Chemistry: analytical and immunological testing – Including sample preparation
Reexamination Certificate
2000-12-19
2004-09-21
Wang, Andrew (Department: 1639)
Chemistry: analytical and immunological testing
Including sample preparation
C436S149000, C436S151000, C435S287100, C435S287800
Reexamination Certificate
active
06794196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to deposited thin films of semiconductors and dielectrics. The present invention further relates to the use of these thin films in detection, analytical, contact, and biomedical applications. Applications of these thin films include desorption-ionization mass spectroscopy, electrical contacts for organic thin films and molecules, optical coupling of light energy for analysis, biological materials manipulation, chromatographic separation, head space adsorbance media, media for atomic molecular adsorbance or attachment, and substrates for cell attachment.
2. Description of Related Art
There is a great deal of interest in semiconductor and semiconductor-based (e.g., oxides, nitrides) materials with large surface to volume ratios; i.e., with large surface area. The reasons for this are two-fold. First, because of the large surface area, such materials are open to widespread surface chemical attack and, therefore, can be used as separation or release layers. These are needed in a variety of applications including MEMS (microelectro-mechanical devices), interconnect dielectric, micro-sensor, micro-fluidic and wafer separation applications. Secondly, these materials can be used as cell and molecule attachment layers, contacts and sensor materials. In addition, such materials can be very compatible with microelectronics. There are various approaches to producing large surface to volume (i.e., large surface area) materials. The technique attracting the most attention today is based on electrochemical etching. When electrochemical etching is used to produce large surface area silicon, the resulting material is commonly termed porous silicon. Porous Si was first obtained in 1956 electrochemically by Uhlir [A. Uhlir, Bell Syst. Tech. J. 35, 333 (1956).] at Bell Labs but it was not until 1970 that the porous nature of the electrochemically etched Si was realized [Y. Watanabe and T. Sakai, Rev. Electron. Commun. Labs. 19, 899 (1971). Recent discussions can be found in R. C. Anderson, R. C. Muller, and C. W. Tobias, Journal of Microelectro-mechanical System, vol. 3, (1994), 10.
The starting material for this wet etched conventional porous Si material is either conventional silicon wafers or thin film Si produced by some deposition process such as low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). In the electrochemical wet etching process the sample is exposed to a wet solution and a current is passed through a contact to the etching sample, through the etching sample, through the solution (e.g., a mixture of hydrofluoric acid, water and ethanol), and through an electrode contacting the solution (the cathode; e.g., platinum). This current causes the “pitting” or etching of the Si producing a porous network structure.
In the electrochemical (anodic) etching process the structure (e.g., pore size and spacing) and the porous-Si layer thickness are controllable by the resistivity of the silicon itself (magnitude and type), current density, applied potential, electrolyte composition, application of light, temperature, and exposure time. For sufficiently long exposures and for sufficiently thick starting material, this electrochemical etching process can be continued to the point where nanoscale structure (i.e., features of the order of nanometers) is obtained. The silicon features are a continuous single crystal when the sample is etched from a single crystal wafer, as is usually done, or polycrystalline silicon when the sample is etched from a deposited film. All these conventional (electrochemically etched) porous silicon materials are distinguished by (1) being the result of a wet, electrochemical etching process, (2) requiring a contact on the sample during this wet etching, (3) having generally disconnected pore regions which can be connected after extensive etching, and (4) being the result of a sequential processing first necessitating formation of the silicon and then necessitating subsequent wet etching. Besides the complexity of having to prepare, use, and dispose of wet chemical etching baths, these wet etched porous materials suffer from a problem of residual etching species and products remaining in the pores.
An alternative approach to producing a porous silicon thin film was shown by Messier (R. Messier, S. V. Krishnaswamy, L. R. Gilbert, and P. Swab, J. Appl. Phys. 51, 1611 (1980).). In this approach, a film with a spatially varying density was deposited. This film was subsequently wet etched, which removed at least some of the low-density region. As a result of this wet etch step, there was an increase in the film surface to volume ratio.
Intense research activity in porous semiconductors has been stimulated over the last decade by the discovery of room temperature visible light emission from electrochemically prepared porous Si in 1990 by Canham (L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990)). Soon after Canham's discovery, further intriguing properties of electrochemically prepared porous silicon were also realized, such as gas sensitivity, bio-compatibility and ease of micromachining, etc. (I. Schecter et al., Anal. Chem. 67, 3727 (1995); J. Wei et al. Nature 399, 243 (1999); L. T. Canham et al., Thin Sold Films, 297, 304 (1997); P. Steiner et al., Thin Solid Films 255, 52 (1995)). All these demonstrated applications to date have been based on the porous silicon material produced by electrochemically etching a wafer or deposited film of silicon.
The approach to producing a high surface area to volume ratio material in the present invention is to use deposition to grow as-deposited high surface-area films. In fact, we show that, with careful control of deposition parameters and techniques, we can attain a spectrum of films with tunable, surface area to volume ratios. This tunability allows morphology from continuous (surface area is the film area; i.e., void free) films to materials with up to about 90% porosity. We show such films have tunable chemical and physical properties such as variable species adsorption, light reflectance, and light absorption properties. Depending on the deposition technique and parameters, these thin films may be continuous (void free) or may have voids between columns and clusters. The present approach uses deposition performed at low temperature and tailored to attain the required morphology. There is no specific etching step involved and no wet processing. The present inventors have demonstrated that the present invention can be used to control void size and void fraction, that a columnar/void network morphology can be produced and that the columns can be polycrystalline material. In the demonstrations provided here for these controlled morphology films, plasma enhanced chemical vapor deposition (PECVD) is used to give continuous (void free) films, physical vapor deposition (PVD) is used to give an intermediate morphology, and PECVD is used to produce high void density (high surface area) material. Due to the deposition approach, high porosity (of up to approximately 90%) is attainable in the high void density material without any specific etching step. None of the controlled morphology films of this invention requires contacts, wet processing, or both. Also unique to the present invention is its ability to fabricate these deposited films, with designed morphology matched to the application, on various types of substrates including glass, metal foils, insulators, plastic, and semiconductor-containing materials including substrates with circuit structures.
As noted, the high void density morphology material is demonstrated using PECVD. In particular a this columnar/void network silicon was demonstrated by use of a high density plasma tool (e.g., Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) tool (PlasmaTherm SLR-770)) using hydrogen diluted silane (H
2
:SiH
4
) as the precursor gas at substrate deposition temperatures less than or equal to about 250° C.
Bae Sanghoon
Cuiffi Joseph
Fonash Stephen J.
Hayes Daniel J.
Borghetti Peter J.
Lopez Orlando
Perkins Smith & Cohen LLP
The Penn State Research Foundation
Tran My-Chau T
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