Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
2002-02-22
2003-08-12
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S492210
Reexamination Certificate
active
06605812
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates ion implanters and more particularly to a method for reducing the undesirable effects of N
2
ion source contamination in an ion implanter during Silicon ion implants.
BACKGROUND OF THE INVENTION
Ion beam implanters are used to implant or “dope” silicon wafers with impurities to produce n or p type doped regions on the wafers. The n and p type material regions are utilized in the production of semiconductor integrated circuits. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type material. If p type material is desired, ions generated with source materials such as boron, gallium or indium are typically used.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The beam is formed and shaped by apparatus located along the beam path en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
During ion implantation a surface is uniformly irradiated by a beam of ions or molecules, of a specific species and prescribed energy. The size of the wafer or substrate (e.g. 8 inches or greater) is typically much larger than the cross-section of the irradiating beam which deposits on the wafer as a spot or “ribbon” of about 1 inch. Commonly, in high current machines, the required uniform irradiance is achieved by moving the wafer through the beam.
Operation of an ion implanter results in the production of certain contaminant materials. These contaminant materials adhere to surfaces of the implanter including beam forming and shaping structures adjacent the ion beam path and also on the surface of the wafer support facing the ion beam. Contaminant materials also include undesirable species of ions generated in the ion source, that is, ions having the either the wrong atomic mass or undesired ions of with the same atomic mass as a desired ion.
In a conventional ion implanter, an ion beam is emitted from an ion source and passed through a pre-analyzing magnet to remove undesired types of ions. Ions having identical energies but different masses experience a different magnetic force as they pass through the magnetic field due to their differing masses thereby altering their pathways. As a result, only those desired ions of a particular atomic mass unit (AMU) are allowed to pass through a prepositioned orifice in the pre-analyzing magnet.
After passing through the pre-analyzing magnet the ion beam is accelerated to a desired energy by an accelerator. Negative ions are changed into positive ions by a charge exchange process involving collisions with a chemically inert gas such as Argon. The positive ions then pass through a post-analyzing magnet and finally reach a wafer where they impact the wafer and are implanted.
Ion implantation has the ability to precisely control the number of implanted dopant atoms into substrates to within 3%. For dopant control in the 10
14
-10
18
atoms/cm
3
range, ion implantation is generally superior to chemical diffusion techniques. The implantation may be performed through materials that may already be in place while other materials may be used as masks to create specific doping profiles. Furthermore, more than one type of dopant may be implanted at the same time and at the same position on the wafer. Other advantages include the fact that ion implantation may be performed at low temperature which does not harm photoresist and in high vacuum which provides a clean environment.
With respect to impurities generated in an ion implanter, among the most troublesome are those where the product of the mass M and the energy E is the same as that of the desired species in the ion beam. In such cases, since the impurities have the same radius of curvature as the desired ion beams, they are likely to pass through both the pre-analyzing and the post-analyzing magnet and reach the wafer.
In such cases there is frequently no way to remove impurities before they reach the wafer. The passage of even a small amount of impurities can have substantial degrading effects on the electrical characteristics of the wafer. For example, in the manufacture of gate oxide films, even if only a very small amount of undesired impurities reach the wafer the quality of a gate oxide film is degraded and in subsequent processing may cause the gate oxide film to grow to an undesired thickness. As a result, semiconductor device reliability is reduced.
One particularly troublesome impurity is N
2
, especially when carrying out an ion implantation process with silicon ions. For example, silicon implanting is used in silicidation processes forming e.g., TiSi
x
in connection with gate structure formation and pre-amorphous implanting of silicon into polysilicon to alter its crystallization behavior, are a few of the many semiconductor formation processes using silicon ion implantation. Since silicon and N
2
ions have the same atomic mass unit (AMU) of
28
they are not differently affected or distinguished when passing through the pre-analyzing or the post-analyzing magnet. As a result, both species are passed through to the wafer where the N
2
adversely affecting silicon implantation. Device electrical characteristics are typically extremely sensitive to implant concentrations making implant stability and repeatability of the utmost importance in quality control considerations.
One example where the presence of the impurity N
2
can undesirably affect the performance of an ion implanter is in the calibration of the ion implanter by the use of a metrology instrument known as a thermawave tool to detect ion implantation damage in the target material. Generally, a measured dose of an implanted test species (measured by monitoring a physical property change in the implanted material) is compared with a previously recorded dose to determine the calibration state of the ion implanter. Consistency between test ion implantations with low mass ions may be used to provide information about the proper operation of the ion implanter. Silicon is frequently used as a test species that is implanted, causing measurable implantation damage which is subsequently measured by a thermawave tool. Generally, the thermawave tool measures a change in the surface reflectivity of the target material which corresponds to a known dose of implanted species. Comparing a present dose to a previously recorded dose indicates whether the ion implanter is performing properly within specifications. Clearly, where the impurity N
2
reaches the wafer together with silicon ions in a calibration state test procedure, the calibration state will be altered resulting in faulty information concerning the operation of the ion implanter. For example, the thermawave results may erroneously indicate that the ion implanter is operating within specifications resulting in an under dosage of implanted silicon. Furthermore, an unpredictable source of contamination leads to instability and irreproducibility between different implanting procedures.
FIG. 1
, for example, shows a typical plot of implant monitoring results to assess the concentration of implanted ions following an ion implantation procedure performed under substantially identical conditions. The vertical axis is a measure of implant concentration and the horizontal axis represents time over a time period of several ion implantations. The upper and lower lines A and B respectively, indicate the window of acceptable variation of ion implantation concentrations for the implantation procedure. It can be seen that the monitor measurements indicated by data line C have a high degree of variability and are poorly reproducible, frequently being out of specification, e.g., above line A or below line B.
One source of N
2
contamination is atmo
Chen Chi-Bing
Chen Lu-Chang
Huang Cheng-Yi
Lee Hsing-Jui
Tsai Chao-Jie
Nguyen Kiet T.
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
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