Patent Application
20100047476
1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a silicon (Si) nanoparticle precursor that can be sintered at low temperatures to form Si thin-films.
2. Description of the Related Art
Silicon, thin-film transistors (TFTs) are commonly used as active-matrix devices in flat-panel displays. Due to severe competition within the display industry, cost reduction in the fabrication process is always an essential goal in the design of new products. Conventionally, a TFT fabrication process uses several chemical vapor deposition (CVD) and/or sputtering steps to deposit semiconductor, insulator, and conductor materials, which require vacuum systems, gas delivery, and control units. These methods result in blanket coated films, which require patterning, typically by multiple photolithography and etch steps. The cost of these processes could be reduced significantly if these steps could be replaced with a simple printing technique.
It has been reported that silicon films can be formed from silane-based liquid precursor (“Solution-processed silicon films and transistors”, Nature 440, 783-786, Apr. 6, 2006). Shimoda et al. reported that TFTs with mobilities of 108 cm2V−1s−1 and 6.5 cm2V−1s−1 were achieved on polycrystalline silicon prepared from spin-coating and ink-jet printing using liquid silicon precursors. Tanaka et al. reported on the formation of n-type silicon films using phosphorous-doped polysilanes (“Spin-on n-type silicon films using phosphorous-doped polysilanes”, Japanese J. Appl. Phys., 46, L886-L888, 2007). Shiho has claimed a polysilane compound with at least one from cyclopentasilane, cyclohexasilane, and silylcyclopentasilane (U.S. Pat. No. 7,067,069). Si particles of 5 nm to μm sizes, with 0.1 to 100 wt %, were dispersed in the silane composition. Si films were then formed with lamp or laser exposure at room temperature to 300° C., in a non-oxidizing atmosphere.
Zurcher has claimed a Si nanoparticle ink (<100 ma sizes) which comprise a molecular precursor, such as a polysilane, silylene, or organo-silane. A Ge-based molecular precursor such as polygermane, germylene, or organo-germane can also be combined with Si-based precursor (U.S. Pat. No. 7,078,276). He has also claimed the use of hydrogen capped nanoparticles of Si or Ge dispersed in a solvent medium to form nanoparticle ink, and Si-based or Ge-based molecular precursors (U.S. Pat. No. 7,259,101).
Bet et al. has reported that Si film can be formed from nanoparticles after laser annealing without using liquid silane (“Laser forming of silicon films using nanoparticle precursor”, S. Bet et al., J. Electron. Mat., 35, 993, 2006, and US 2007/0218657).
Most, of the above-mentioned researchers claims that liquid Si-containing precursors can be applied to substrate as an ink-like material, and through heating or irradiation, are converted into amorphous or polycrystalline silicon films. However, such a practice is limited by considerations of cost and safety. It is known that many of the claimed materials are flammable and in some cases, such as germanium-containing precursor, can also be toxic. In fact, some high molecule silicon hydrides have been suggested for use in a combustion chamber as missile propellant (U.S. Pat. No. 5,775,096).
Although a safe operation can be maintained in an enclosure using sophisticated safety precautions, the manufacturing costs associated with flammable materials are high. The end result may be that the cost of a so-called “low-cost” Si printing process will become too high for actual practice.
Since the safety issue associated with the use of Ge and liquid silane is related to the concentration of these materials, it would be advantageous to minimize the amount of liquid Si or Ge compounds used in a Si precursor. However, none of the above-mentioned methods provide an analysis of the amount or percentage of Si-containing compound required to form silicon films.
It would be advantageous if the safety of Si ink or printable materials could be enhanced by reducing the required amount of the liquid silane used in Si precursors.
The Si precursor disclosed herein minimizes, or completely eliminates the amount of a liquid silane compound needed to form a printable Si precursor, which is referred to herein as a Si nanoparticle precursor. In one aspect, the Si nanoparticle precursor is a Si-containing solution, containing Si nanoparticles, liquid silane compounds, and solvents. The major constituent of the solution is Si nanoparticles. Liquid silane compounds serves as an agent to form channels between Si nanoparticles, to form a continuous Si film after heating or light irradiation. The Si nanoparticle precursor minimizes the amount of liquid silane needed to form the proper channels by maximizing the packing of Si nanoparticles.
After deposition, the sintering of Si nanoparticles requires an elevated temperature. The sintering temperature can be reduced significantly if the connection channels are formed from liquid silane or from Ge nanoparticles. The Si nanoparticle precursor uses a designed size ratio of Si nanoparticles, to minimize the amount of liquid silane, or to reduce the use of Ge nanoparticles.
