Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2000-05-12
2002-10-08
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
Reexamination Certificate
active
06462817
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to an ion implantation process. Certain embodiments relate to monitoring and/or evaluating an ion implantation process by measuring the optical properties of a wafer during processing.
2. Description of the Related Art
Ion implantation is typically used to introduce impurity materials, or dopant ions, into the surface of a semiconductor device. Because ion implantation offers several advantages over diffusion doping, it is becoming an integral part of many semiconductor fabrication processes. Ion implanters, however, are among the most sophisticated and complex systems in semiconductor manufacturing. In order to be utilized efficiently, ion implanters may require frequent monitoring and careful operation. For example, ion implantation systems may introduce a number of defects (e.g., non-uniformities) into the semiconductor process. Such defects may cause significant yield problems. A defect may result from contamination, such as material that is sputtered from the semiconductor substrate or wafer surface as a result of ion bombardment. The accumulation of contaminants over time may adversely affect the performance of the ion implanter and may reduce the wafer yield below acceptable levels.
In order to take advantage of the benefits of ion implantation processes, extensive characterization is typically performed to ensure that the process is within design tolerance. Ideally, extensive characterization of the process takes place both during process development and during process control of manufacturing processes. Typically, ion implantation processes are characterized by implant dose, uniformity of implant dose across the wafer, uniformity of dose across several wafers, and implantation depth profiles. Accurate measurement of the implant dose, however, may be a difficult task because the measurement is generally based on integrating the beam current. Error sources may be introduced into the measurement of the integrated beam current by interactions between the beam and electrons, neutrals, and negative ions as well as secondary particles which may be emitted as a result of ion bombardment of the target.
One process control method that may be used to monitor and assess an ion implantation process involves determining the sheet resistance of implanted wafers using a four-point probe technique. The four-point probe technique involves using a colinear probe arrangement which is arranged to contact the implanted regions on the semiconductor wafer. In operation, a current is passed between the two outer probes and the voltage drop across the two inner probes is measured. The test is typically performed twice in order to eliminate thermoelectric heating and cooling errors from the measurements. The first test involves passing a current in a first direction, referred to as the forward direction. The second test then involves passing the current in a second direction, opposite to the first direction, referred to as the reverse direction. The two voltage readings may then be averaged. The test may also be performed at several different current levels because testing at an improper current may cause the forward and reverse test results to differ or to cause the readings to drift.
Because the impurity regions must be electrically activated prior to electrical testing, this process control method may introduce several additional processing steps to the fabrication of a monitor or test wafer. For example, the impurity regions are typically electrically activated by rapid thermal processing. During this processing, masking materials such as photoresist may volatilize or reflow which may cause contamination or removal problems in subsequent processing. Therefore, the photoresist or other masking material is typically removed prior to electrically activating the impurity regions. Consequently, the time required to perform these additional processing steps increases the processing time and cost associated with electrical testing. Furthermore, long test times may be extremely costly if additional wafers have been processed incorrectly before the electrical test results were available. Suspending processing until the electrical test results are available, however, may also be costly due to production delays and idle production tools.
Optical dosimetry may also be used to monitor and control ion implantation processes. This technique measures the darkening, or increased optical absorption, of photoresist that occurs due to exposure to ion beams. Monitor wafers may be prepared by coating photoresist onto a transparent substrate. The monitor wafer is then scanned with a dosimeter to determine its background optical absorption. The wafer may then be subjected to an ion implantation process. After implantation, the wafer may be scanned again using the dosimeter, and the background optical absorption may be subtracted from this data. In this manner, the distribution of implanted ions across the entire wafer may be measured and plotted on a contour plot. From the plot of this data, variations and trends in the distribution of implanted ions across the wafer may be discerned. Additionally, the extent of the variations and trends may be analyzed to determine if the ion implantation process is within design tolerances.
Optical dosimetry may be particularly useful when making qualitative assessments of the performance of an ion implantation process. In addition, this technique allows the diagnosis of an ion implantation step to be done with greater resolution and sensitivity. For example, scan lock-up, non-linear scanning, and loss of beam diameter control may be detected using the optical dosimetry technique. Scan lock-up is a common problem which may result from overlap of individual Gaussian beam traces and may cause dopant non-uniformities across the wafer. Scan lock-up may be particularly problematic at low dopant doses. For example, low dopant doses typically have decreased beam currents and scan times which may lead to significant overlap in some regions of the wafer and doping level gaps in other regions of the wafer. Non-linear scanning may result from beam neutralization caused by collisions between ions and residual gas atoms in the beam chamber and neutralization from thermal electrons caught in the beam. Positive charge build-up on an insulating layer on the wafer may also result in non-linear scanning because the build-up may alter the charge balance in the ion beam and lead to significant dose variations across the wafer. Loss of beam diameter control, or defocusing, is another common problem in ion implantation which may result from separation of the beam due to repulsion of like charges. Defocusing of the beam may also cause uneven ion density and non-uniform implant concentrations in the wafer.
There are several disadvantages, however, in using the optical dosimetry technique to monitor ion implantation processes. For example, because only a photoresist-coated transparent substrate may be used in this technique, additional processing steps and materials are typically required. Furthermore, the testing method may not accurately predict the ion implantation performance of a product wafer which may have a topography which differs dramatically from a substrate coated with a planar resist layer. For example, positive charge build-up on a wafer may be particularly problematic when implanting into an insulating layer, such as resist or silicon dioxide, which may lead to significant dose variations across the wafer. The topography or patterning of the masking layer on a product wafer may cause additional localized positive charge build-up. A wafer having a planar resist layer may not accurately show the localization of positive charge build-up. Therefore, using a dissimilar test wafer may not accurately detect all of the potential problems that may occur in an ion implantation process.
Another process control method which may be used to monitor and control ion implantation processes involves the use of modulated
Conley & Rose & Tayon P.C.
Mewherter Ann Marie
Meyertons Eric B.
Stafira Michael P.
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