Patent Application
20050062066
The present invention relates to semiconductor devices and circuits containing semiconductor devices.
Semiconductor devices are typically formed as multilayer structures in which the semiconductor materials are present in a patterned array that defines channels for the transport of charge carriers. For example, an inorganic semiconductor layer may be applied to a dielectric substrate surface. A mask may then be applied to protect regions of the semiconductor layer, intended to constitute charge carrier channels, from a subsequently applied etchant. The etchant then removes the semiconductor in the unmasked regions, leaving behind a finely patterned array of semiconductor channels on the substrate. In the absence of such patterning, the semiconductor devices may be inoperable or be subject to excessive crosstalk.
Inorganic semiconductors typically are rigid and brittle at ambient temperatures. Hence, semiconductor devices formed with inorganic semiconductors generally are rigid as well. As the myriad end use applications for semiconductor devices have evolved, availability of semiconductor devices that can be flexed and bent without damage is desirable. Flexible semiconductor device structures also offer potential capability for bulk processing in the fabrication of large area device arrays, such as a continuous web for example, at low unit costs.
Much work has been done to develop organic semiconductors for applications where flexible semiconductor devices are needed. However, organic semiconductors generally cannot survive the harsh conditions required in order to carry out an etching step to generate a patterned array of channels for charge carrier transport. Printing processes have accordingly been sought in order to directly provide a patterned semiconductor channel array without a need to remove regions of a continuous layer of material. Unfortunately, the fine feature definition that is needed to generate microarrays of semiconductor channels has not been attained by printing of organic semiconductors.
Pentacene, for example, has become an organic semiconductor of great interest due to its high conductivity when formed into a crystalline film. Single dendritic crystals of pentacene having dimensions as large as 2 millimeters long, 1 millimeter wide, and 0.5 millimeter thick have been produced. For example, individual thin film field effect transistors comprising single pentacene crystals have been made with high channel mobilities within a range of between about 1 centimeter squared per volt-second (cm2/Vs) and about 5 cm2/Vs, at room temperature. However, these individual thin film field effect transistors often have low on/off current ratios, and are not integrated into a circuit.
Pentacene is known to have high conductivity when formed on substrates comprising poly(vinylphenol). However, direct patterning of pentacene leads to poorly defined and irregular channel boundaries that cannot be used in fabricating semiconductor devices such as transistors or arrays comprising transistors.
One effort to generate a suitably patterned array of semiconductor channels comprising pentacene involved the direct printing of a pentacene precursor, which was then converted into pentacene. See, for example, Dimitrakopoulos et al. U.S. Pat. No. 5,981,970, entitled, “Thin-film field-effect transistor with organic semiconductor requiring low operating voltages.” However, the performance of semiconductor devices made by this process was unsatisfactory. For example, the process required a high temperature annealing step that degraded the semiconductor devices. Further, two different material patterning methods were needed. The stability of the resulting semiconductor devices was also unreliable.
Another process for forming devices utilizing patterned organic semiconductor films, disclosed in Katz U.S. Pat. No. 6,403,397 issued on Jun. 11, 2002 and entitled “Process For Fabricating Organic Semiconductor Device Involving Selective Patterning,” involved treating a surface to selectively provide regions of greater affinity and lesser affinity for an organic semiconductor or an organic semiconductor solution. When the organic semiconductor, or solution comprising the semiconductor, was deposited on the treated surface, either the organic semiconductor or the organic semiconductor solution dewetted from the lesser affinity regions or the resultant film adhered only weakly to the lesser affinity regions such that selective removal was readily performed. Even where such removal was not performed, the portions of the organic semiconductor film overlying the greater affinity regions exhibited higher conductivity and better film continuity relative to the other portions of the film.
There remains a need for semiconductor devices comprising semiconductors having finely patterned regions of high and low conductivity. There further is a need for methods of making semiconductor devices employing semiconductors that are not easily patterned.
The present invention provides semiconductor apparatus in which regions of high and low conductivity are induced in a semiconductor by regions of materials placed in contact with the semiconductor, and methods of making such semiconductor apparatus. Exemplary embodiments employ organic semiconductors and utilize polymers as such materials. Semiconductor devices can be made with organic and inorganic semiconductors according to the present invention without patterned placement or removal of the semiconductors.
In one embodiment according to the present invention, a semiconductor apparatus is provided, comprising: a substrate having a substrate surface; a layer of a first material overlying a first region of the substrate surface; a layer of a semiconductor overlying the layer of first material and overlying a second region of the substrate surface; a first region of the layer of semiconductor, overlying the layer of first material and having a first conductivity; a second region of the layer of semiconductor, overlying the second region of the substrate surface and having a second conductivity; and the first conductivity being substantially different from the second conductivity.
