Abstract:
A switch comprises a set of electrodes with a nanocrystal channel disposed between the electrodes. The nanocrystal channel has bridges between conductive nanocrystals. A gate electrode is disposed above the nanocrystal channel and is insulated there from. Voltage applied to the gate can modulate electrical conductivity of the bridges between the nanocrystals, thus modulating current flowing between the electrodes.

Description:
BACKGROUND  
       [0001]     Many processes for forming electronic circuitry rely on very expensive semiconductor fabrication processes and facilities. There is a desire to find less expensive methods of manufacturing electronic circuitry. Semiconductor fabrication processes may also use significant amounts of heat to produce the electronic circuitry. Some of the circuitry may be sensitive to excess heat. If circuitry is formed early in a process, heat produced by subsequent process steps may cause changes in earlier formed circuitry. It can be difficult to control and plan for the effects of such heat. The amount of heat prior to adverse damage is referred to as a thermal budget. Thermal budgets may vary, and may be difficult to control. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0002]      FIG. 1  shows a nanocrystal switch device structure in an example embodiment.  
         [0003]      FIG. 2  shows an electrical conduction mechanism between adjacent nanocrystals connected with a material bridge in an example embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0004]     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.  
         [0005]     In  FIG. 1 , a channel  110  for a switch  115  is formed with a layer of nanocrystals. In one embodiment, metallic nanocrystals  117  are used for fabricating the channel. In further embodiments, semiconductor nanocrystals may be used to form the channel, especially if they exhibit conductivity in certain conditions.  
         [0006]     Many commercially available metallic nanoparticles may be used as the nanocrystals, such as Cr, Fe, Ni, Co, Cu, Mo, Ga, In, Sn, Zn, Au, Pd, Ag, and Pt to name a few. In addition, semiconductor nanoparticles with low enough resistance, such as for example Si, GaSe, InSe, InP, CdSe, PbS, and PbSe may also be used. In further embodiments, semiconductor nanocrystals may be any conductive semiconductors in groups W, III-V, II-VI, and III-VI.  
         [0007]     Employed nanoparticles may be coated with a protective layer of organic molecules. These organic molecules may prevent random close contact between the nanoparticles that may induce electrostatically induced attraction between the nanoparticles causing them to “stick” to each other and making their deposition difficult. The organic coating may be used to make handling of the nanoparticles easier.  
         [0008]     In one embodiment, the channel  110  comprises multiple nanocrystals  117  that include material bridges  118  between them. The bridges may provide for electrical conductivity of the nanocrystals channel. The protective layer of organic molecules may provide the material used in formation of the material bridges  118  between the nanoparticles.  
         [0009]     Formation of the material bridges  118  can be accomplished in a number of ways. In one embodiment, the nanoparticles are heat-treated causing particle sintering in the form of partial melting accompanied by reaction with the surface organic coating, and subsequent formation of the random network of material bridges between the adjacent nanoparticles. Similar effect can be obtained with alternative energy sources, like laser illumination, RF beam, plasma exposure, or particle bombardment.  
         [0010]     In another embodiment bridges  118  can be formed by a catalytic process. The material bridges between adjacent nanoparticles are formed by chemical reaction(s) involving nanoparticles&#39; surface and ambient surrounding nanoparticles. In a further embodiment selected parts of the nanoparticle&#39; surface could catalyze reaction, causing formation of the material bridges.  
         [0011]     In one embodiment voids  112  within the network of nanoparticles and bridges remain empty. In another embodiment they are filled with insulator such as, for example, highly resistive polymers (polyethylene, polyester, polyvineldifluorine, polystyrene, polycarbonate, polysiloxane, polypropylene, polyacrylate, polydimethylsiloxane, polymethylmethacrylate). This filling material can provide additional function of stabilizing and mechanically protecting and supporting potentially fragile network of nanoparticles  117  and bridges  118 .  
         [0012]     The size of the nanoparticles may be varied. In one embodiment, the nanoparticles have a diameter small enough to be consistent with a process for forming the channel. In one embodiment, the channel is printed, using a fluidjet printer, such as an inkjet printer. For example, the fluid jet printer ejects the nanoparticles onto a substrate  120  as described in more detail below.  
         [0013]     In one embodiment when low processing temperatures are a concern, the temperature to form the material bridges can be decreased by using very small nanoparticles exhibiting suppression of the melting point. This way, sintering processing to form the bridges may be accomplished at very low temperatures. Very small nanoparticles exhibit melting point suppression, meaning that they melt at temperatures below the bulk melting point. For example, 2 nm particles of Au melt at around 300° C.  
         [0014]     Switch  115  in one embodiment comprises the substrate  120 , such as a silicon wafer in one embodiment. Other substrates may also be used. A dielectric layer  125  is then formed on the substrate by one of many different known techniques. Electrodes  130  and  135  are supported by the dielectric layer  125 , with the channel  110  formed between them. A further dielectric layer  140  (gate dielectric) is formed over the corresponding electrodes  130  and  135 , and the channel  110 . Then, gate electrode  150 , aligned with respect to the electrodes  130 ,  135 , and channel  110  is formed on the top of gate dielectric  140 . The electrodes  130 ,  135  may be formed of conductive material, such as metal or highly doped semiconductor materials.  
         [0015]     Voltage applied horizontally, between electrodes  135  and  130 , causes electric current flow across the channel  110 .  