Accordingly, a method is provided for forming a Si nanoparticle precursor. The method provides a plurality of nanoparticle classes, including at least one Si nanoparticle class. The nanoparticles in each nanoparticle class are defined as having a predetermined diameter. A predetermined amount of each nanoparticle class is measured and combined. For example, a first Si nanoparticle class may be provided having a largest diameter and a second Si nanoparticle class having a second-largest diameter equal to about (0.43)×(the largest diameter). As another example. Si nanoparticle classes may be provided having a diameter ratio of about 77:32:17.
In some aspects, the method measures a predetermined amount of liquid silane, which is combined with a plurality of Si nanoparticle classes. In other aspects, at least one class of germanium (Ge) nanoparticles is provided, which is combined with a Si nanoparticle class and liquid silane.
Additional details of the above-described method, as well as a method for forming a Si thin-film from a Si nanoparticle precursor are provided below.
Even when liquid silane is used, as shown in
However, in 3 dimensions, the calculation of the optimal combination of sphere sizes becomes quite complicated. In a three dimensional closest packed assembly of spheres, the volume occupied by the largest spheres is calculated to be 74% of the total volume, while the pores occupy 26% of the total volume. The size of next size sphere that optimally fits into the pores can be estimated by the following relationship;
(2-D size ratio)/(2-D volume ratio)=(3_D size ratio)(3-D volume ratio)
Since the 2-D size ratio is 15.5, the 2-D volume ratio is 9.3, and the 3-D volume ratio is 26, the 3-D size ratio is estimated to be (15.5×26)/9.3-43%. Therefore, the radius of the next size sphere that can optimally fit into the pores between the large spheres is estimated here to be about 43% of the radius of the large spheres.
From the table it is observed that the size ratio of the largest and second largest spheres is 32/77, which is about 42%. This result is very close to calculation above. Most of the volume is occupied by a few (4) of the largest spheres, with a much smaller volume for the medium size and small size particles. With the proper distribution of nanoparticle sizes, the pores only occupy a small portion of the total volume.
In M. Rahaman's book, Ceramic Processing and Sintering, 2nd edition. Marcel Dekker, Inc., 2003, the maximum packing density of a binary mixture is stated to be 86.8%. When the interstitial holes in the binary mixture are filled with a large number of very fine spheres in dense random packing, the maximum packing density becomes 95.2%. The maximum packing density of quaternary mixtures is stated to be 98.3%.
By using nanoparticles of mixed sizes, and adding liquid silane to fill in the remaining pores, the amount of liquid silane can be reduced to a minimum, approximately <5-15% of the total volume, depending on the combination of the size and distribution of the nanoparticles.
Liquid silane precursors can be formed from silane based monomers including, but not limited to, cyclotrisilane, cyclobutasilane, cyclopentasilane, cyclohexasilane, and cycloheptasilane. These cyclic monomers have a high propensity towards photo-polymerized resulting in silane precursor materials with an increase in molecular weight and concurrent boiling point. Other silane based monomers include, but not limited to, monosilane, disilane or trisilane. These silane based monomers can be polymerized through homogeneous or heterogeneous catalytic reactions. Linear or branched polymers can be formed depending on the reaction conditions. The silane precursors can be dissolved in a variety of hydrocarbon solvents such as n-hexane, n-heptane, n-octane, n-decane, benzene, toluene, xylene, and ether solvents such as dipropyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, tetrahydrofuran, and polar solvents such as, H-methyl-2-pyrrolidone, dimethylformamide, acetonitrile and dim ethyl sulfoxide.
An alternative method is to add smaller Ge nanoparticles to the interstitial sites of Si particles. Since the melting point of Ge is much lower than Si, and Ge can absorb near IR radiation, liquid sintering occurs, forming Ge or SiGe channels to connect the Si nanoparticles.
In summary, mixed sizes of Si nanoparticles are used in the Si nanoparticle precursor to increase packing density. In some aspects, liquid silane is added to fill in the remaining voids. The amount of liquid silane can be reduced to only a portion of the total volume, for example 5-15%. Diluting liquid silane in a proper solvent permits the liquid silane to settle into the pores among the nanoparticles. The liquid silane can be replaced or augmented with Ge nanoparticles, or liquid germane. Using nanoparticles of mixed sizes, under proper annealing condition, the use of liquid silane or germanium can be completely eliminated. In this case, the mixed nanoparticle sizes increase the packing density and enhance the direct contacts among the Si nanoparticles.
Mix Si particles in a size ratio of around 77:32:17 or 77:32:17:D, where D=12˜14. As an example, the sizes of the particles can be: 39 nm, 16 nm, and 8˜9 nm. The ratio of wt. % is: 956 gm (39 nm):69 gm (16 nm):21 gm (8˜9 nm). Since the nanoparticle material is silicon in this example, the weight ratio is directly proportional to the volume ratio as shown in the table of
Add a liquid silane compound dissolved in an organic solvent, with volume ratio of 5˜15% of the total volume. When liquid silane is added, Si nanoparticles are arranged in two or more classes. For example, 77:32, 77:32:17, or more combinations.