In another embodiment according to the present invention, such a semiconductor apparatus is provided, further comprising a layer of a second material overlying the second region of the substrate surface, the second region of the layer of semiconductor overlying the layer of the second material. In an additional embodiment according to the present invention, such a semiconductor apparatus is provided in which the layer of first material overlies the second region of the substrate surface.
In a further embodiment according to the present invention, such a semiconductor apparatus is provided in which the conductivity in the first region of the layer of semiconductor is at least about 100 times as large as the conductivity in the second region of the layer of semiconductor. In another embodiment according to the present invention, such a semiconductor apparatus is provided in which the layer of semiconductor comprises crystal grains, and in which the average crystal grain size within the first region of the layer of semiconductor is at least about 10 times as large as the average crystal grain size within the second region of the layer of semiconductor. In an additional embodiment according to the present invention, such a semiconductor apparatus is provided in which the layer of semiconductor comprises crystal rains, and in which the average separation between crystal grains within the second region of he layer of semiconductor is at least about 10 times as large as the average separation between crystal grains within the first region of the layer of semiconductor.
In a further embodiment according to the present invention, a method of making a semiconductor apparatus is provided, comprising the steps of: providing a substrate having a substrate surface; providing a layer of a first material overlying a first region of the substrate surface; and providing a layer of a semiconductor overlying the layer of first material and overlying a second region of the substrate surface; a first region of the layer of semiconductor, overlying the layer of first material and having a first conductivity; a second region of the layer of semiconductor, overlying the second region of the substrate surface and having a second conductivity; and the first conductivity being substantially different from the second conductivity.
In another embodiment according to the present invention, such a method of making a semiconductor apparatus is provided, further comprising the step of providing a layer of a second material overlying the second region of the substrate surface, the second region of the layer of semiconductor overlying the layer of the second material. In an additional embodiment according to the present invention, such a method of making a semiconductor apparatus is provided, in which the layer of first material overlies the second region of the substrate surface.
In further embodiments according to the present invention, integrated circuits comprising semiconductor apparatus, and methods for making such integrated circuits, are provided.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following detailed description and the accompanying drawings.
The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:
The drawings of this specification are not to scale but are merely schematic representations, and thus are not intended to portray the specific dimensions of the various embodiments according to the invention, which may be determined by skilled artisans through examination of the disclosure herein.
The present invention will now be described more fully with reference to the accompanying drawings, in which several presently preferred embodiments of the invention are shown. This invention may, however, be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
The present invention provides active semiconductor devices, in which regions of relatively high and relatively low conductivity are induced in the semiconductor by a selected adjacent layer.
The terms “overlying”, “overlies”, “overlie” and the like as used in the present specification mean and refer to the relative orientation of the subject structural elements to each other, which elements may either be in direct mutual contact or may be separated by additional interposed elements as in context are indicated. For example, designated regions of exemplary semiconductor layer 110 may overlie and be in direct contact with exemplary insulating substrate 124. Alternatively, designated regions of exemplary semiconductor layer 110 may overlie exemplary insulating substrate 124, but be separated from direct contact with such substrate by interposed exemplary high conductivity inducing layer region 120 or 122. Analogous structures, for example, comprise interposed exemplary low conductivity inducing layer regions, as discussed below.
In one exemplary embodiment according to the present invention, the semiconductor layer 110 is formed by an organic semiconductor. Organic semiconductors are compounds comprising hydrocarbon moieties, and compositions comprising such compounds, that can be formed into a structure that is capable of transporting charge carriers. Pentacene is one specific exemplary organic compound that is capable of so transporting charge carriers. Exemplary embodiments according to the present invention will be discussed in the context of pentacene as an organic semiconductor. However, it will be understood that the teachings according to the present invention likewise extend to other organic semiconductors as well as to inorganic semiconductors, and to mixed and hybrid organic and inorganic semiconductors. Conductivity in an organic semiconductor is dependent upon its crystallinity. Electrical conductivity is given by the formula σ=enμd where μd is the electron mobility, e is the charge on the electron, and n is the density of free electrons. Conductivity accordingly is proportional to mobility. Mobility can readily be measured. For example, a polycrystalline film of pentacene provides a low mobility, within a range between about 0.3×10−7 cm2/Vs and about 1.5×10−5 cm2/Vs at room temperature. In contrast, where crystallization of pentacene is encouraged as by its thermal evaporation in a vacuum or by growing a pentacene film from the vapor phase in a stream of inert gas, mobilities within a range between about 1 cm2/Vs and about 5 cm2/Vs at room temperature can be achieved.
