Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
1999-09-01
2001-08-14
Powell, William (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C216S084000, C216S092000, C216S099000, C438S008000, C438S748000, C438S753000
Reexamination Certificate
active
06274505
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an etching method, etching apparatus and analyzing method. More particularly, the invention relates to an etching method, etching apparatus and analyzing method used for detecting impurities contained in a semiconductor substrate, semiconductor thin film, and so forth, by chemical analysis with a high sensitivity.
Sodium (Na), iron (Fe) and other various metal impurities existing in semiconductor thin films and semiconductor substrates invite deterioration in insulation-to-voltage property of oxide films and production of crystallographic defects therein, and therefore greatly affect the characteristics of semiconductor devices. Taking it into consideration, device manufacturing processes usually include a step of removing metal impurities from device-making regions by washing or gettering. Recently, however, the industry is under rapid progress toward higher integration, miniaturization and higher performance of semiconductor devices. Along this tendency, allowance to metal impurity concentration is getting lower and lower, and much higher sensitivity and reliability are required in impurity analysis for controlling contamination. Simultaneously, there is a strong demand for a method not only capable of conventional surface analysis but also capable of analyzing impurities contained in bulk of semiconductor thin films and substrate surface layers for actually making devices.
As one of methods for analysis of impurities contained in bulk of substrates, SIMS (secondary ion mass spectroscopy) is now being used widely. This method has large advantages, e.g., high resolution as high as 1 &mgr;m or less, ability of analysis in a depth direction, and availability for analyzing various substances from hydrogen (H) to uranium (U). However, its detectable limit for Fe, representative contaminant, is as high as 10
15
atoms per cm
3
, which is insufficient as the sensitivity required today for appreciating contamination.
On the other hand, electric methods such as DLTS (deep level transient spectroscopy), &mgr;-PCD (photoconductivity decay) or SPV (surfacephoto voltage) have very high sensitivities, as their detectable limit as high as 10
10
atoms per cm
3
represents. However, there is a large restriction in measurable substrates and elements, and their application is limited to certain purposes.
Chemical analysis, which etches a substrate and analyzing impurities in the etching solution, is somewhat complex and requires a sill to conduct it. However, it is greatly advantageous in promising a high sensitivity with its detectable limit being as high as 10
11
to 10
14
atoms per cm
3
and in having no restriction in detectable substrates and elements. Representative one of such methods is disclosed by Takenaka et al. in “Analytic Chemistry” (Vol. 43, pp. 173-176 (1994)). This is a step etching method which etches a substrate fixed in a Tefion-made decomposition vessel by injecting a HF/HNO
3
mixed solution on the substrate surface. There is another method disclosed by M. Shabani et al. in Proceedings of 41st Joint Symposium of Japan Society of Applied Physics No. 2, p 598. This is a drop etching method in which a HF/HNO
3
mixed solution is sandwiched between a Teflon plate and a silicon substrate, and the surface of the silicon substrate is etched.
All of these techniques are characterized in dissolving silicon (Si) by exothermic reaction expressed by three formulas shown below, then changing the etching depth by changing the HF concentration in the mixed solution (the HF concentration and the etching depth are proportional), and thereby enabling analysis in the depth direction.
Si+2HNO
3
→SiO
2
2N
2
O
3
↑+H
2
O
SiO
2
+4HF→SiF
4
↑
SiO
2
+2HF→H
2
SiF
6
↑
Additionally, by further concentrating the etching solution, these techniques can further increase the sensitivity.
In order to lower the detectable limit and increase the sensitivity in chemical analysis, it is necessary to reduce contamination from outside and increase the sensitivity of an analyzer. Since the requirement to analyzer is not easy to realize, current development is centrally directed toward techniques for reducing external contamination. “External contamination” herein pertains to the following three modes of contamination.
(1) contamination from reagents used (contamination contained in reagents)
(2) contamination from environments of analysis (clean room and booth for analysis)
(3) contamination from tools for analysis Among these modes of contamination, (1) contamination from reagents is currently the most important issue. If the amount of impurities contained in a reagent, the back ground level of impurities becomes high and the detection sensitivity decreases. To prevent this problem, a reagent with a super-high purity must be used for analysis. However, as the used amount increases, the amount of contamination also increases so much, and the back ground level of impurities increases accordingly. Therefore, it is important to minimize the used amount of reagents in impurity analysis.
Regarding (2) contamination from environments, even when works are conducted in a clean booth of the class 10 or higher installed in a clean room having a cleanness of the current class 100 or higher, there is still a possibility of contamination, depending on elements. Therefore, works of analysis (decomposition and concentra-tion) must be finished in minimum time. Regarding tools for analysis, by using Teflon as the material and washing it well before analysis, reduction of contamination to zero has been accomplished. In relation to the above-made explanation, advantages and problems of step etching and drop etching are summarized in Table 1.
TABLE 1
Advantages and Problems of Step Etching and Drop Etching
Step Etching
Drop Etching
Etching depth
0.01~10 &mgr;m
0.1~1 &mgr;m
(6 inches)
Amount of
10 ml
1 ml
Chemicals
Decomposition
5 minutes
5 minutes
Time
Advantages
•Etching to a deep re-
•No contamination by
gion is possible (etching
chemicals (using very
under over-supply of
small amount of chemi-
HNO
3
)
cals)
•No contamination from
environments (time for
concentration is short (<1
hr))
Problems
•Contamination by rea-
•Maximum etching depth
gents (relatively large
is 1 &mgr;m (because of short-
amount of reagents is
age of HNO
3
, reaction does
used)
not progress beyond 1 &mgr;m)
•Contamination from
•Uneven etching due to
environments (due to a
existence of a plurality of
long time for concentra-
mountains on the etching
tion)
surface (FIG. 8)
•Over-etching in form of
an inverted cone getting
deeper toward the cen-
tral portion of the sub-
strate (FIG. 7)
Today's most serious problems in Table 1 is unevenness of etching. Such etching unevenness becomes a serious problem especially in case of epitaxial wafers and SOI (silicon on insulator) wafers which need analysis of respective layers. A solution for increasing surface evenness of etching would be executing gentle etching by decreasing HF (hydrofluoric acid) concentration. However, this is not practical because it invites an increase of the amount of etching solution and causing the above-indicated problem, i.e., increase of the background level of impurities. Additionally, each technique optimizes the amount of the reagent it uses upon development thereof, and it is undesirable to change the amount of liquid also from the viewpoint of etching accuracy.
It would be also possible to decrease the temperature for reaction, taking account of etching being exothermic reaction, as means for increasing the surface evenness. However, it has been known to the Inventor through experiments that this retards the etching reaction itself and largely increases the time required. The Inventor further made a trial for uniform etching by continuously stirring the etching solution. However, this was also ineffective to improve the surface evenness, and stirring was rather liable to invite contamination from environments and could not overcome the pr
Ito Shoko
Kageyama Mokuji
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Powell William
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