# Patent

## Title
Systems and methods for forming apertures in microfeature workpieces

## Abstract

Systems and methods for forming apertures in microfeature workpieces are disclosed herein. In one embodiment, a method includes directing a laser beam toward a microfeature workpiece to form an aperture and sensing the laser beam pass through the microfeature workpiece in real time. The method can further include determining a number of pulses of the laser beam and/or an elapsed time to form the aperture and controlling the laser beam based on the determined number of pulses and/or the determined elapsed time to form a second aperture in the microfeature workpiece.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of U.S. patent application Ser. No. 10/839,457, filed May 5, 2004, which is incorporated herein by reference in its entirety.


TECHNICAL FIELD
The present invention is related to systems and methods for forming apertures in microfeature workpieces. More particularly, the invention is directed to systems and methods for forming apertures with laser beams.
BACKGROUND
Microelectronic devices are used in cell phones, pagers, personal digital assistants, computers, and many other products. A die-level packaged microelectronic device can include a microelectronic die, an interposer substrate or lead frame attached to the die, and a molded casing around the die. The microelectronic die generally has an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The bond-pads are coupled to terminals on the interposer substrate or lead frame. The interposer substrate can also include ball-pads coupled to the terminals by conductive traces in a dielectric material. An array of solder balls is configured so that each solder ball contacts a corresponding ball-pad to define a “ball-grid” array. Packaged microelectronic devices with ball-grid arrays are generally higher grade packages that have lower profiles and higher pin counts than conventional chip packages that use a lead frame.
Die-level packaged microelectronic devices are typically made by (a) forming a plurality of dies on a semiconductor wafer, (b) cutting the wafer to singulate the dies, (c) attaching individual dies to an individual interposer substrate, (d) wire-bonding the bond-pads to the terminals of the interposer substrate, and (e) encapsulating the dies with a molding compound. Mounting individual dies to individual interposer substrates is time consuming and expensive. Also, as the demand for higher pin counts and smaller packages increases, it becomes more difficult to (a) form robust wire-bonds that can withstand the forces involved in molding processes and (b) accurately form other components of die-level packaged devices. Therefore, packaging processes have become a significant factor in producing semiconductor and other microelectronic devices.
Another process for packaging microelectronic devices is wafer-level packaging. In wafer-level packaging, a plurality of microelectronic dies are formed on a wafer and a redistribution layer is formed over the dies. The redistribution layer includes a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and a plurality of traces coupled to individual ball-pads of the ball-pad arrays. Each ball-pad array is arranged over a corresponding microelectronic die, and the traces couple the ball-pads in each array to corresponding bond-pads on the die. After forming the redistribution layer on the wafer, a stenciling machine deposits discrete blocks of solder paste onto the ball-pads of the redistribution layer. The solder paste is then reflowed to form solder balls or solder bumps on the ball-pads. After forming the solder balls on the ball-pads, the wafer is cut to singulate the dies. Microelectronic devices packaged at the wafer level can have high pin counts in a small area, but they are not as robust as devices packaged at the die level.
In the process of forming and packaging microelectronic devices, numerous holes are formed in the wafer and subsequently filled with material to form conductive lines, bond-pads, interconnects, and other features. One existing method for forming holes in wafers is reactive ion etching (RIE). In RIE, many holes on the wafer can be formed simultaneously. RIE, however, has several drawbacks. For example, RIE may attack features in the wafer that should not be etched, and the RIE process is slow. Typically, RIE processes have removal rates of from approximately 5μ/min to approximately 50μ/min. Moreover, RIE requires several additional process steps, such as masking and cleaning.
Another existing method for forming holes in wafers is laser ablation. A conventional laser ablation process includes forming a series of test holes in a test wafer to determine the time required to form various through holes in the test wafer. The test holes are formed by directing the laser beam to selected points on the wafer for different periods of time. The test wafer is subsequently inspected manually to determine the time required to form a through hole in the wafer. The actual time for use in a run of identical wafers is then calculated by adding an overdrill factor to the time required to drill the test holes to ensure that the holes extend through the wafer. A run of identical wafers is then processed based on the data from the test wafer. A typical laser can form more than 10,600 holes through a 750 Å wafer in less than two minutes.
Laser ablation, however, has several drawbacks. For example, the heat from the laser beam creates a heat-affected zone in the wafer in which doped elements can migrate. Moreover, because the wafer thickness is generally non-uniform, the laser may not form a through hole in thick regions of the wafer or the wafer may be overexposed to the laser beam and consequently have a large heat-affected zone in thin regions of the wafer. Accordingly, there exists a need to improve the process of forming through holes or deep blind holes in microfeature workpieces.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a system for forming an aperture in a microfeature workpiece in accordance with one embodiment of the invention.
FIG. 2 is a schematic side cross-sectional view of the system of FIG. 1 with the laser directing a laser beam toward the microfeature workpiece.
FIG. 3 is a top plan view of the microfeature workpiece without a redistribution layer.
FIG. 4 is a schematic side cross-sectional view of the system of FIG. 1 with the laser forming a production aperture in the microfeature workpiece,