Conductivity in an organic semiconductor depends on the size and separation of crystal grains. The size distribution of crystal grains determines how many of them must be effectively traversed by a charge carrier in order to be transported from an origin to a destination. The separation between crystal grains determines the impact of non crystalline regions on conductivity. For example, crystal grains separated by a distance greater than the tunneling limit for a particular material may constitute a nonconductive pathway for charge carriers. The tunneling limit is that distance, determined by the uncertainty principle of quantum mechanics, across which molecules may move over time in relation to each other without application of external forces.
Conductivity within a crystal grain of an organic semiconductor depends on charge carrier energy levels and molecular overlaps in the crystal. Energy levels of charge carriers at the boundaries between adjacent crystals along a pathway cannot interpose undue burdens to the transport of charge carriers.
Referring to
In one exemplary embodiment according to the present invention, the semiconductor layer 110 shown in
In operation of the thin film transistor 102, a voltage applied to the gate electrode 112 can be used to control a flow of charge carriers between the source electrode 106 and the drain electrode 108. Similarly, in operation of the thin film transistor 104, a voltage applied to the gate electrode 118 can be used to control a flow of charge carriers between the source electrode 114 and the drain electrode 116. In thin film transistor 102, charge carriers are transported between the source electrode 106 and the drain electrode 108 through the several layers of pentacene molecules nearest the interface 132 between the high conductivity inducing layer region 120 and the semiconductor layer 110. Similarly in thin film transistor 104, charge carriers are transported between the source electrode 114 and the drain electrode 116 through the several layers of pentacene molecules nearest the interface 134 between the high conductivity inducing layer region 122 and the semiconductor layer 110. Although these several layers of pentacene molecules typically are not monocrystalline, they nevertheless have substantially increased grain size and substantially reduced grain separation as compared with a pentacene film deposited directly onto a conventional insulating substrate.
Referring to
In one exemplary embodiment according to the present invention, the semiconductor layer 310 shown in
The high conductivity inducing layer regions 520 and 521 are formed of dielectric materials that increase the conductivity of the semiconductor layer 510 in the semiconductor regions 528 and 530, respectively. The resulting enhanced transconductivity increases the capacity of the thin film transistors 502 and 504 to transport charge carriers, thus directly improving the performance of such thin film transistors 502 and 504. Regions 522, 523 and 524 of low conductivity inducing layer 519 are formed of dielectric materials that decrease the conductivity of the semiconductor layer 510 outside of the semiconductor regions 528 and 530. The resulting decreased conductivity of the semiconductor layer 510 results in decreased transconductivity outside of the semiconductor regions 528 and 530. The capability of the semiconductor layer 510 overlying regions 522, 523 and 524 of the low conductivity inducing layer 519 to transport charge carriers accordingly is reduced. Low conductivity inducing layer region 523, for example, is interposed between the insulating substrate 525 and a region 540 of the semiconductor layer 510 that is between the drain electrode 508 of thin film transistor 502 and the source electrode 514 of thin film transistor 504. The reduced capability of region 540 of the semiconductor layer 510 to transport charge carriers results in less crosstalk through the semiconductor layer 510 between thin film transistors 502 and 504. Hence, the performance of the thin film transistors 502 and 504 is improved both by reducing crosstalk and by increasing the capacity of the thin film transistors 502 and 504 to transport charge carriers.
In one exemplary embodiment according to the present invention, the semiconductor layer 510 shown in
The high conductivity inducing layer regions 720 and 722 are formed of dielectric materials that increase the conductivity of the semiconductor layer 710 in the semiconductor regions 728 and 730, respectively. The resulting enhanced transconductivity increases the capacity of the thin film transistors 702 and 704 to transport charge carriers, thus directly improving the performance of such thin film transistors 702 and 704. Low conductivity inducing layer 726 makes contact with regions 732, 734 and 736 of semiconductor layer 710. Regions 728 and 730 of the semiconductor layer 710 are protected from such contact by the interposed high conductivity inducing layer regions 720 and 722. Low conductivity inducing layer 726 can be a continuous layer overlying the insulating substrate 724 and covering the gate electrodes 712 and 718, potentially providing added flexibility in fabrication procedures. The low conductivity inducing layer 726 is formed of dielectric materials that decrease the conductivity of the semiconductor layer 710 in regions 732, 734 and 736. The resulting decreased conductivity of the semiconductor layer 710 results in decreased transconductivity in regions 732, 734 and 736 of semiconductor layer 710. The capability of regions 732, 734 and 736 of the semiconductor layer 710 to transport charge carriers accordingly is reduced. Hence, the performance of the thin film transistors 702 and 704 is improved both by reducing crosstalk and by increasing the capacity of the thin film transistors 702 and 704 to transport charge carriers.
