Patent Application
20080083926
For a better understanding of the nature and objects of the present invention, reference is made to the following detailed description taken in conjunction with the following drawings, in which:
a and 1b are schematic representations (side and top views, respectively) of a printed Si-based p-n-p bipolar transistor with electrodes on top of a Si print, according to an embodiment of the present invention;
A new method and apparatus are presented for printing transistor or diode structures using nanoparticles (e.g., silicon nanoparticles). According to embodiments of the present invention, Si-based electronic structures (e.g., transistors, diodes) can be printed in a simple low cost process and thus being a potential alternative to obtain a low cost manufacturing process for, e.g., Si-based active matrix (AM) backplanes as well as other applications such as processors requiring a very large-scale integration (VLSI) level integration and performance.
According to an embodiment of the present invention, the process can comprise:
Step 1: Formation of doped and un-doped nanoparticles (e.g., Si-nanoparticles);
Step 2: Formation of a solution with said nanoparticles; and
Step 3: Printing of various transistor or diode structures onto a substrate using solutions containing the Si-nanoparticles as well as other relevant materials, and printing of other relevant materials (e.g., conducting and insulating materials).
In step 1, the creation of nanoparticles (such as Si-nanoparticles) can be done, e.g., by an electrochemical etching of silicon wafers, as done by Professor Nayfeh's group at the University of Illinois (e.g., see Akcakir et al, “Detection of Luminescent Single Ultrsmall Silicon Nanoparticles Using Fluctuation Correlation Spectroscopy”, Applied Physics Letters, 76, pp. 1857-1859 2000; Chaieb et al., “Assemblies of Silicon Nanoparticles Roll up into Flexible Nanotubes”, Applied Physics Letters, 87, pp. 062104 2005).
Although Professor Nayfeh's group manufactures intrinsic (undoped) Si-nanoparticles primarily for optical applications (e.g., see Nayfeh M H, Rao S, Nayfeh O M, Smith A, and Therrien J, “UV Photodectors with Thin-Film Si Nanoparticle Active Medium”, IEEE Transactions on Nanotechnology 4, pp. 660-668, 2005, and Nayfeh O M, Rao S, Smith A, Therrien J, and Nayfeh, M H, “Thin Film Silicon Nanoparticle UV Detectors”, IEEE Photonics Technology Letters 16, pp. 1927-1929, 2004), the manufacturing technique may be extended to manufacturing of doped Si-nanoparticles as well, by starting with a doped Si-wafer. Other techniques to obtain doped and undoped Si-nanoparticles, such as mechanical grinding, can be utilized as well.
Step 2: although individual atoms/molecules of pure silicon may not be utilized for printing, extremely small particles of silicon, i.e., nanoparticles (ranging from approximately 1 mm to hundreds of nanometers, e.g., to one hundred nanometers) can be dispersed into a suitable solvent and printed, e.g., with an ink-jet printer. The use of ultrasound to obtain a dispersion of nanoparticles is well known and equipment for obtaining such dispersions is manufactured, e.g., by the company HIELSCHER (see http://www.hielscher.com/ultrasonics/index.htm, downloaded Sep. 7, 2006). The method is well known, e.g., in the printing industry for dispersing inks.
By combining an ultrasound disperser with an ink-jet printing head, the dispersed Si-nanoparticles can be printed in a simple printing process using a suitable solvent with an ultra sound generator continuously mixing the solution in the solution reservoir. However, other printing techniques such as Screen printing (with a higher concentration of active material in the “paste”), Gravure printing and others may be also used.