DETAILED DESCRIPTION
A. Overview
The present invention is directed toward systems and methods for forming apertures in microfeature workpieces. The term “microfeature workpiece” is used throughout to include substrates in or on which microelectronic devices, micromechanical devices, data storage elements, and other features are fabricated. For example, microfeature workpieces can be semiconductor wafers, glass substrates, insulated substrates, or many other types of substrates. Several specific details of the invention are set forth in the following description and in FIGS. 1-4 to provide a thorough understanding of certain embodiments of the invention. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that other embodiments of the invention may be practiced without several of the specific features explained in the following description.
Several aspects of the invention are directed to methods for forming apertures in microfeature workpieces. In one embodiment, a method includes directing a laser beam toward a microfeature workpiece to form an aperture and sensing the laser beam pass through the microfeature workpiece in real time. In one aspect of this embodiment, the method further includes determining a number of pulses of the laser beam and/or an elapsed time to form the aperture and controlling the laser beam based on the determined number of pulses and/or the determined elapsed time to form a second aperture in the microfeature workpiece. In another aspect of this embodiment, an electromagnetic radiation sensor senses the laser beam. The method can further include positioning the microfeature workpiece between a laser and an electromagnetic radiation sensor before directing the laser beam.
In another embodiment, a method includes ablating a microfeature workpiece by directing pulses of a laser beam to form a test aperture in the microfeature workpiece and automatically determining a number of pulses of the laser beam and/or an elapsed time to form the test aperture. The method further includes automatically controlling the laser beam based on the determined number of pulses and/or the determined elapsed time to form a plurality of production apertures in the microfeature workpiece. In one aspect of this embodiment, automatically controlling the laser beam includes directing the laser beam toward the microfeature workpiece for an adjusted number of pulses and/or an adjusted time to form at least one of the production apertures. The adjusted number of pulses can be different from the determined number of pulses, and the adjusted time can be different from the determined elapsed time. For example, if the production aperture is a blind hole, the adjusted number of pulses can be less than the determined number of pulses and/or the adjusted time can be less than the determined elapsed time by an underdrill factor. Alternatively, if the production aperture is a through hole, the adjusted number of pulses can be greater than the determined number of pulses and/or the adjusted time can be greater than the determined elapsed time by an overdrill factor.
Another aspect of the invention is directed to systems for forming apertures in microfeature workpieces. In one embodiment, a system includes a laser configured to produce a laser beam along a beam path, an electromagnetic radiation sensor positioned along the beam path to sense the laser beam, and a workpiece carrier configured to selectively position a microfeature workpiece in the beam path before the electromagnetic radiation sensor to form an aperture in the microfeature workpiece. The system can further include a controller operably coupled to the laser, the electromagnetic radiation sensor, and the workpiece carrier. The controller can have a computer-readable medium containing instructions to perform any one of the above-described methods.
B. Embodiments of Systems for Forming Apertures in Microfeature Workpieces
FIG. 1 is a schematic view of a system 100 for forming an aperture in a microfeature workpiece 160 in accordance with one embodiment of the invention. In the illustrated embodiment, the system 100 includes a laser 110, a workpiece carrier 130, a sensor 140, and a controller 150. The laser 110, the workpiece carrier 130, and the sensor 140 are operatively coupled to the controller 150. The laser 110 selectively generates a laser beam 120 to form apertures in the microfeature workpiece 160 by ablating the workpiece material. The system 100 can also include a metrology tool 102 (shown schematically in broken lines) to determine the thickness of portions of the microfeature workpiece 160.
The laser 110 can include an illumination source 112, a galvo mirror 114, and a telecentric lens 116. In one embodiment, the laser 110 can be a solid-state laser that produces a laser beam with a wavelength of approximately 355 nm and a pulse frequency of approximately 10 kHz to approximately 75 kHz. In one aspect of this embodiment, the power generated by the laser 110 can be approximately 7 watts, and the laser beam can have a pulse frequency of approximately 20 kHz to approximately 30 kHz. In additional embodiments, other lasers may be used with different configurations.
The workpiece carrier 130 is configured to hold and properly position the microfeature workpiece 160. More specifically, the workpiece carrier 130 positions the microfeature workpiece 160 relative to the laser 110 so that the laser beam 120 forms an aperture at a desired location on the workpiece 160. The workpiece carrier 130 can be moveable along three orthogonal axes, such as a first lateral axis (X direction), a second lateral axis (Y direction), and/or an elevation axis (Z direction). In other embodiments, the workpiece carrier 130 may not be movable along all three orthogonal axes, and/or the laser 110 may be movable.