The high conductivity inducing layer 920 makes direct contact with regions 922 and 924 of semiconductor layer 910. The high conductivity inducing layer 920 is formed of dielectric materials that increase the conductivity of the semiconductor layer 910 in regions 922 and 924. The resulting increased conductivity of the semiconductor layer 910 results in increased transconductivity in regions 922 and 924 of semiconductor layer 910. High conductivity inducing layer 920 can be a continuous layer overlying the insulating substrate 925 and covering the gate electrodes 912 and 918, potentially providing added flexibility in fabrication procedures. The low conductivity inducing layer regions 932, 934 and 936 are formed of dielectric materials that decrease the conductivity of the semiconductor layer 910 in the low conductivity semiconductor regions 926, 928 and 930, respectively. The resulting decreased conductivity of the semiconductor layer 910 results in decreased transconductivity outside of the semiconductor regions 922 and 924. The capability of the semiconductor layer 910 in the low conductivity semiconductor regions 926, 928 and 930 to transport charge carriers accordingly is reduced. Low conductivity semiconductor region 928, for example, is located between the drain electrode 908 of thin film transistor 902 and the source electrode 914 of thin film transistor 904. Hence, the performance of the thin film transistors 902 and 904 is improved both by reducing crosstalk and by increasing the capacity of the thin film transistors 902 and 904 to transport charge carriers.
The embodiments shown in
Further embodiments according to the invention will be discussed below in connection with thin film transistors. However, it is to be understood that the teachings according to the present invention can be further extended to devices other than thin film transistors that require lateral patterning of a semiconductor film or layer into regions of relatively high and low conductivity. For example, diodes can be produced in an analogous manner. Other exemplary semiconductor electronic devices that can be fabricated by use of the teachings according to the present invention include light emitting devices, photodetecting devices, photovoltaic cells, photoconductive cells, photoresistive cells, photodiodes, and photoswitches.
Appropriate materials for forming high and low conductivity inducing layers to be employed in making thin film transistors according to the present invention are selected depending upon the composition of the semiconductor to be employed. Materials for forming high conductivity inducing layers are selected that encourage crystal grain growth in an adjacent layer of the semiconductor. Materials for forming low conductivity inducing layers are selected that impede and reduce crystal grain size in an adjacent layer of the semiconductor. Where materials for forming high and low conductivity inducing layers are both employed, the affinity of the chosen semiconductor for such respective materials may be uniform or different as desired. Affinity is a measure of the ability of two materials to bind together, and does not affect crystal grain growth.
In one embodiment according to the present invention, the selected semiconductor is pentacene. Pentacene, an organic compound, is a linear acene having five rings. Films of pentacene are capable of transconductive transport of holes. Pentacene has the highest mobility among the linear acenes. Although the mobility of pentacene in polycrystalline organic thin films is within a low range between about 0.3×10−7 cm2/Vs and about 1.5×10−5 cm2/Vs at room temperature, the mobility of pentacene in monocrystalline organic thin films is within a high range between about 1 cm2/Vs and about 5 cm2/Vs. In another embodiment according to the present invention, another linear or nonlinear acene is substituted for pentacene.
Suitable polymers for making layers for use in contact with portions of an organic semiconductor layer comprising pentacene to induce high transconductivity include, for example, poly(para-vinyl phenol), poly(4-vinylpyridine), and poly(2-vinylnaphthalene). Poly(para-vinyl phenol) is also known as poly(hydroxystyrene). Poly(meta-vinyl phenol) or poly(ortho-vinyl phenol) may also be effective. Alternatively, poly(para-vinyl phenol-co-2-hydroxyethylmethacrylate) can be employed. In addition to poly(4-vinylpyridine), poly(2-vinylpyridine) may also be effective. In addition to poly(2-vinylnaphthalene), poly(2-vinylnaphthalene-co-2-ethylhexyl acrylate) can be used. In addition to poly(2-vinylnaphthalene), poly(1-vinylnaphthalene) may also be effective. Blends of these polymers, and of other suitable polymers, if available, can also be used. The term “blends” as used in this specification broadly encompasses and includes copolymers formed from the corresponding monomers or oligomers, mixtures of polymers, and other compositions comprising the monomers, however produced.
