The present invention relates generally to porous membranes, and more particularly to semipermeable membranes and techniques for fabrication thereof.
The development of practical microfluidic devices is currently hampered by phenomena that effect the reliability and manufacturability of these devices. Among these troublesome phenomena are processes associated with the introduction of unwanted constituents into microscopic flow channels, thereby interfering with the intended functionality of the device. These constituents include such entities as air bubbles and solid microparticles in liquid flow channels. A filtration system that can eliminate such unwanted constituents, and which can be co-fabricated with surface micromachined devices would greatly enhance the reliability of microfluidic apparatus.
A natural candidate for the heart of such a filtration system is a semipermeable membrane. Although such membranes are commonly thought of as being able to pass gases while preventing the passage of fluids. More generally, however, the term can be used to describe any membrane for which the membrane permeability differs significantly for fluids of interest. Semipermeable membranes can thus be used to separate fluid components, perform dialysis or osmosis. The selectivity of a membrane can be induced by physical configuration, including size (small molecules pass, large ones don""t) and shape (skinny molecules pass, fat ones don""t), by surface chemistry (different species bond to the membrane surface with different strengths), or by a combination of these factors.
Semipermeable membranes are currently made in many ways. Perhaps the oldest approach is to make a thin sheet of a binary alloy, and then expose the sheet to a chemical agent which preferentially dissolves one of the two metals making up the alloy. If the alloy concentration and the degree of attach by the chemical agent are correct, the result will be a porous metallic sheet, where the pores are nearly on an atomic scale.
It is more common today to make semipermeable membranes from polymers. Although polymer alloy fabrication techniques similar to those used on the metallic alloy membranes above can be used, often a controlled stretching process is all that is required.
The practical problem is that both polymer-based and metal-based semipermeable membranes are incompatible with current surface micromachining techniques, making it very difficult to incorporate these semipermeable membranes into such devices. However, if a filtration device can be co-fabricated with a surface micromachined microfluidic device, the overall process is greatly simplified.
A porous polysilicon-based membrane has been previously fabricated. In this process, a layer of silicon dioxide is deposited, and patterned into small localized deposits. These deposits are then embedded in polysilicon by growing a polysilicon layer atop the deposits. The membrane is then made porous by removing the silicon dioxide using a preferential etch. However, this technique is quite difficult to implement, and cannot produce semipermeable membranesxe2x80x94the pores which result from the patterning process are simply too wide and lack the convolutions which allow the length of the pores to be large.
There is thus a well-established need for a semipermeable membrane which can be readily co-fabricated with a surface micromachined microfluidic device. The fabrication process should allow production of membranes with a wide range of permeabilities.
The invention is of a new type of semipermeable membrane and fabrication processes therefor. These membranes comprise porous silicon nitride sheets, where the porosity is induced by a dry reactive ion etch. The permeability of such membranes can be controlled during fabrication by changing the degree of exposure to the etching process. Semipermeable membranes have been made which allow gases to freely pass, but not fluids. Such membranes have potential applications in gas/fluid/particle filtration, and in separation of physically and chemically distinct species.