The behavior of semiconductor devices is highly dependent upon the concentrations of impurity elements either intentionally added (e.g., doping with As, B, or P), present due to incomplete purification of the semiconductor base material or contaminated by other manufacturing steps. Small quantities of impurity elements can degrade charge carrier lifetimes in the active regions of integrated circuits. Many impurities exhibit significant electrical activities at contaminant concentrations below 1 part per billion. Measurements of element concentrations in semiconductor materials are essential for the development of devices and for maintenance of quality control during their manufacture.

Because the amounts of impurities are generally very small, they cannot typically be determined by conventional methods. The demand for an analytical technique with very high sensititives has led to the uti liz ation of neutron activation analysis (NAA), which usually offers the following advantages:

(1) simultaneous analysis of approximately forty elements,
(2) detection limits in the ppb and ppt range,
(3) nondestructive analysis (although samples will be radioactive after analysis).

Someof the elements routinelymeasured in semiconductor materials include the transitionmetals (i.e., Cr, Fe, Co, Ni, Cu, Zn, Mo, Ag, Cd, and Au) and the rare earths (i.e., La, Ce, Sm, Eu, Tb, and Yb).

The University of Missouri Research Reactor (MURR) has been performing NAA on silicon and other semiconductior materials since the early 1980's for customers in Denmark , Italy , Japan and the United States . Table 1 below lists the elements and detection sensitivities for elements reported in a typical analysis of semiconductor silicon by NAA at MURR.

 

Table 1. Approximate detection limits and sensitivities for impurities in an undoped silicon specimen weighing 15 grams after a 40-hour irradiation in a thermal neutron flux of 2.5E13 neutrons per square centimeter per second. Determined by counting the sample twice with a high-resolution HPGe detector. The first count was for thirty minutes after a 2-day decay, and the second count was for 6 hours after a 15-day decay.

Element	Parts per Billion (by Weight)	Contaminant Atoms per cc of Silicon	Element Atoms per billion Si Atoms
Ag	1.5E-3	2E+10	4E-4
As	2.5E-3	5E+10	1E-3
Au	1.5E-5	1E+8	2E-6
Ba	1.0E-1	1E+12	2E-2
Br	3.0E-3	5E+10	1E-3
Ca	6.0E+1	2E+15	4E+1
Cd	2.5E-2	3E+11	5E-3
Ce	2.0E-2	2E+11	4E-3
Co	4.0E-4	2E+10	2E-4
Cr	2.0E-2	5E+11	1E-2
Cs	1.0E-4	1E+9	2E-5
Cu	3.0E-2	6E+11	1E-2
Eu	4.5E-4	4E+9	8E-5
Fe	8.0E-1	2E+13	4E-1
Ga	4.0E-3	8E+10	2E-3
Hf	1.5E-3	1E+10	2E-4
Hg	4.0E-3	3E+10	6E-4
In	8.0E-3	1E+11	2E-3
Ir	1.5E-5	1E+8	2E-6
K	5.5E-1	2E+13	4E-1
La	1.0E-3	1E+10	2E-4
Mo	1.5E-1	2E+12	4E-2
Na	3.5E-2	2E+12	4E-2
Ni	4.0E-1	1E+13	2E-1
Pt	7.0E+0	5E+13	1E+0
Rb	1.5E-2	3E+11	6E-3
Sb	2.0E-3	2E+10	4E-4
Sc	6.5E-5	2E+9	4E-5
Se	6.0E-3	1E+11	2E-3
Sn	2.0E-1	2E+12	4E-2
Sr	2.0E-1	3E+12	6E-2
Ta	2.5E-4	2E+9	4E-5
Tb	2.0E-4	2E+9	3E-5
Th	6.5E-4	4E+9	8E-5
Ti	2.0E+2	5E+15	1E+2
U	1.5E-3	1E+10	2E-4
W	1.5E-3	1E+10	2E-4
Yb	6.0E-3	5E+10	1E-3
Zn	4.5E-2	1E+12	2E-2
Zr	3.0E-1	5E+12	1E-1

 

Several important elements which cannot be measured in the silicon matrix by NAA include the light mass elements H, Li, Be, B, C, N, O, and F because they do not produce radionuclides or they have extremely short halflives. Other elements not normally measured in silicon include Al, Mg, P, S, Y, Nb, Tl, Pb, and Bi. Recently, we have expanded our analytical capabilities to enable the analysis of other high-purity materials such as quartz, graphite, and specialized materials such as enriched Boron-11.

If you would like to learn more about applications of our analytical program to semiconductors and other high-purity materials, please contact:

Mr. Henry Newcomb
Silicon Analysis Program
Research Reactor Center
University of Missouri, Columbia, MO 65211
Phone No: (573) 882-5356
or
FAX No: (573) 882-6360