Suitable polymers for making layers for use in contact with portions of an organic semiconductor layer comprising pentacene to induce low transconductivity include, for example, poly(n-butyl methacrylate), poly(vinylidene difluoride-co-methyl vinyl ether), polystyrene, poly(p-methoxystyrene), poly(vinylidene difluoride), poly(vinyl acetate), poly(vinyl propionate) and poly(methoxy acetate). In one embodiment according to the present invention, poly(vinylidene difluoride-co-methyl vinyl ether) comprising about 85% by weight of vinylidene difluoride and about 15% by weight of methyl vinyl ether, having a dielectric constant of about 10, is employed. In addition to poly(n-butyl methacrylate), a polymer such as poly(n-propyl methacrylate), poly(isopropyl methacrylate), or poly(n-pentyl methacrylate) may also be effective. Acrylates, ethacrylates, and halogenated analogs of poly(n-butyl methacrylate) may further be effective. In addition to poly(vinylidene difluoride-co-methyl vinyl ether), a polymer such as poly(vinylidene difluoride-co-ethyl vinyl ether) or poly(vinylidene difluoride-co-propyl vinyl ether) may be effective. Blends of these polymers, blends being broadly defined as above, and of other suitable polymers, if available, can also be used.
In another embodiment according to the present invention, a polymer for making layers to be placed in contact with portions of an organic semiconductor layer comprising pentacene to induce low transconductivity is selected for its high content of charge carrier traps. Such a polymer, when interposed between selected regions of a substrate surface and an overlying semiconductor layer, effectively reduces the conductivity of those portions of the semiconductor layer overlying the selected regions. For example, poly(dimethylaminoethyl methacrylate), poly(dimethylaminopropyl methacrylate), poly(aminopropyl methacrylate) and poly(diethylaminoethyl methacrylate) can be so used. Each of these polymers has a high concentration of amino groups and a glass transition temperature approximating that of poly(n-butyl methacrylate). The polymers discussed above for making layers that induce high and low conductivity in films comprising pentacene are commercially available from the Sigma-Aldrich Chemical Company, St. Louis, Mo.
In one embodiment according to the present invention, thin film transistors 102 and 104 shown in
At step 1110, gate electrodes 112 and 118 are provided on the insulating substrate 124. For example, a steel shadow mask can be placed over the surface of the insulating substrate 124, leaving exposed those portions of the surface where gate electrodes 112 and 118 are to be located. The insulating substrate 124 is then placed in a suitable vacuum chamber such as a bell jar, which is then evacuated. A source of aluminum metal is also placed in the vacuum chamber and heated to vaporize and deposit aluminum over the shadow mask to produce the gate electrodes 112 and 118. Other suitable materials for fabricating gate electrodes can be substituted for aluminum. For example, gold, silver, gallium, indium, platinum, nickel, titanium, copper, or conductive ceramics such as indium tin oxide can be used. Any metal that can be evaporated or sputtered can be used for the gate.
At step 1115, high conductivity inducing layer regions 120 and 122 are provided on the insulating substrate 124, centered over the gate electrodes 112 and 118, respectively. A suitable solution of a high conductivity inducing polymer is needed, as the high conductivity inducing polymers discussed above generally are commercially provided in a powder form. For example, a selected polymer such as poly(4-vinylpyridine) is suitably purified, such as by reprecipitation or chromatography, and dissolved in a suitable solvent. A solution comprising between about 5% and about 10% of poly(4-vinylpyridine) in methyl ethyl ketone, for example, can be prepared. The polymer solution can, for example, be applied to the insulating substrate 124 by a spin coating process. A poly(4-vinylpyridine) film is spin coated on the insulating substrate 124, having a thickness, for example, of about one micron. The film of poly(4-vinylpyridine) is then removed from the insulating substrate 124 except for portions to constitute the high conductivity inducing layer regions 120 and 122, for example by a wiping the polymer away with a swab dipped in methyl ethyl ketone. The gate electrodes 112 and 118 are then exposed by the same wiping process.
In another embodiment according to the present invention, a lamination process is substituted for the spin coating process in order to deposit the high conductivity inducing polymer in step 1115. Suitable lamination processes for this purpose are disclosed in U.S. patent application Ser. No. ______, filed concurrently herewith, entitled “Process for Laminating a Dielectric Layer into a Semiconductor.” This patent application is assigned to E. I. du Pont de Nemours and Company, docket No. CL-2181, and is hereby incorporated herein by reference in its entirety. It is to be understood that such lamination processes can be used in substitution for spin coating processes in all of the instances where spin coating is discussed in this specification.
