing section of the force sensor10is adapted to detect a reaction force acting on a part of the collet assembly that is generated upon application of a bonding force onto a surface. The force sensor10is preferably coupled to the collet assembly62axially opposite a point of contact between the collet assembly62and the bonding surface. Further, the collet assembly62preferably exerts a preload force onto the force sensor10, the need for which was explained above in relation toFIG. 5.

The collet assembly62and force sensor10are supported on a slider mount66that is slidable on a slider or ball brushing68. The slider mount66allows the collet assembly62to slide relative to a bond force actuation unit bracket64in order to modulate the bonding force so as to obtain greater control of the bonding force and to avoid damaging dice that are picked and placed by the pick and place tool60.

A bond force motor coil70is mounted onto the bond force actuation unit bracket64by a bond force motor coil mount72. Located adjacent the bond force motor coil70are a bond force motor ferromagnetic plate74, bond force motor magnet76and bond force motor ferromagnetic core78. The various components of the bond force motor70,72,74,76,78essentially form a linear motor that imparts a controllable bonding force onto the collet assembly62. A compression spring80serves to provide a preload force to the collet assembly62as against a support datum in the form of a bonder shaft82. The pick and place tool60is connected to a bond head of a bonding machine through the bonder shaft82.

Using this arrangement, the force sensor10can continuously monitor a force exerted on it by the collet assembly62. When the collet assembly62is not pushing against a surface, the force sensor10experiences a preload force generated from the collet assembly62acting on it. As the collet assembly62pushes against a flat horizontal surface, such as a die on a pick-up site or a bonding site, a distribution of forces on each sensing section of the force sensor10can be detected. If the pick and place tool60is not properly aligned because of an unequal distribution of forces beyond a certain tolerance, an alarm can be generated immediately and the pick and place tool60can be realigned either manually, or automatically with suitable additional mechanisms that are configured to realign the pick and place tool60.

It should be appreciated that this embodiment has an advantage over the separate alignment station50ofFIG. 5and the devices of the prior art since real-time closed-loop feedback of the alignment of the pick and place tool can be obtained and any misalignment can be remedied immediately. Time required for alignment of the pick and place tool is also reduced by not having to move the pick and place tool to a separate station each time alignment is to be checked.