Step 3: by printing using the dispersed nanoparticles (e.g., Si-nanoparticles) with a suitable printing technique, one can obtain various transistor and diode structures on practically any substrate. In the case of an active matrix (AM) backplane for displays, the structure of main interest is a transistor structure. To obtain transistors (or other structures) suitable for the AM-backplane one can use several different approaches demonstrated in
a and 1b show an example among many others of schematic representations (side and top views, respectively) of a printed Si-based p-n-p bipolar transistor (which is a part of a module 10) with electrodes 20, 22 and 24 on top of a silicon print, according to an embodiment of the present invention. Here, in its simplest form, the p-n-p bipolar transistor could be formed by printing three parallel lines 14, 16 and 18 of p+, n, and p doped Si, respectively, on a substrate 12. In addition to the three printed lines 14, 16 and 18 of Si-nanoparticles, only the conducting lines 20, 22 and 24 that are connected to the p+, n and p regions, respectively, would be needed. The printing (e.g. by ink-jet, screen printing, etc.) of such conducting lines is also well known, e.g., by using an ink or a paste of a metal, carbon particles, conducting polymers, etc. In the example of
Thus, according to one embodiment of the present invention, all components of the transistor or diode structure can be disposed on the substrate using the printing technique.
Since devices made by printing nanoparticles (e.g., Si-nanoparticles) are based on the properties of Si and other materials, all different structures that have been demonstrated in these materials using the traditional lithography processes can be also possible to manufacture using printing as the manufacturing technique. Thus, other options to the bipolar transistor technology would be the MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) structures, either as NMOS(N-channel MOSFET), PMOS (p-channel MOSFET) or CMOS (Complementary MOSFET). Other alternatives may be (but are not limited to): pn junction diodes, e.g., Thin Film Diodes (TFD), AM-backplane applications, etc.
The various structures of MOSFETs and Diodes are well known to a person skilled in the art, and the structures (in their various configurations) could be realized by using printable nanoparticles (e.g., doped or undoped Si-nanoparticles), conducting materials (e.g., metal, carbon particles or conducting polymers), and various insulating materials (organic materials and/or inorganic oxides, e.g., in a form of nanoparticles).
By using the printing method, according to embodiments described herein, it is possible to print, e.g., Si-based transistors, as well as other electronic elements/components. However, the performance of said components may not be optimized due to a limited contact area between the individual nanoparticles. To improve the performance of the printed components two additional approaches, thermal annealing (or annealing by radiation at different wavelengths) and the use of an active “filler” material, can be used.
The thermal annealing (or even crystallization) can be performed by applying a direct heat, or by applying a laser light of an appropriate wavelength (a similar process that is used for obtaining low temperature poly silicon, LTPS). By annealing the nanoparticle based material, the connection between the individual nanoparticles and the device performance can be improved.
Furthermore, if the annealing temperature needs to be lowered, it is also possible to use surface activated Si-nanoparticles to reduce the energy required for the annealing process. Such surface activated Si-nanoparticles could have, e.g., Ni, Al, or other suitable metals on their surface (e.g., by electrochemically “attaching” metal atoms to the surface), i.e., said metals deposited as a separate layer that through diffusion at elevated temperatures is incorporated into, and interacting with the nanoparticles. Reducing the crystallization temperature in Si by using various metals (e.g., in the form of NiSi2) is well known to a person skilled in the art.
By using an active “filler” material, the connection between the individual nanoparticles may also be improved. Such filler materials could be conducting and/or semiconducting organic molecules and/or polymers, and thus the approach would be more of a hybrid approach between, e.g., traditional Si-transistors and organic transistors (OTFTs). By blending the active “filler” material(s) in suitable portions with the Si-nanoparticle solution, the device performance may thus be improved. No thermal annealing would be needed then, which could be highly desirable if plastic based substrates are used. The printable “ink” would thus contain the nanoparticles, the active “filler” and the solvent. Furthermore, in line with the use of an active “filler” of, e.g., a conjugated polymer/molecular material, the printed structures may also be so called hybrid structures where some of the inorganic materials are completely replaced with organic counterparts. For example, in the transistor structures the insulating layer could be based on an organic insulator such as PMMA (polymethyl methacrylate) or its precursor, or another insulating polymeric material.
The flow chart of
It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the scope of the present invention, and the appended claims are intended to cover such modifications and arrangements.
