Etching a substrate: processes – Etching to produce porous or perforated article
Reexamination Certificate
2000-10-27
2002-10-08
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Etching to produce porous or perforated article
C216S017000, C216S041000, C216S047000, C210S490000
Reexamination Certificate
active
06461528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
2. Description of the Prior Art
Media with long pores are of interest because of their capabilities to sort, direct and mechanically fix smallest chemical or biological units. Various attempts have been undertaken to incorporate porous silicon or porous alumina in microelectronic or microfluidics devices. A remaining problem is that up to now pure aluminum substrates are required and that in all cases anodized pore growth was perpendicular to the aluminum surface. If one were able to make lateral or directed pores in some material, they could be integrated into bio-analytical systems. Additional possible applications arise if pore growth could be directed or pores jointed together in a well-controlled way.
Porous media are also of interest because of their filtering capabilities. Various attempts have been undertaken to incorporate porous silicon or porous alumina in microelectronic or microfluidics devices. Again in regard to porous alumina, one remaining problem is that up to now pure aluminum substrates are required and that in all used cases pore growth was perpendicular to the aluminum surface. If one were able to have lateral or directed pores in some material, they could be integrated into cheap nanofluidic systems.
Much work has been done to explore the growth of vertical pores in alumina and filter membranes using vertical pores in alumina have been developed. All studies use unstructured anodization of pure aluminum sheets as a substrate, as its purity, compactness and polycrystalline arrangement are prerequisite for well-ordered pore growth. One publication reports anodization on a niobium-masked Al substrate and shows short pores that grow isotropically under a mask layer forming random branches under constant voltage and lifting the mask's rim upward.
What is needed is to find a technical way to growing long pores underneath a mask, direct them, leave a vertical entrance direction and obtain well ordered, well-defined spreading or joining/branching pore structures.
What is further needed is a method to fabricate in-plane filters by opening the endings of lateral pores. The elements should be formed in a monolithic, compatible process and combined with fluidic inlets and outlets. Finally methods of integration these filters into planar fluidic systems are needed to make them suitable for use in cheap nanofluidic systems.
BRIEF SUMMARY OF THE INVENTION
The invention is a method for forming a lateral pore in a film having an in-plane extent and a vertical direction perpendicular thereto comprising the steps of disposing a stress compliant mask on the film, defining a vertical hole through the stress compliant mask and into the film, and forming a lateral pore in the film by anodization.
The method further comprises the step of disposing a polymer layer on the stress compliant mask. The film has regions which will be porous and nonporous, which regions have at least one boundary between them. The stress compliant mask disposed on the film comprises is a planarized stress compliant mask disposed over or on the boundary of the film between of nonporous regions and porous regions. The step of disposing the stress compliant mask on the film comprises disposing on the film multiple composite mask layers. The multiple composite mask layers comprise at least a first layer bearing high intrinsic tensile stress and adjacent thereto at least a second layer bearing compressive stress. In one embodiment the multiple composite mask layers comprise at least a SiO
2
layer and disposed adjacent thereto at least a Si layer. In another embodiment the multiple composite mask layers comprise at least a SiC layer and disposed adjacent thereto at least a Si layer. In still a further embodiment the multiple composite mask layers comprise at least a Si
3
N
4
layer and disposed adjacent thereto at least a SiO
2
layer. In any case the stress compliant mask on the film is a mechanically stable mask which withstands stress during anodization and counteracts pore formation stress to lead to pore ordering and directed growth. In other words the multilayer mask on the film has a composition of materials with different elastic properties such that tensile stress in the film is at least approximately matched to counteract compressive stress in the film caused by porous material growth. The disposition of the planarizing mask material provides locally increased masking layer thickness at the boundary between nonporous and porous material in the film.
The method further comprises annealing the film to improve its polycrystalline structure and prepare it for ordered pore growth, and/or disposing a passivating layer on the film to avoid oxidation during annealing.
The step of forming the lateral pore in the film by anodization comprises defining a start hole through the stress compliant mask and then anodizing the film through the start hole with an approximately constant anodizing voltage. In another embodiment the step of forming the lateral pore in the film by anodization comprises defining a start hole through the stress compliant mask with a nonrectangular geometry of a pore formed thereby and then anodizing the film through the start hole with a time varying anodizing voltage dependent on the nonrectangular geometry. When the nonrectangular geometry is trapezoidal, the anodizing voltage, V, is varied as determined by the equation dy/dx (V
0
v)/y
0
=dV/dt, where dy/dx is the change of width of the pore with respect to length of the pore, V
0
is the starting anodizing voltage, v is the rate of pore growth, and y
0
is the starting width of the pore. When the nonrectangular geometry is circular, the anodizing voltage, V, is varied as determined by the equation, dV/dt=&pgr;V
0
v/y
0
, where V
0
is the starting anodizing voltage, v is the rate of pore growth, and y
0
is the starting diameter of the pore.
In another embodiment the step of disposing a stress compliant mask on the film comprises disposing a stress compliant mold on the film, or disposing a stress compliant mold on the stress compliant mask.
The method further comprises the step of removing the stress complaint mask including the vertical hole defined therein and all other structures adjacent to the lateral pore except for a wall a nonporous material adjacent to the lateral pore to create at least one lateral test tube. The test tube can be loaded with a microsample by electromigration. The microsample can be read, marked, modified or cut in the test tube by means of scanning electron microscopy. The microsample in the test tube can also be read by means of a near field optical microscope. An aperture can be defined in the test tube for disposition of a tip of an atomic force microscope therein and the microsample read, or modified in the test tube by means of atomic force microscopy.
The lateral pore has a first and second end and the method further comprises the steps of opening the first and second end of the pore, and disposing a wire in the pore. The wire has a first and second opposing end. The first and second opposing end of the wire is connected to electrical contacts. The first and second opposing end of the wire may be connected with the electrical contacts either by forming the electrical contacts adjacent to the first and second opposing end of the pore prior to the wire being disposed therein and contacting the first and second opposing end of the wire with the previously formed electrical contacts, or by forming the electrical contacts adjacent the first and second opposing end of the pore after to the wire is disposed therein and contacting the first and second opposing end of the wire with the subsequently formed electrical contacts.
The method further comprises forming at least two interconnected lateral pores in the film. In one embodiment the interconnected lateral pores in the film are formed by selectively disposing prior to anodization at least two interconnected nonporous channels of anodizable material in the film. In another embodiment
Doll Theodore
Hoffman Thomas
Scherer Axel
Ahmed Shamim
California Institute of Technology
Dawes Daniel L.
Gulakowski Randy
Myers Dawes & Andras LLP
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