At step 1120, a layer 110 of pentacene is provided on the high conductivity inducing layer regions 120 and 122, and on the exposed portions of the insulating substrate 124. The pentacene is applied by a process that is suitable to allow the high conductivity inducing layer regions 120 and 122 to affect the crystallinity of the applied pentacene. For example, pentacene can be applied onto the high conductivity inducing layer regions 120 and 122 by employing a vacuum sublimation process. The insulating substrate 124 having the high conductivity inducing layer regions 120 and 122 is placed in a suitable vacuum chamber such as a bell jar, which is then evacuated. A source of pentacene is also placed in the vacuum chamber and heated to sublimate the pentacene and deposit a pentacene layer 110 over the high conductivity inducing layer regions 120 and 122 and over the exposed portions of the insulating substrate 124. High conductivity regions 126 and 128 are thus generated within the pentacene layer 110.
At step 1125, source electrodes 106 and 114, and drain electrodes 108 and 116, are provided on the semiconductor layer 110 in alignment over the high conductivity inducing layer regions 120 and 122. For example, a steel shadow mask can be placed over the surface of the semiconductor layer 110, leaving exposed those portions of the surface where the source electrodes 106 and 114 and the drain electrodes 108 and 116 are to be located. The insulating substrate 124 is then placed in a suitable vacuum chamber such as a bell jar, which is then evacuated. A source of gold metal is also placed in the vacuum chamber and heated to vaporize and deposit gold over the shadow mask to produce the source electrodes 106 and 114 and the drain electrodes 108 and 116.
In another embodiment according to the present invention, thin film transistors 302 and 304 shown in
At step 1210, gate electrodes 312 and 318 are provided on the insulating substrate 324 in the same manner as discussed above in connection with
At step 1215, low conductivity inducing polymer layer regions 321, 322 and 323 are provided on the insulating substrate. A suitable solution of a low conductivity inducing polymer is needed, as the low conductivity inducing polymers discussed above generally are commercially provided in a powder form. For example, a selected polymer such as poly(n-butyl methacrylate), is dissolved in a suitable solvent. A solution comprising between about 5% and about 10% of poly(n-butyl methacrylate) in methyl ethyl ketone, for example, can be prepared. The polymer solution can, for example, be applied to the insulating substrate 324 by the same spin coating process that was discussed above in connection with
In the embodiment shown in
At step 1220, gate dielectric regions 326 and 330 are provided on the insulating substrate 324. The function of gate dielectric regions 326 and 330 is to insulate the gate electrodes from the semiconductor layer 310. Accordingly, any organic or inorganic dielectric material that is compatible with the other materials with which it comes into contact in thin film transistors 302 and 304 can be used. In one exemplary embodiment according to the present invention, the gate dielectric layers 326 and 330 can be made barely large enough to effectively cover the gate electrodes 312 and 318. In another exemplary embodiment according to the present invention, the gate electrodes 312 and 318 are fabricated from a metal provided with an oxidized surface, and the oxidized surface serves as gate dielectric layers 326 and 330. For example, the gate electrodes 312 and 318 can be fabricated from aluminum, and the exposed surfaces are anodized to provide them with a dielectric coating.
In the embodiment shown in
At step 1225, a layer 310 of pentacene is provided on the low conductivity inducing layer regions 321, 322 and 323, on the gate dielectric regions 326 and 330, and on the exposed portions of the insulating substrate 324. The pentacene can be applied using the same process as discussed above in connection with
At step 1230, source electrodes 306 and 314, and drain electrodes 308 and 316, are provided on the semiconductor layer 110 in alignment over the gate electrodes 312 and 318, respectively. These source electrodes 306 and 314, and drain electrodes 308 and 316 can be prepared in the same manner as was discussed above in regard to
In another embodiment 1300 according to the present invention, thin film transistors 502 and 504 shown in
At step 1310, gate electrodes 512 and 518 are provided on the insulating substrate 524 in the same manner as discussed above in connection with
At step 1315, low conductivity inducing polymer layer regions 522, 523 and 524 are provided on the insulating substrate in the same manner as discussed above in regard to
At step 1320, high conductivity inducing layer regions 520 and 521 are provided on the insulating substrate 525. The high conductivity inducing layer regions 520 and 521 may as shown in
In the embodiment shown in
At step 1325, a layer 510 of pentacene is provided on the high conductivity inducing layer regions 520 and 521, and on the exposed portions of the low conductivity inducing layer regions 522, 523 and 524. The pentacene is applied using the same process as discussed above in connection with
At step 1330, source electrodes 506 and 514, and drain electrodes 508 and 516, are provided on the semiconductor layer 510 in alignment over the gate electrodes 512 and 518, respectively. These source electrodes 506 and 514, and drain electrodes 508 and 516 can be prepared in the same manner as was discussed above in regard to
In another embodiment according to the present invention, thin film transistors 702 and 704 shown in
At step 1410, gate electrodes 712 and 718 are provided on the insulating substrate 724 in the same manner as discussed above in connection with
At step 1415, a low conductivity inducing polymer layer 726 is provided on the insulating substrate. A suitable solution of a low conductivity inducing polymer such as poly(n-butyl methacrylate) is provided in the same manner as discussed above in connection with
At step 1420, high conductivity inducing layer regions 720 and 722 are provided on the low conductivity inducing polymer layer 726. A suitable solution of a high conductivity inducing polymer such as poly(4-vinylpyridine), is provided in the same manner as discussed above in connection with
At step 1425, a layer 710 of pentacene is provided on the high conductivity inducing layer regions 720 and 722, and on the exposed portions of the low conductivity inducing polymer layer 726. The pentacene is applied by the same process as discussed above, in a manner that is suitable to allow the high conductivity inducing layer regions 720 and 722 as well as the exposed portions of the low conductivity inducing polymer layer 726 to affect the crystallinity of the applied pentacene. High conductivity regions 728 and 730 and low conductivity regions 732, 734 and 736 are thus generated within the pentacene layer 710.