         [0016]     When voltage is applied to the gate electrode  150 , by circuitry  155  coupling the gate electrode  150  and the substrate  120 , a vertical electric field modifies properties of defects present within the bridges  118  between the nanocrystals  117  of the channel  110 . Thus, a horizontal current is modulated along the network of nanocrystals  117  in the channel  110 , as observed by circuitry  160  coupling the electrode  130  and the electrode  135 .  
         [0017]     In one embodiment, the channel  110  exhibits valve, or switch-like properties. A sharp transition between substantially conductive versus non-conductive behavior occurs over a fairly narrow change in electric field or voltage applied to the electrode  150 . The switch  115  is configured much like a field effect transistor, with electrodes  130  and  135  serving as source and drain, and electrode  150  serving as the gate.  
         [0018]     In one embodiment, the bridges can be crafted by proper selection of temperature and organic molecules used in formation of the bridges  118  as above, to provide an I-V relationship different from one obtainable for classic Si transistors. It may operate in different voltage regimes and high or low current ranges for a given voltage. A multi-state device may also be produced. The channel conductivity may be continuously or quasi-continuously varied as gate voltage is increased. Realization of regions of negative resistance may also be obtained. Further, the switch may be integrated with nanocrystal-based optoelectronic devices fabricated adjacent to the switch.  
         [0019]      FIG. 2  illustrates electrical conduction mechanism within a material bridge  200  formed between adjacent nanocrystals  205  and  210 . For illustration nanocrystal  205  represents a semiconductor nanocrystal, while nanocrystal  210  represents a metallic nanoparticle. In various embodiments, both nanocrystals are either semiconductor or metal. Three potential components of the material bridge are shown: complete organic molecule  215 , organic molecule residue  220  and defective, inorganic crystalline region  225 . Any combination of these three components can constitute the material bridge  200 .  
         [0020]     The material bridge  200  is formed in at least two different ways. In one embodiment, the material bridges are formed from organic molecules  215  or molecule residue  220 . In one embodiment, nanocrystals  205  and  210  are coupled directly with molecules  215 . Nanocrystals are packed into two dimensional or three dimensional arrays, with the organic molecules  215 ,  220  providing a bridge  200  between adjacent nanocrystals  205 ,  210 . They can also form less organized structure with organic molecules  215 ,  220  providing a network of randomly connected nanoparticles. Amines are common organic molecules that are used, and others may also be used.  
         [0021]     A further type of material bridge  200  is formed at the onset of melting of the nanocrystals, which typically have a fairly low melting point. At the melting point, formation of highly defective nanocrystal crystalline regions  225  of the same material as the nanocrystals  205  and  210  occurs. With organic molecules  215  and  220  present in the original nanocrystals, annealing of a packed array forms highly defective bridges containing both nanocrystal material  225  and remains of the organic molecules  215  and  220 .  
         [0022]     Nanocrystals  205 ,  210  formed with such bridges  200  may be coupled to electrodes as indicated at  230  and  235 , which may further be coupled to circuitry  240  to provide a voltage gradient to observe conductivity. A voltage gradient of 1 to 2 volts may be sufficient.  
         [0023]     Transport across the material bridges appears to be the sum of transport processes mediated by defects and allowed states within the bridges. They appear to provide hopping conductivity as observed in both metallic and semiconductor nanocrystals arrays. Hopping transport is described as a combination of conduction via mixture of discrete allowed states (as in  215 ,  220 ) and a large number of trap-release processes (as in  225 ), where each trapping center has finite energy, capture cross-section and release probability that are determined by its origin.  
         [0024]     Most of the individual transport processes illustrated in  FIG. 2  are a function of electric field. For example, most of the traps can exhibit Frankel-Poole effect, where application of an electric field changes trap parameters by orders of magnitude. Similarly, transport through allowed discrete states can be modified by the electric field.  
         [0025]     The switch  115  may be manufactured in many different ways. In one embodiment blank layer of nanoparticles is deposited by spraying or spinning, followed by photolithographic patterning and removal of undesired parts of the original nanoparticle layer. Alternatively, desired pattern of nanoparticles can be obtained by deposited through a shadow mask. In another embodiment, the switch is manufactured using jetted particles to print the desired, nanoparticle coated regions. Ink jet printer can be used to deposit the nanoparticles. This can provide a very low temperature deposition of the channel. An ink jet printing process may be used that utilizes particles suspended in a liquid medium, such as a solvent that evaporates after printing, leaving the particles behind in the shape printed.  
         [0026]     In one embodiment dielectric material filling voids  112  can be deposited in conjunction with nanoparticles. Further heat treatment may be required to polymerize the oligomers filling the voids.  
         [0027]     To form the switch  115 , electrodes  130  and  135  are first printed on the dielectric layer  125  of substrate  120 . The electrodes may be formed of many different conductive materials, such as metals or highly conductive semiconductors. The nanoparticles of the channel  110  are then printed, with organic material also included in the liquid medium. Dielectric layer  140  is then printed, and may be formed of SiO 2  or other type of insulative material, such as those commonly used in integrated circuit processing methods. Finally, the gate electrode  150  is printed. The gate electrode may also be formed of a conductive metal or other conductive material. In one embodiment, dielectric layer  125  is also printed prior to printing of electrodes  130  and  135 .  
         [0028]     After or during printing of the nanoparticles of the channel  130 , material bridges  118 ,  200  are formed by the aforementioned processes. In further embodiments, switch  115  may be formed in a hybrid fashion. For example, metal and dielectric layers may be formed using classical integrated circuit processes such as oxide growth, patterning, etc. The remainder of the switch  115  may be printed. Many other combinations of the use of printing and integrated circuit processing may be utilized.  
         [0029]     The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.