In the illustrated embodiment, the workpiece carrier 130 engages and supports the perimeter of the microfeature workpiece 160. More specifically, the microfeature workpiece 160 has a first surface 166, a second surface 168 opposite the first surface 166, and a perimeter edge 169. The workpiece carrier 130 can have an edge-grip end effector configured to engage the perimeter edge 169 of the microfeature workpiece 160 without contacting the first and second surfaces 166 and 168. In other embodiments, the workpiece carrier 130 may contact a portion of the first and/or second surfaces 166 and/or 168 of the microfeature workpiece 160. For example, the workpiece carrier 130 may engage the perimeter edge 169 and a perimeter region of the second surface 168 to carry the microfeature workpiece 160 without obscuring the laser beam 120 from passing through the desired points on the workpiece 160.
The sensor 140 senses electromagnetic radiation to determine when the aperture has been formed in the microfeature workpiece 160. More specifically, the sensor 140 detects when the laser beam 120 passes through the microfeature workpiece 160 and sends a signal to the controller 150 indicating that an aperture has been formed. The sensor 140 can be an electromagnetic radiation sensor, such as a photodiode, selected to respond to the wavelength of the laser beam 120. The laser 110 and the sensor 140 can be arranged so that the workpiece carrier 130 can position the microfeature workpiece 160 between the laser 110 and the sensor 140. The sensor 140 can be movable relative to the microfeature workpiece 160 to be aligned with the laser beam 120. For example, the sensor 140 can be moveable along the three orthogonal axes X, Y and Z. In other embodiments, the sensor 140 can be fixed relative to the laser 110 such that they can move together.
FIG. 2 is a schematic side cross-sectional view of the system 100 with the laser beam 120 directed toward the microfeature workpiece 160 (shown enlarged for illustrative purposes). In the illustrated embodiment, the microfeature workpiece 160 includes a substrate 170 having a plurality of microelectronic dies 180 and a redistribution layer 190 formed on the substrate 170. Each microelectronic die 180 can have an integrated circuit 184 (shown schematically) and a plurality of bond-pads 182 coupled to the integrated circuit 184. The redistribution layer 190 includes a dielectric layer 192 and a plurality of ball-pads 196 in the dielectric layer 192. The ball-pads 196 are arranged in ball-pad arrays relative to the microelectronic dies 180 such that each die 180 has a corresponding array of ball-pads 196. The redistribution layer 190 also includes a plurality of conductive lines 194 in or on the dielectric layer 192 to couple the bond-pads 182 of the microelectronic dies 180 to corresponding ball-pads 196 in the ball-pad arrays. In other embodiments, the microfeature workpiece 160 may not include microelectronic dies 180 and/or the redistribution layer 190. For example, the microfeature workpiece 160 can be a circuit board or other substrate.
C. Embodiments of Methods for Forming Apertures in Microfeature Workpieces
FIG. 2 also illustrates an embodiment of a method for forming apertures in microfeature workpieces. The controller 150 generally contains computer operable instructions that generate signals for controlling the laser 110, the workpiece carrier 130, and the sensor 140 to form a single aperture or a plurality of apertures in the microfeature workpiece 160. In one embodiment, the controller 150 controls the laser 110 to form a test aperture 162 in the microfeature workpiece 160 and determines the number of pulses of the laser beam 120 and/or the time required to form the test aperture 162. In this embodiment, the laser 110 directs the laser beam 120 toward the first surface 166 of the microfeature workpiece 160 at a test location to form the test aperture 162. The laser beam 120 locally ablates the workpiece material and produces a vapor 161 that can be convected away from the region adjacent to the test aperture 162. The laser 110 directs the laser beam 120 toward the microfeature workpiece 160 until the sensor 140 senses the laser beam 120. The sensor 140 detects the laser beam 120 when the test aperture 162 extends through the microfeature workpiece 160. When the sensor 140 detects the laser beam 120, it sends a signal to the controller 150 which in turn sends a control signal to the laser 110 to stop generating the laser beam 120. The controller 150 stores the elapsed time and/or the number of pulses of the laser beam 120 required to form the test aperture 162.
The test aperture 162 can be formed in a noncritical portion of the microfeature workpiece 160. For example, FIG. 3 is a top plan view of the microfeature workpiece 160 without the redistribution layer 190. Referring to FIGS. 2 and 3, test apertures 162 can be formed in a perimeter region of the microfeature workpiece 160 proximate to the perimeter edge 169 and/or along the singulation lines A-A (FIG. 3) where the workpiece 160 is cut to separate the packaged microelectronic dies 180. Accordingly, in the illustrated embodiment, the portion of the microfeature workpiece 160 that includes the test aperture(s) 162 is not used for circuitry or other components of the dies 180 or workpiece 160. A plurality of test apertures are generally formed in a microfeature workpiece, but in many applications only a single test aperture may be formed in a workpiece. In other embodiments, not every workpiece in a run of workpieces needs to include a test aperture.
