Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
1999-08-30
2001-04-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
C356S445000, C356S417000, C356S319000, C073S800000, C250S226000
Reexamination Certificate
active
06211961
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for characterizing a sample using electromagnetic radiation and, in particular, relates to a system for determining at least one electrical property of ion-implanted semiconductors, and semiconductors doped by other methods, and the electrical properties of films deposited onto semiconductor material.
BACKGROUND OF THE INVENTION
Currently in the semiconductor industry there is great interest in monitoring the presence of charge in insulating layers deposited onto semiconductor surfaces. Such charges may be indicative of contamination or defects in these layers which may affect the performance of semiconductor devices.
Current techniques for measuring charge in insulating layers include the following.
A first technique is capacitance-voltage profiling in which the capacitance of an electrode in intimate contact with a sample is measured as a function of an applied bias voltage and, possibly, frequency. References in this regard are S. M. Sze, “Physics of Semiconductor Devices”, New York: John Wiley and Sons, 1969, and also A. S. Grove “Physics and Technology of Semiconductor Devices”, New York: John Wiley and Sons, 1967. In a variation of this method a small amount of liquid mercury may be placed in contact with the sample through the use of a small capillary.
A disadvantage of this technique is that it is necessary to have an electrode in contact with the sample.
A second known approach employs a surface photo-voltage technique. In this technique a voltage is applied to the surface of the sample by means of an electrode, and the sample is illuminated with a low-intensity light source, such as a light-emitting diode, whose intensity is modulated at a low frequency, for example 10 kHz.
A disadvantage of this technique is that either a contacting electrode is required or, alternatively, an electrode which can deposit charge onto the surface of the sample.
A further approach is known as Deep Level Transient Spectroscopy (DLTS). In this technique the temperature is slowly swept and the charges are progressively released from their trapping sites. The resulting change in capacitance is measured to infer the density of charge trapping centers.
However, this technique also requires that an electrical contact be made to the sample.
Furthermore, none of techniques are well-suited to the study of very small areas of a sample because the sensitivity decreases with decrease in the area that is probed.
In the semiconductor industry certain materials such as silicon, germanium, and gallium arsenide are frequently doped with impurities so as to change their electrical or mechanical properties. These impurities may be introduced by means of ion implantation or by means of in-diffusion from a solid, liquid or gas source. Associated with the introduction of such impurities is an amount of crystalline damage whose characteristics depend on the method by which they are introduced. A variety of ions are commonly used for this purpose including B, P, Ga, Ge, F, Si, B11, BF2, Sb, In, As and hydrogen. In the case of implantation, these ions are accelerated to an energy which may be as low as a few keV or as high as several hundred keV, and are then directed at the surface of the material. After entering the material an ion loses energy by collisions with the atoms of the material. These collisions result in damage to the material, such as displacements of atoms from their normal crystalline positions. For sufficiently high ion doses parts of the material may become amorphous rather than crystalline. The material is thus modified as a result of the damage that occurs (also referred to as the generation of defect sites) and as a result of the introduction of the ions themselves, even if no damage occurs. For in-diffused species, crystal damage in the sample, such as a substrate, may occur as the diffusing atoms displace sample atoms from their lattice sites. The extent of the damage depends on the size of the sample and the diffusing atoms, the nature of the diffusion source (solid, liquid, gas), the concentration of diffusion species in the source, and the details of the thermal process used to drive them into the substrate. It is also possible for there to be no crystal damage (e.g. if the diffusing atoms are small compared to the lattice constant of the sample). In such cases, diffusing atoms may occupy interstitial sites in the sample, and so may alter the local electronic and optical characteristics of the sample.
The material modification generally occurs in a surface layer or region the depth of which can vary from less than 100 Angstroms for low energy ions to several microns (e.g. when high energy ions are used). The dosage, i.e. the number of ions introduced per unit area of the surface of the material, can be varied over a wide range for implanted species by controlling the ion beam current and the time for which the ion beam is directed at the material. For the in-diffusion case the dosage can be controlled varying the thermal cycle or the source concentration. Currently in the semiconductor industry, implant doses as low as 10
10
ions per cm
2
and as high as 10
18
ions per cm
2
are used for different purposes. Both the material damage and the introduction of the ions results in a change in the electrical properties of the material in the vicinity of the surface where the ions are introduced. Some of the damage to the crystalline structure can be removed by thermally annealing the material.
In the fabrication of semiconductor chips, ion implantation or in-diffusion may be used at a number of stages of the process. Typically, an implant is restricted to predetermined areas, i.e. the implant is patterned. Similarly, in-diffused species may be added in a pattern by masking regions with an impenetrable, heat resistant layer such as SiO
2
or nitride. It is important to be able to monitor the dosage and to confirm that the correct regions have been implanted or doped by in-diffusion. Since these regions may be very small, it is important for a measurement technique to have very high spatial resolution. Also, and to avoid unintentionally contaminating the sample during the measurement, it is desirable that a non-contact measurement method be used.
A number of different techniques have been used or proposed for the evaluation of ion-implanted materials, including Rutherford back-scattering, Raman spectroscopy, and sheet resistance measurements. Some of these techniques have also been used to characterize samples to which foreign atoms have been introduced by in-diffusion.
Yet another technique which has been used to characterize ion implants employs a 100% intensity modulated laser beam with modulation frequency &ohgr; that is directed at a semiconductor surface, as described by Opsal et al., Method and Apparatus For Evaluating Surface and Subsurface Features in a Semiconductor”, U.S. Pat. No. 4,854,710. The light that is absorbed in the sample generates an electron-hole plasma, and also a heavily damped thermal wave close to the surface of the sample. Both the plasma and the thermal wave oscillate at frequency &ohgr;. These forced plasma and thermal oscillations give rise to small oscillations in the optical reflectivity of the sample which can be measured by means of a probe laser directed onto the same spot as the modulated laser. The amplitude and phase of the small oscillatory component at frequency &ohgr; arising in the intensity of the reflected probe beam depend strongly on &ohgr;, and also can be affected by the presence of ion implants and related damage in the semiconductor. Thus a measurement of this oscillatory component can be used as a defect or ion implant monitor.
Reference in this regard can also be had to J. Opsal, “Method and Apparatus for Evaluating Ion Implant Levels in Semiconductors”, U.S. Pat. No. 5,074,669. In this technique, both the unmodulated component of the reflected probe beam, and the component modulated at frequency &ohgr;, are measured and analyzed. In all of the above describe
Brown University Research Foundation
Font Frank G.
Ohlandt Greeley Ruggiero & Perle LLP
Punnoose Roy M.
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