Patent Application
20140021373
This invention relates to ion implantation and, more particularly, to uniformity during ion implantation.
Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
There are many different solar cell architectures. Two common designs are the selective emitter (SE) and the interdigitated backside contact (IBC). A SE solar cell has high-dose stripes across the lightly doped surface impinged by sunlight. An IBC solar cell has alternating p-type and n-type stripes across the surface not impinged by sunlight. Both a SE and IBC solar cell may be implanted with ions to dope the various regions.
“Glitches” may occur during the ion implantation process. A glitch is defined as a sudden degradation in the beam quality during an ion implantation operation, typically due to a variation in an operating voltage. Such a glitch is typically caused by interactions between components along the beam path, which affect one or more operating voltages, and can be caused at various locations along the beam path. For example, ion implanters generally employ several electrodes along this beam path, which accelerate the beam, decelerate the beam, or suppress spurious streams of electrons that are generated during operation. Each of these electrodes is maintained at a predetermined voltage. Often, electrodes of different voltage are located near each other and therefore an arc may occur between electrodes. Generally, arcs occur across acceleration gaps, deceleration gaps, or suppression gaps, although arcs may occur elsewhere. Interaction between, for example, a source extraction voltage, source suppression voltage, and source beam current may cause a glitch. These glitches may be detected as a sharp change in the current from one of the power supplies. If the implantation is interrupted or affected by a glitch, the implanted solar cell or other workpiece may be negatively affected or even potentially rendered unusable. For example, a solar cell may have a lower efficiency due to the lower implanted dose caused by a glitch. This may have a cost impact on the implanted workpieces. Thus, steps are usually taken to both minimize the occurrence of such glitches and to recover from the glitches if possible.
Repairing the dose loss caused by the glitch in such a manner may take over 30 seconds, which may be too time-consuming for the throughput demands of the solar cell industry. Ion beam stability and implant uniformity within the ion implanter are controlled by the speed of the voltage and current sources connected to the ion implanter.
Therefore, there is a need in the art for an improved method of glitch recovery during the implantation of workpieces and, more particularly, solar cells.
An ion implantation system and method are disclosed in which glitches in voltage are minimized by use of a modulated power supply system in the implanter. The modulated power supply system includes a traditional power supply and a control unit associated with each power supply, where the control unit is used to isolate the power supply from an electrode if a glitch or arc is detected. The control unit then restores connectivity after the glitch condition has been rectified.
For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
The embodiments of this method are described herein in connection with an ion implanter. Beam-line ion implanters, plasma doping ion implanters, or flood ion implanters may be used. Any n-type and p-type dopants may be used, but the embodiments herein are not limited solely to dopants. Furthermore, embodiments of this process may be applied to many solar cell architectures or even other workpieces such as semiconductor wafers, flat panels, or light emitting diodes (LEDs). Thus, the invention is not limited to the specific embodiments described below.
As noted above, glitches may cause non-uniformity of the ion beam or non-uniformity of the implantation of the workpiece. However, the extent of the non-uniformity is related to the duration of the glitch.
In general, the beam-line ion implanter 200 includes an ion source 280 to generate ions that form an ion beam 281. The ion source 280 may include an ion chamber 283. A gas is supplied to the ion chamber 283 where the gas is ionized. This gas may be or may include or contain, in some embodiments, hydrogen, helium, other rare gases, oxygen, nitrogen, arsenic, boron, phosphorus, aluminum, indium, antimony, carborane, alkanes, another large molecular compound, or other p-type or n-type dopants. The ions thus generated are extracted from the ion chamber 283 to form the ion beam 281. The ion beam 281 passes through an extraction electrode 284a.
An end station 211 supports one or more workpieces, such as workpiece 138, in the path of ion beam 281 such that ions of the desired species are implanted into workpiece 138. The end station 211 may include a platen 295 to support one or more workpieces 138. The end station 211 also may include a scanner (not shown) for moving the workpiece 138 perpendicular to the long dimension of the ion beam 281 cross-section, thereby distributing ions over the entire surface of workpiece 138. Although the ion beam 281 is illustrated, other embodiments may provide a spot beam. It will be understood to those skilled in the art that the entire path traversed by the ion beam is evacuated during ion implantation. The beam-line ion implanter 200 may include additional components known to those skilled in the art and may incorporate hot or cold implantation of ions in some embodiments.
In the path between the ion source 280 and the workpiece 138, the ion beam 281 passes through various components. These components may include, for example, a suppression electrode, a ground electrode, a mass analyzer and an angle corrector magnet. The mass analyzer may include a resolving magnet and a masking electrode having a resolving aperture. The resolving magnet deflects ions in the ion beam 281 such that ions of a desired ion species may pass through the resolving aperture. Undesired ion species do not pass through the resolving aperture, but may be blocked by the masking electrode. Ions of the desired ion species may pass through the resolving aperture to the angle corrector magnet. The angle corrector magnet may deflect ions of the desired ion species and convert the ion beam from a diverging ion beam to ribbon ion beam, which has substantially parallel ion trajectories. In other embodiments, a mass analyzer or angle corrector magnet is not included in the ion implanter 200. The beam-line ion implanter 200 may further include acceleration or deceleration units in some embodiments.