FIG. 4 is a schematic side cross-sectional view of the system 100 with the laser 110 forming a production aperture 164 in the microfeature workpiece 160. Based on the data gathered from forming the test aperture 162, the controller 150 and/or an operator develops a recipe to form the production aperture 164. The controller 150 can use the number of pulses of the laser beam 120 and/or the elapsed time to form the test aperture 162 in forming the production aperture 164. For example, the controller 150 can calculate an expected number of pulses of the laser beam 120 required to form the production aperture 164 based on the stored number of pulses required to form the test aperture 162. Additionally, the controller 150 can calculate an expected time required to form the production aperture 164 based on the stored elapsed time to form the test aperture 162.
In one embodiment, the expected number of pulses of the laser beam 120 and the expected time required to form the production aperture 164 are determined by multiplying the stored number of pulses and the stored elapsed time to form the test aperture 162 by a correction factor. The correction factor can adjust for differences in the thickness across the microfeature workpiece 160. For example, the metrology tool 102 (FIG. 1) can measure the thickness of the workpiece 160 at the test aperture 162 location and at the production aperture 164 location. The correction factor can account for the difference in the thickness of the workpiece 160 at the two locations. The correction factor can also adjust for differences in the workpiece material at the test aperture 162 location and at the production aperture 164 location. For example, in the illustrated embodiment, the test aperture 162 is formed through material adjacent to a first die 180a, and the production aperture 164 is formed through the first die 180a including the bond-pads 182. The correction factor can also increase the reliability of the process. For example, if the production aperture is a through hole, the correction factor can include an overdrill factor to increase the likelihood that the production aperture is formed completely through the microfeature workpiece.
After the controller 150 calculates the expected number of pulses of the laser beam 120 and/or the expected time required to form the production aperture 164, the system 100 forms the production aperture 164 in the microfeature workpiece 160. The workpiece carrier 130 properly positions the microfeature workpiece 160 relative to the laser 110, and then the laser 110 directs the laser beam 120 toward the workpiece 160 for the expected number of pulses of the laser beam 120 and/or for the expected time required to form the production aperture 164. In this embodiment, the sensor 140 does not need to be aligned with the production aperture 164 because the controller 150 controls the laser 110 based on the data gathered from forming the test aperture 162. However, in other embodiments, the system 100 may form the production aperture 164 without first forming the test aperture 162. In these embodiments, the sensor 140 can be aligned with the production aperture 164 to signal the controller 150 when the production aperture 164 has been formed, as described above with reference to FIG. 2. In any of these embodiments, the system 100 can form a plurality of production apertures in the microfeature workpiece 160.
In additional embodiments, the system 100 can also form blind apertures that do not extend completely through the microfeature workpiece 160. In these embodiments, the controller 150 can calculate the expected number of pulses and/or the expected time required to form the blind production aperture based on the data gathered from forming the test aperture 162 in a process similar to that described above. More specifically, the expected number of pulses of the laser beam 120 and the expected time required to form the blind production aperture can be determined by multiplying the stored number of pulses and the stored elapsed time to form the test aperture 162, respectively, by a correction factor. The correction factor in this application can adjust for differences in the workpiece material and thickness as described above to underdrill the workpiece for forming a blind production aperture. The correction factor also adjusts for the difference between the depth of the test aperture 162 and the desired depth of the blind production aperture. In other embodiments, the correction factor can also adjust for other factors.
One feature of the system 100 of the illustrated embodiment is that it provides good control of the exposure time that the microfeature workpiece 160 is subject to the laser beam 120. The laser beam 120 can be shut off after an aperture is formed because either the sensor 140 provides real-time feedback to the controller 150 or the controller 150 is able to accurately predict when the aperture has been formed. An advantage of this feature is that the heat-affected zone in the microfeature workpiece 160 is mitigated because the laser beam 120 is shut off in a timely manner. In prior art systems, the laser beam continues to pulse even after an aperture is formed and consequently increases the size of the heat-affected zone in the workpiece; such sizable heat-affected zones are detrimental to microelectronic devices because doped elements can migrate within the zone. Another advantage of the illustrated system 100 is that it enables high throughput using lasers and prolongs the life of the laser 110 because the number of pulses of the laser beam 120 required to form the apertures is reduced.
Another feature of the system 100 of the illustrated embodiment is that the system 100 consistently forms accurate apertures in the microfeature workpiece 160. An advantage of this feature is that apertures are consistently formed with a desired depth. The ability of the system 100 to more precisely determine the number of pulses of the laser beam 120 and/or the elapsed time to form a through hole allows the system 100 to avoid overdrilling and underdrilling.
From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

