1 2
metal. The silicon film has pores with diameters of less METHOD OF PRODUCING A SILICON FILM than about 60 angstroms, in order to effect separations WITH MICROPORES in the molecular regime. Pores as small as about 10-30
angstroms are contemplated, as well as pores which are BACKGROUND OF THE INVENTION 5 generally straight and uniform in size.
1. Field of the Invention In a preferred aspect, a monocrystalline or polycrysThis invention relates to the formation of a micropo- talline silicon substate is anodized to form a porous
rous silicon film. More particularly, it concerns the silicon film on the substrate. The porous film is then
controlled anodization and oxidation of silicon to create peeled or otherwise removed from the substrate and a porous film. By isolating and mounting the film on a 10 mounted on a coarser, stronger filter medium. A pre
porous substrate, it is possible to effect separation pro- ferred filter medium is sintered stainless steel, although
cesses in a molecular regime. other sintered metals, ceramics and the like, may be
2. Brief Description of the Prior Art employed depending upon the operating fluids and Silicon is an abundant element, relatively inexpen- conditions to be employed. Conveniently, an epoxy
sive, and commonly used in semiconductors. Refine- 15 resin or cement may be used to attach the porous film to
ments in silicon technology have had applications in the the coarser filter support. The outer periphery of the
computer and integrated circuit industries. flhn wiu normally be attached in this manner.
Silicon tends to exist in crystalline form at room tem- It is contemplated that a filter device of the present
perature. It is well known in the art that a silicon surface invention may ^ be made be depositing a silicon film
may be etched, oxidized, anodized, and bombarded 20 on a relatively coarse filter support and then anodizing
with ions. Advances in silicon processing have concen- ^ mm m Jace Pol talline silicon fllmSj for exam.
trated on improved control of surface modification j ^ ited ... &i_
steps, because silicon s good physical properties allow f. ,A\m\ * T> • (4l n u
^ ?^ £ i tions (CVD) reactor. Doduiks of the film mav be re
custom-tailonng of its surface contours. For example, "u"av lco" "i""60" llia*"
the formation of pores has been of particular interest. 25 ^u5f' . „ .,.
Pore sizes have decreased substantially over time. With , ^ Porer surfaces °f a Porous smcon film ^
these decreases, the potential uses for silicon have in- P^ted, as for example with copper, silver, or other'
creased metal. The plating may be conducted following anod
A variety of silicon methods for making pores are Nation in a liquid electrolyte by transferring the anod
known in the prior art. One process for forming aper- 30 ized, porous film with electrolyte in the pores from the
tures in silicon (Abbas et al.; U.S. Pat. No. 3,962,052) anodizing cell into a plating cell. When plating copper,
requires the use of photolithographic machinery and the electrical polarity of the cell is reversed from an
materials on both sides of the silicon. The holes pro- anode to a cathode.
duced by this process, however, are no smaller than one Once a porous silicon film has been formed, it may be
micron, a size far above molecular. 35 generally desirable to stabilize the film against oxidation
A bombardment method disclosed in Price (U.S. Pat. if the film is to be used or stored for an extended period.
No. 3,303,085) requires a vacuum chamber and a parti- Otherwise, the film may tend to oxide and the pores to
cle beam. There would be great utility and benefit in close with time. Controlled oxidation may be used to
eliminating the need for such enormous support facili- reduce pore size.
ties. 40 A particular feature of the invention is to provide
SUMMARY OF THE INVENTION filters with pore sized capable of use in catalysis, hydrocarbon gas separations, artificial organs such as kidneys
Porous silicon has been of interest in silicon-on- m£ iungSj biotechnology, and in the manufacture of
insulator technology. A porous silicon layer is formed pharmaceuticals, food products, chemicals, and the like,
on a silicon wafer, and single crystal silicon islands are 45 Films with pore sizes between about 20 and 60 ang
then formed on the porous silicon by various tech- stroms are contemplated to be particularly suited for
niques. A porous layer is then oxidized to provide di- application of the invention.
electric isolation for the silicon islands The mean sizes roduced in single crystal silicon
Porous silicon layers have been produced by anodiz- wiU d d ^ the ... of the starti
ing crystalline silicon wafers m hydrofluoric acid solu- 50 ^£ current densit ^ to a less<fr
tions. During anodization, the wafer has served as the , u J 5 ■ -j * • *i.
, , , , j it. A J degree, on the hydrofluoric acid concentration in the
anode, and platinum has served as the cathode. °.. :' , _r . .„ „ , . , ...
It has been proposed that laser beams be used to drill anod!zm.S bfh. Dopmg will normally be required, with
small holes in layers of silicon. It has also been proposed less doping for larger pore sizes. Boron has been found
that various etching, and bombardment techniques be 55 t0Jf especially effective as a dopant, used for this general purpose. All of these techniques, DoPmS may al*°.be generally desirable in depositing
however, involve complex or expensive procedures. doPed polycrystalhne or amorphous silicon films on a
A continuing need exists for filters capable of separat- substrate. Again, boron has appeared to be an effective
ing materials in the molecular regime. Thus, filters with dopant.
pore sized in the range of 20 to 200 angstroms could be 60 In apparatus form, the present invention comprises a
useful in fluid processing applications ranging from smcon film containing micropores having straight sides
hydrocarbon separations to biomedical molecular syn- attached to a substrate material having pores of larger
thesis. Materials such as zeolites and sintered metals diameter than the pores in the silicon film. The substrate
have been used in such service, but these materials suf- material may be any material having sufficient struc
fer from non-uniformity and random flow paths. 65 tural properties to enhance the strength and rigidity of
The present invention comprises a method for mak- the silicon film to which it is attached. Graphite and
ing very fine filters wherein a film of porous silicon is stainless steel are suitable substrate materials. Such an
supported on a coarser filter medium, notably sintered invention may be used as a filter in applications requir