At step 1430, source electrodes 706 and 714 and drain electrodes 708 and 716 are provided on the semiconductor layer 710 overlying the high conductivity inducing layer regions 720 and 722. These electrodes may also partially overlie the portions of the low conductivity inducing polymer layer 726 that are in contact with the semiconductor layer 710. The source electrodes 706 and 714 and drain electrodes 708 and 716 can be prepared in the same manner as was discussed above in regard to
In another embodiment according to the present invention, thin film transistors 902 and 904 shown in
At step 1510, gate electrodes 912 and 918 are provided on the insulating substrate 925 in the same manner as discussed above in connection with
At step 1515, a high conductivity inducing polymer layer 920 is provided on the insulating substrate. A suitable solution of a high conductivity inducing polymer such as poly(4-vinylpyridine) is provided in the same manner as discussed above in connection with
At step 1520, low conductivity inducing layer regions 932, 934 and 936 are provided on the high conductivity inducing polymer layer 920. A suitable solution of a low conductivity inducing polymer such as poly(n-butyl methacrylate) is provided in the same manner as discussed above in connection with
At step 1525, a layer 910 of pentacene is provided on the low conductivity inducing layer regions 932, 934 and 936, and on the exposed portions of the high conductivity inducing polymer layer 920. The pentacene is applied by the same process as discussed above, in a manner that is suitable to allow the low conductivity inducing layer regions 932, 934 and 936 as well as the exposed portions of the high conductivity inducing polymer layer 920 to affect the crystallinity of the applied pentacene. High conductivity regions 922 and 924 and low conductivity regions 926, 928 and 930 are thus generated within the pentacene layer 910.
At step 1530, source electrodes 906 and 914 and drain electrodes 908 and 916 are provided on the semiconductor layer 910 overlying regions of the high conductivity inducing polymer layer 920 that are in contact with the semiconductor layer 910. These electrodes may also partially overlie low conductivity inducing polymer layer regions 932, 934 and 936. The source electrodes 906 and 914 and drain electrodes 908 and 916 can be prepared in the same manner as was discussed above in regard to
In another embodiment according to the present invention, thin film transistors 1002 and 1004 shown in
At step 1625, source electrodes 1006 and 1012 and drain electrodes 1008 and 1014 are provided on the high conductivity inducing layer regions 1010 and 1016. The source electrodes 1006 and 1012 and drain electrodes 1008 and 1014 can be prepared in the same manner as was discussed above in regard to
At step 1630, a layer 1010 of pentacene is provided on the high conductivity inducing layer regions 1010 and 1016, on the source electrodes 1006 and 1012 and drain electrodes 1008 and 1014, and on the exposed portions of the low conductivity inducing polymer layer 1024. The pentacene is applied by the same process as discussed above, in a manner that is suitable to allow the high conductivity inducing layer regions 1010 and 1016 as well as the exposed portions of the low conductivity inducing polymer layer 1024 to affect the crystallinity of the applied pentacene. High conductivity regions 1028 and 1030 and low conductivity regions 1032, 1034 and 1036 are thus generated within the pentacene layer 1010.
