Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-06-02
2001-11-27
Niebling, John F. (Department: 2812)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100
Reexamination Certificate
active
06323497
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to controlling ion implantation during vacuum fluctuation. In particular, the invention relates to controlling an ion beam implantation process to compensate for vacuum fluctuation based on a measured beam current and not a measured pressure.
BACKGROUND OF THE INVENTION
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A typical ion implantation process uses an energetic ion beam to introduce impurities into the semiconductor wafers. As is well-known, introducing the impurities at a uniform depth and density into the wafers is important to ensure that the semiconductor devices being formed operate within specification.
One factor in the ion implantation process that can affect the uniformity of the impurity dose in the wafer is vacuum fluctuations during the implantation process. The vacuum fluctuations can be caused by photoresist or other materials coated on a semiconductor wafer that outgas, volatilize or sputter when the ion beam impacts the semiconductor wafer. The outgassing, volatilization or sputtering releases gas particles, which cause a pressure rise in the normally high vacuum condition along the beam line and can result in collisions between ions in the beam and released particles. These collisions can cause ions in the beam to experience a charge change. For example, singly-charged positive ions in an ion beam may collide with residual gas atoms produced by photoresist outgassing during implantation, and experience a charge exchange without a significant change in kinetic energy. The singly-charged positive ions may be neutralized by the collisions and impact the semiconductor wafer in the neutral charge state. In contrast, when outgassing, volatilization or sputtering does not occur from the semiconductor wafer surface, the vacuum level can remain relatively high and constant along the beam line, thus resulting in fewer ion charge exchanging collisions.
The charge exchanging collisions that result when the vacuum level along the beam line drops can cause problems because the detectors used to determine and control the ion beam current (and also the total dose of the wafer) during implantation typically only detect charged particles, but not neutral particles. The neutral particles that are implanted in the wafer are the desired implantation species and have the desired energies for implantation and thus, should be counted in the total implant dose. Since the typical ion beam current detector, such as a Faraday cup, is not capable of detecting the neutral particles, neutral particles that should be counted as contributing to the wafer dose are not detected. As a result, a beam current that is less than the actual beam current is detected, thereby prompting an increase in the beam current and overdosing of the wafer.
Previous methods for controlling implantation uniformity during vacuum fluctuations include detecting both the ion beam current and the vacuum level in the implantation chamber and controlling the ion beam accordingly, as disclosed in U.S. Pat. No. 4,587,433 to Farley and U.S. Pat. No. 5,760,409 to Chen. Such systems have drawbacks, including improper control caused by a delay between an actual change in vacuum along the beam line and the time when the vacuum change, i.e., a pressure change, is detected. This delay between actual vacuum change and detection can cause a delay in ion beam control and result in improper wafer dosing. This type of method also has the disadvantage of requiring the empirical correlation of a plurality of detected gas pressure and beam current values with a corresponding correction value for a plurality of sets of ion implantation parameters, such as gas composition, beam energy, implant species, amount of dose, photoresist type, etc.
Accordingly, a method for controlling implantation during vacuum fluctuation that is independent of such implantation parameters and that can rapidly respond to vacuum fluctuation is needed.
SUMMARY OF THE INVENTION
The invention provides methods and apparatus for controlling an ion beam implantation process in the presence of vacuum fluctuation along the beam line. In one aspect of the invention, vacuum fluctuations can be detected based on a detected ion beam current, and not a detected pressure. Thus, an ion implantation apparatus and method may correct for vacuum fluctuations based on beam current without detecting pressure within an implantation chamber.
In one illustrative embodiment, a method for controlling an ion implantation process includes generating an ion beam, and directing the ion beam along a beamline. A beam current is detected along the beamline, and a material is implanted with ions in the ion beam. Vacuum fluctuations during implantation are compensated for based on detected beam current and not based on a detected pressure.
In one illustrative embodiment, an ion beam including energetic particles for implantation into a semiconductor wafer is generated. A reference value for the ion beam current is then determined. The reference value for the ion beam current can be determined by actually measuring the ion beam current while a vacuum level along the beam line is at a desired level, e.g., is at a relatively high and stable level before implantation of the semiconductor wafer begins. The reference value for the ion beam current can also be a stored value that is retrieved from memory or input by a human operator, for example. Once the reference value for the ion beam current is determined, implantation of the semiconductor wafer is performed. The ion beam current is measured during implantation, and a difference between the reference value for the ion beam current and the measured ion beam current is determined. The ion beam current, a wafer scan rate or other implantation process parameters can be adjusted based on the difference between the reference value and the measured ion beam current. Since the measured ion beam current includes information regarding vacuum fluctuation, e.g., a decrease in measured beam current may be assumed to be caused mainly by vacuum fluctuation, the ion implantation process can be adjusted accordingly, e.g., a wafer scan rate may be decreased in response to a decrease in detected beam current.
In another illustrative embodiment of the invention, a difference between the reference value and the measured beam current can be scaled and the scaled difference value used to control ion implantation process parameters. For example, the ion implantation system can include an angle corrector magnet that bends and collimates the ion beam before the beam is incident on a semiconductor wafer. In this type of arrangement, vacuum fluctuations along the beam line can cause an increase in ion charge exchanging collisions that occur in two regions along the beam line, i.e., a line-of-sight region and a non-line-of-sight region. Charge exchanging collisions in the non-line-of-sight region that neutralize a particle cause the particle to not be deflected by the angle corrector magnet along the path to the wafer. Instead, the neutral particles follow a path that does not impact the wafer. Thus, although the non-line-of-sight collisions cause a decrease in detected ion beam current at the wafer, the collisions also cause a decrease in the dose rate at the wafer. In contrast, charge exchanging collisions that occur in the line-of-sight region neutralize energetic particles, but since the particles are traveling along a path toward the wafer, the neutral particles still are implanted in the wafer and contribute to the total dose. However, since the particles are neutralized, they are not detected as part of the total ion beam current. Thus, collisions that neutralize particles in the line-of-sight region cause a decrease in detected ion beam current, but not necessarily a change in the wafer dose rate. Scaling of the difference value between the reference value for the ion beam current and the measured ion beam current may be used to adjust the implantatio
Niebling John F.
Stevenson André C
Varian Semiconductor Equipment Assoc.
Wolf Greenfield & Sacks
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