1063-7397/02/3102- $27.00 © 2002 MAIK “Nauka /Interperiodica”
Russian Microelectronics, Vol. 31, No. 2, 2002, pp. 88–96. Translated from Mikroelektronika, Vol. 31, No. 2, 2002, pp. 104–114.
Original Russian Text Copyright © 2002by Starostina, Starkov, Vyatkin.
Locally porous silicon layers have proven to be a
promising means of lateral isolation for shallow-junc-
tion active components of VLSIs [1, 2]. This applica-
tion requires that the layers be fabricated in a reproduc-
ible manner and have desired dimensions and isolating
Locally porous layers can be produced by a kind of
selective chemical etching known as stain etching.
O etchants are used, pore forma-
tion may be accompanied by the partial dissolving of
silicon . The thickness of the layer obtained depends
on the ratio of rates for the two processes. To insure the
reproducibility of thickness, one should know how the
kinetics of pore formation depends on etchant compo-
To mark substrate areas intended for pore formation,
one modiﬁes their resistivity. This can be done by the
implantation of an electrically active dopant with a
ﬁnely focused ion beam  or a wide ion beam passed
through a mask . Alternatively, one can employ the
thermal diffusion of such dopant through a mask.
As regards the etchant itself, its ingredients must be
taken in a correct proportion lest it impair surface pla-
narity. Regrettably, the available data on the composi-
tion of stain etchants [2, 5–7] have a limited ﬁeld of
uses. The formation of stain ﬁlms was investigated for
narrow ranges of acid concentrations [8–10]. However,
we know from experience that the chemical formation
of porous silicon is possible for a much wider variety of
HF and HNO
proportions in an HF–HNO
etchant. This consideration dictated the subject of the
With respect to etch polishing, it can be effected in
very wide ranges of HF and HNO
proportions . If
one of them or both are decreased below certain level,
etch polishing gives way to stain etching and a layer of
porous silicon is formed on the surface .
This paper presents an experimental investigation
into the etching of silicon in HF–HNO
differing from each other in the proportions of the
ingredients. The focus is on (1) the transition from etch
polishing to stain etching, (2) the kinetics of porous-sil-
icon formation, and (3) the properties of porous ﬁlms.
The experiment was conducted on (1) p-Si(100)
substrates of resistivity
= 0.01, 7.5, 12, or 1000
and (2) high-resistivity Si(111) substrates covered with
m heavily doped n
the layer produced by the deep thermal diffusion of
The etchants were mixtures of a 70% nitric acid, a
49% hydroﬂuoric acid, and water. Two types of HF–
O etchant were employed. In the etchants of
type 1, the proportion of nitric acid is larger than that of
hydroﬂuoric acid. Speciﬁcally,
= 1 : 20 : 0, 1 : 15 : 0, 1 : 6 : 2,
1 : 6 : 3, 1 : 6 : 5, 1 : 6 : 6, 1.2 : 6 : 15, 1.3 : 6 : 15,
1.3 : 6 : 25, 1 : 7 : 20.
For the etchants of type 2, the converse is true, with
, and H
O being in a proportion of
= 1500 : 1 : 0, 1200 : 1 : 0,
900 : 1 : 0, 250 : 1 : 0, 4 : 1 : 5.
After etching at
, specimens were rinsed in
deionized water and dried in air.
The front surface of substrates was partly shielded
with picein in order to obtain steps on the surface (Fig. 1)
by the selective etching of silicon and removing the
resultant porous silicon with a 1-N solution of KOH.
Picein was then removed with hexane. Having mea-
sured the height of the steps, we were able to quantify
of stain etching and the thickness
porous silicon (Fig. 1). The difference between
is the depth to which porous silicon was removed dur-
Porous-Silicon Formation in HF–HNO
E. A. Starostina, V. V. Starkov, and A. F. Vyatkin
Institute of Microelectronic Technologies and High-Purity Materials, Russian Academy of Sciences, Chernogolovka,
Moscow oblast, Russia
Received April 23, 2001
—The chemical formation of porous silicon in HF–HNO
O etchants is studied experimentally. A
technique is devised to determine the ranges in which the proportions of the acids should be varied in order to
change from etch polishing to pore formation. The structure and properties of porous layers are examined in
relation to the proportion of etchant ingredients.