Mix Si nanoparticles with Ge nanoparticles, in a size ratio of 77 (Si):32 (Ge) or 77 (Si):32 (Si):17 (Ge). There are multiple ways to form the mixture. The weight ratio is adjusted according to the density of silicon and germanium, and follows the volume ratio of the table in
Annealing can be performed in an inert environment using a furnace, laser, rapid thermal annealing (RTA), or by flash lamp annealing method.
Although the addition of liquid silane or Ge can help form conduction channels among Si nanoparticles at a much lower temperature, it is also possible to form Si films by arranging the nanoparticles in proper size ratio without the addition of liquid silane or Ge. Si nanoparticles with size ratio of 77:32:17 or 77:32:1.7:12˜14 can be mixed in a dispersion solution and applied onto substrates. Sintering is then performed using one of the above-mentioned annealing methods.
Step 702 provides a plurality of nanoparticle classes, including at least one Si nanoparticle class. The nanoparticles in each nanoparticle class having a predetermined diameter. In one aspect, the diameter tolerance is ±10%. However, the method is not necessarily limited to any particular range of tolerances. Step 704 measures a predetermined amount of each nanoparticle class. Step 706 combines the nanoparticle classes.
In one aspect. Step 705 measures a predetermined amount, of liquid silane, and Step 706 combines a plurality of Si nanoparticle classes with the liquid silane. For example, Step 705 may measure liquid silane with a volume in the range of about 5 to 15%, as compared to the combined volume of the Si nanoparticle classes. Some examples of liquid silane include cyclotrisilane, cyclobutasilane, cyclopentasilane, cyclohexasilane, and cycloheptasilane. Alternately, Step 705 measures a volume of liquid germane in the range of about 0 to 15%, as compared to the combined volume of the Si nanoparticle classes.
In another aspect, Step 702 provides at least one class of germanium (Ge) nanoparticles, and Step 705 measures a predetermined amount of liquid silane. Then, Step 706 combines a Si nanoparticle class, liquid silane, and the Ge nanoparticle class. More explicitly, providing the Si nanoparticle class and the Ge nanoparticle class in Step 702 may includes providing diameter ratio of about 77(Si):32(Ge), or a fourth ratio of about 77(Si):32(Si):17(Ge).
In one example, providing the Si nanoparticle class in Step 702 includes providing a first Si nanoparticle class having a largest diameter and a second Si nanoparticle class having a second-largest diameter equal to about (0.43)×(the largest diameter).
In another example. Step 702 provides Si nanoparticle classes having a of first diameter ratio of about 77:32:17, or a second diameter ratio of about 77:32; 17:D, where D is in a range of about 12-14. To continue the example, measuring the predetermined amount of each Si nanoparticle class (Step 704) includes measuring the first ratio in a corresponding weight % ratio of about 956:69:21.
If Step 804 deposits a Si nanoparticle precursor formed exclusively from Si nanoparticle classes. Then, sintering the Si nanoparticle precursor in Step 806 includes sintering at the first temperature.
In one aspect, depositing the Si nanoparticle precursor in Step 804 includes depositing a Si nanoparticle precursor with a plurality of Si nanoparticle classes and a predetermined amount of liquid silane. Some examples of liquid silane include cyclotrisilane, cyclobutasilane, cyclopentasilane, cyclohexasilane, and cycloheptasilane. Then, sintering the Si nanoparticle precursor in Step 806 includes sintering at a second temperature, less than the first temperature. For example, Step 804 may deposit a volume of liquid silane in the range of about 5 to 15%, as compared to the combined volume of the Si nanoparticle classes.
In another aspect. Step 804 deposits a Si nanoparticle precursor including a predetermined amount of at least one germanium (Ge) nanoparticle class and a predetermined amount of liquid silane. Then, sintering the Si nanoparticle precursor in Step 806 includes sintering at a third temperature, less than the first temperature. Typically, the third temperature is greater than the second temperature.
In one example, Step 804 deposits a first Si nanoparticle class having a largest diameter and a second Si nanoparticle class having a second-largest diameter equal to about (0.43)×(the largest diameter).
As a second example, Step 804 deposits Si nanoparticle classes having a first diameter ratio either about 77:32:17 or a second diameter ratio of about 77:32:17:D, where D is in a range of about 12-14. Alternately, the first ratio may be expressed as a weight % ratio of about 956:69:21.
As a third example, Step 804 may deposit Si nanoparticle classes and a Ge nanoparticle class in a size ratio of either about 7(Si):32(Ge), or about 77(Si):32(Si):17(Ge).
A Si nanoparticle precursor, precursor fabrication process, and precursor deposition process have been presented. Examples of particular size ratios and material combinations have been presented as examples. However, the invention is not necessarily limited to these examples. Other variations and embodiments of the invention will occur to those skilled in the art.