Some of these components may be at differing voltages, and therefore require power supply systems to provide those voltages. In
In one specific embodiment, only two power supply systems are used. In this embodiment, only three electrodes 284a-284c are used, where electrode 284a is an extraction electrode, electrode 284b is a suppression electrode and electrode 284c is a ground electrode. In this embodiment, one of the power supply systems is used to negatively bias the suppression electrode 284b relative to ground. The extraction power supply system 230a is used to positively bias the ion source 280 relative to ground. There are three specific instances where the glitches may occur. First, the extraction electrode 284a, which is positively biased, may arc to the suppression electrode 284b, which is negatively biased. Second, the suppression electrode 284b, which is negatively biased, may arc to a ground electrode 284c. Lastly, a positively biased extraction electrode 284a may arc to a ground electrode 284c.
In other embodiments, deceleration and acceleration electrodes may also be used in the ion implanter 200, which requires an additional power supply system for each electrode. This also increases the instances where glitches can occur, as there are more power supply systems in the implanter.
The arcing corresponding to a glitch may be sensed by a voltage collapse to a value below the voltage threshold value or a current rise above the current threshold value. By improving arc detection of the voltage sources, it is possible to better control glitch duration. Faster arc detection and voltage recovery may be used to keep glitch durations below 1 ms. This allows a workpiece to be implanted to within 6% of the desired dose, which may be acceptable for workpieces such as solar cells.
As described above, glitches of sufficiently short duration may not impact the efficiency of a solar cell and will not reduce the manufacturing throughput. Thus, it is desirable to reduce glitches is about 1 ms. Most currently available high voltage power supplies have slow arc detection and very slow recovery. In fact, in some embodiments, a power supply may take hundreds of milliseconds to return to its nominal value after a glitch.
The source switch 321 and the discharge switch 331 are actuated by a control unit 310. The control unit 310 may be any processing unit, such as a microprocessor, microcontroller, or special purpose computing device. The control unit 310 may have an associated storage element. The storage element contains the computer readable instructions necessary to implement the algorithms and routines described herein. In addition, the control unit 310 also has at least one input 311, which is used to detect glitches or arcs associated with the power supply 300. In some embodiments, the input 311 is an analog input, such that the input signal represents the current sourced by the power supply 300. For example,
In each of these embodiments, the control unit 310 monitors the input 311 and determines whether it is within a predetermined range. In the case of a current monitor 231, the control unit 310 may set an allowable range, such that any value outside this range is considered to be a glitch. In some embodiments, this range has both positive and negative thresholds, as current may flow in either direction depending on the polarity of the arc or glitch.
Thus, the control unit 310, through the use of programmable parameters in the storage element, can be configured to control a number of parameters. These parameters include the threshold value at which a glitch is detected. By adjusting this threshold, faster glitch detection can be achieved. In addition, the time between the opening of the source switch 321 and the closing of discharge switch 331 (i.e. t1) may be controlled by the control unit 310, if desired. Similarly, the time between the opening of the discharge switch 331 and the source switch 321 (i.e. t3) may be controlled by the control unit 310. For example, the time periods, t1 and t3, may be selected based on the switching characteristics of the source switch 321 and the discharge switch 331. It should be noted that, in some embodiments, the source switch 321 and discharge switch 331 may be configured such that a single output from the control unit 310 may be used to simultaneously control both switches. For example, one of the switches may be an N-channel MOSFET, while the other is a P-channel MOSFET. A single output from the control unit 310 can then be used to actuate both switches simultaneously. The control unit 310 can also be programmed with various blanking times, which is the time during which the discharge switch 331 is closed. In some embodiments, the blanking time may be as short as tens or hundreds of microseconds. In other embodiments, the blanking time may be several milliseconds. The blanking time may be adjusted based on the energy stored in the system and thus, the time required to discharge this stored energy. It may also be adjusted depending on process requirements, such as required uniformity. This time may be implementation specific, and no restrictions are placed on these values. In some embodiments, the blanking times may be as short as about 100 microseconds for certain applications. In other embodiments, longer blanking times, such as several milliseconds, may be acceptable.
In some embodiments, the blanking time may be shorter than the recovery time of the power supply 300. In other words, after a glitch, the power supply 300 may require some time, often in the millisecond range, to reestablish a regulated output. It is therefore beneficial if the power supply 300 used has a large output capacitance, such that the stored energy can control the droop that can occur after the source switch 321 is closed. If the output capacitance is smaller, the blanking time may be shortened to minimize droop and the process may be repeated multiple times to eliminate the stored energy.
In some embodiments, each modulated power supply system 230 has a dedicated control unit 310, which is used to control the source switch 321 and discharge switch 331 for a respective power supply 330. However, as shown in
The use of a modulated power supply system allows a high throughput method of manufacturing semiconductors, where exact dose uniformity is not a requirement, such as solar cells. In such an embodiment, a workpiece is placed on the platen 295. Ions are then directed toward the workpiece by energizing the various components of the ion implantation system. The use of the modulated power supply systems serves to minimize the duration of any glitches, thus helping to maintain the dose uniformity to within about 1%. In some embodiment, a supervisory controller (not shown) monitors the beam current being directed at the workpiece. As long as the dips in the beam current are within a certain limit, such as 0.5-3%, preferably 1%, the dose uniformity is acceptable, and the ion implantation is allowed to continue. Dips greater than this may cause an unacceptable change in dose, rendering the workpiece ineffective as a solar cell. In this case, the scanning of the workpiece is stopped, while the beam current is restored to its nominal level.
The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.
