DISCUSSION OF THE PAPER

Hurd: How does the SIMS profiling technique compare to other profiling techniques, such as NAA, spreading resistance, or the four point probe techniques?

Lewis: Actually I should probably refer you to one of the people who has used SIMS and those other techniques, Joe Tsai could tell you more.

Tsai: Our results show very good correlation as a matter of fact.

Morabito: Another point is that the SIMS technique can measure the electrically inactive as well as active species.

Lewis: The electrical techniques only measure the concentration of electrically active impurities whereas SIMS measures the actual chemical concentration. I should also say something about the speed of analysis. A typical SIMS profile can be obtained in a matter of minutes, but your spreading resistance measurement takes how long?

Participant: About a day or so.

Lewis: Right. So I think it is important to point this out. You can, in a day, do easily 40 or 50 SIMS depth profiles. While the high performance SIMS instruments are quite expensive, the speed of analysis also has to be taken in consideration. This is one thing I like to point out when SIMS analysis is compared to electron probe analysis. At the same concentration level SIMS can typically do analysis five to ten times faster. This can readily offset the difference in cost of these machines. The real interest to semiconductor people, however, is that SIMS has the sensitivity to detect the dopant concentration levels and I do not know of any other technique that does as well.

Harrington: You can also tell what the dopant species is, whether it is boron, or

Lewis: You must always take into consideration the amount of material sputtered and the ionization efficiency of the particular species you are looking at. As the area is made smaller and smaller you must sputter to greater depths to get enough ions to achieve an adequate precision. I am talking about a bulk analysis now. In a concentration gradient this does not really apply. The answer is that the lateral spatial resolution is about a half a micron in bulk analyses. However, this is not a practical area to use for a depth profile. A practical limit to depth resolution for a depth profile is a few tens of angstroms for an area 100 μm or so in diameter.

In secondary ion mass spectrometry (SIMS), the sample is bombarded with energetic ions, usually in the range 5 to 30 keV. These primary ions impart their energy to the atoms of the sample via inelastic processes, resulting in the sputtering of atoms lying at or near the surface. The ionized fraction of these sputtered atoms, the secondary ions, are then mass analyzed, forming the basis of this spectrometry.

Concomitant with the sputtering of atoms is the damage to the sample produced by the primary ion solid interaction along the entire range of primary ions. Lattice defects such as vacancies and interstitials are created by the displacement of atoms from lattice sites due to energy imparted by the primary ion collisions. The effect of ion bombardment is to degrade the crystal perfection. Such degradation occurs in a shallow layer near the sample surface, the depth of which is governed by the range of the primary ions. Typically, this range will be of the order of 50 nm or less for +

the ions, for example, Ar+, 02+, o*, N2t Nt, 0, and the beam energies used in SIMS and other surface analysis techniques which make use of ion bombardment for erosion.1

Assessment of the damage induced by ion bombardment is possible by use of the electron channeling pattern technique in the scanning electron microscope (SEM). 2,3,4 Briefly, electron channeling contrast is obtained in SEM because of the influence of the periodic structure of a crystal on the initial electron-specimen interaction. The electron channeling pattern (ECP) is generated by varying the beam incidence angle relative to the crystal through the scanning action. The emitted or absorbed electron current varies depending on the angle between the beam and the crystal lattice. The information carried by electron channeling contrast is confined to a region within 50 nm of the surface since elastic scattering of the beam electrons rapidly randomizes and decollimates the beam with depth. Moreover, the ECP has been found to be sensitive to imperfections in the crystal. 5,6 ECP's can be obtained from areas of 10 um diameter or

less and, thus, the technique offers the possibility of assessing the crystal perfection of shallow layers with micrometer resolution.

In the present study, the ECP technique has been used to qualitatively assess the damage induced in a silicon crystal by the ion bombardment in an SIMS instrument. The ion bombardment conditions used were: 160 primary ions, beam energy 18.5 keV, beam current 10 nA, beam astigmatically focused and scanned in an area 50 μm × 50 μm for 1000 seconds. The resulting eroded crater is shown in figure 1. A region of constant current density was obtained over most of the scanned area, as evidenced by the uniform erosion (figure 1, region I) observed by an optical interference pattern. An ion flux of 2.5 x 1018/cm2 passed through this region. The astigmatically focused beam produced a gradation in ion flux at the edge of the crater (figure 1, region II). ECP's were obtained from the original crystal surface, the region of limited ion bombardment (region II) and the center of the crater by translating the specimen under the beam. The ECP from the original surface, figure 2, shows the fine detail and strong line definition characteristic of highly perfect crystal. The ECP obtained from the region of limited ion bombardment, figure 3, with identical SEM operating conditions as those used to generate figure 2, is greatly degraded. The ECP from the center of the pit was totally lost, indicating that as far as the electron channeling effect is concerned, the specimen is amorphous.

Additional experiments are being carried out to relate ECP quality to the defect concentration. However, from the present qualitative comparison of an ion bombarded region with the original crystal, it is clear that ion erosion leaves behind a severely degraded surface layer. This fact must be considered when questions of solute mobility during analysis or of the physical nature of the region under analysis are important.