## Claims

1. A method for forming a plurality of production apertures in a microfeature workpiece, the method comprising:
ablating the microfeature workpiece by directing pulses of a laser beam to form a test aperture in the microfeature workpiece;
determining a first thickness of the microfeature workpiece at a first location, the first location being the location of the test aperture;
automatically determining a number of pulses of the laser beam and/or an elapsed time to form the test aperture;
determining a second thickness of the microfeature workpiece at a second location different than the first location, the second location being the location of one of the plurality of production apertures; and
automatically controlling the laser beam based on the determined number of pulses and/or the determined elapsed time to form a plurality of production apertures in the microfeature workpiece, wherein automatically controlling the laser beam comprises controlling the laser beam based on the difference between the first thickness and second thickness.

2. The method of claim 1 wherein automatically determining the number of pulses and/or the elapsed time comprises sensing the laser beam pass through the microfeature workpiece in real time.

3. The method of claim 1 wherein ablating the microfeature workpiece comprises forming a through hole.

4. The method of claim 1 wherein automatically controlling the laser beam comprises:
selecting an adjusted number of pulses by changing the determined number of pulses according to a correction factor; and
directing the laser beam toward the microfeature workpiece for the adjusted number of pulses to form at least one of the production apertures.

5. The method of claim 1 wherein automatically controlling the laser beam comprises:
selecting an adjusted time by changing the determined elapsed time according to a correction factor; and
directing the laser beam toward the microfeature workpiece for the adjusted time to form at least one of the production apertures.

6. The method of claim 1 wherein:
the production apertures comprise a plurality of blind holes; and
automatically controlling the laser beam comprises controlling the laser beam based on a selected depth of the blind holes.

7. The method of claim 1, further comprising positioning the microfeature workpiece between a laser and an electromagnetic radiation sensor before directing the laser beam, wherein ablating the microfeature workpiece occurs while the microfeature workpiece is positioned between the laser and the electromagnetic radiation sensor.

8. The method of claim 1 wherein automatically determining the number of pulses and/or the elapsed time comprises sensing the laser beam with an electromagnetic radiation sensor.

9. The method of claim 1 wherein:
the microfeature workpiece comprises a first die and a second die adjacent to the first die; and
ablating the microfeature workpiece comprises forming the test aperture between the first and second dies.

10. The method of claim 1 wherein:
the microfeature workpiece comprises a perimeter region; and
ablating the microfeature workpiece comprises forming the test aperture in the perimeter region of the microfeature workpiece.

11. The method of claim 1 wherein:
the microfeature workpiece includes a first surface and a second surface opposite the first surface; and
the method further comprises supporting the microfeature workpiece with a workpiece carrier so that a center region of the first surface and a center region of the second surface do not contact the workpiece carrier while directing the laser beam.

12. A method for forming a plurality of production apertures in a microfeature workpiece, the method comprising:
measuring a first thickness of the microfeature workpiece at a first location;
ablating the microfeature workpiece by directing pulses of a laser beam to form a test aperture in the first location of the microfeature workpiece;
automatically determining a number of pulses of the laser beam and/or an elapsed time to form the test aperture; and
measuring a second thickness of the microfeature workpiece at a second location different than the first location;
automatically controlling the laser beam based on the determined number of pulses and/or the determined elapsed time, the first thickness, and the second thickness to form a production aperture in the second location of the microfeature workpiece.

13. The method of claim 12 wherein automatically controlling the laser beam includes determining a number pulses of the laser beam and/or an elapsed time to form the production aperture based on the determined number of pulses and/or the determined elapsed time, the first thickness, and the second thickness.

14. The method of claim 12 wherein automatically controlling the laser beam includes determining a number pulses of the laser beam and/or an elapsed time to form the production aperture by multiplying the determined number of pulses and/or the determined elapsed time by a correction factor accounting for a difference in thickness and/or material between the first location and the second location on the microfeature workpiece.