Appropriate polymers for fabricating layers and regions that induce high and low conductivity in an adjacent semiconductor layer are selected for a particular semiconductor chosen for use. In one embodiment according to the present invention, given a voltage of −50V from source to drain across a test transistor, the mobility in the high conductivity regions of the semiconductor is desirably at least about 0.01 cm2/Vs, preferably at least about 0.1 cm2/Vs, and more preferably at least about 1.0 cm2/Vs, at room temperature. The structure of the high conductivity regions of the semiconductor desirably has an average crystal grain size of at least about 50 nanometers, and an average grain separation of less than about 20 angstroms. In one embodiment according to the present invention, the high conductivity regions of the semiconductor have an average crystal grain size of less than about 500 nanometers. In one embodiment according to the present invention, the mobility in the low conductivity regions of the semiconductor is desirably within a range between about 0.3×10−7 cm2/Vs and about 1.5×10−5 cm2/Vs at room temperature. In one embodiment according to the present invention, the conductivity in the high conductivity regions of the semiconductor is at least about 100 times as large as the conductivity in the low conductivity regions of the semiconductor. In another embodiment according to the present invention, the average crystal grain size within the high conductivity regions of the semiconductor desirably is at least about 10 times as large the average crystal grain size within the low conductivity regions of the semiconductor. In a further embodiment according to the present invention, the average separation between crystal grains within the low conductivity regions of the semiconductor desirably is at least about 10 times as large the average separation between crystal grains within the high conductivity regions of the semiconductor.
In one embodiment according to the present invention, a raster scanning method is employed to deposit a uniform coating of a selected semiconductor material onto a substrate having test regions of selected high and low conductivity inducing polymers. This method provides control over the processing conditions and facilitates testing of the compatibility and performance of the selected materials. The substrate can be provided with test regions of selected high and low conductivity inducing polymers as detailed in steps 1305, 1310 and 1320 discussed above in connection with
According to the raster scanning method, a source of semiconductor material is provided, such as pentacene for example. The semiconductor material is then heated to a suitable vaporization temperature and is sprayed onto the test regions of selected high and low conductivity inducing polymers by the raster scanning method. A spray source such as a nozzle is indexed back and forth in a zigzag raster pattern over the surfaces of the test regions of selected high and low conductivity inducing polymers in order to systematically cover such surfaces with a uniform coating of the selected semiconductor. Control of the volume over unit time of the heated semiconductor material that is emitted from the spray source, and control over the raster pattern and speed, further allow control over the thickness of the semiconductor coating layer. The distance between the spray source and the surfaces of the test regions of selected high and low conductivity inducing polymers can be controlled for uniformity in preparing a test device, and can be systematically varied from one test device to another. In this manner, uniformity of the semiconductor coating in a given test device is encouraged, and the optimum distance between the spray source and the surfaces of the test regions for the selected materials can readily be determined. Further, the temperature of the surfaces of the test regions of the polymers and of the semiconductor material upon contact with the surfaces of the test regions of selected high and low conductivity inducing polymers can be controlled in order to adjust the resulting crystallinity of the semiconductor coating.
In one embodiment according to the present invention, high and low conductivity inducing polymer layers and regions are provided on insulating substrates by a process comprising laser induced thermal transfer imaging. For example, a laser can be used to induce the patterned transfer of a high or low conductivity inducing polymer material from a donor film to a receiver film. Each type of polymer, inducing either high or low conductivity, typically is individually printed in a separate printing step, and one donor film at a time is printed. In another embodiment according to the present invention, multiple polymers are simultaneously printed. A suitable donor film may comprise a MYLAR polyethylene terephthalate base having a thickness of 2 mils, having a semitransparent coating of sputtered chromium, nickel or aluminum with a thickness that allows transmittance of about 50% of visible light. The desired high or low conductivity inducing polymer to be transfer printed is then suitably coated onto the sputtered metal surface of the donor film. The receiver film may be constituted by a film of the selected insulating substrate material, and may comprise a supportive backing for the substrate that can later be removed. The donor and receiver films are then assembled together with the desired high or low conductivity inducing polymer facing the substrate. The assembled films are then sized and fitted onto a printer cylinder with the donor film facing out. Alternatively, if the receiver film is transparent then the receiver film can face out. Typically, the receiver film is wrapped around the drum. Then the donor film is wrapped around the receiver. The laser is directed in through the back of the donor film. In this manner, a donor film can be unloaded and another donor film can then be loaded in its place. The printer cylinder may, for example, have a diameter of about 14 inches.
A laser head is provided adjacent to the printer cylinder, aimed to direct laser light onto the cylinder surface through the assembled films. The laser head may, for example, include 200 lasers having a collective beam width of 500 microns, providing a resolution of 2×5 microns. The exemplary laser spot size has a length of 5 microns across the head, and a width of 2 microns in the direction of printer cylinder rotation. The laser head includes a lead screw for guiding the laser head over the longitudinal axis of the printer cylinder. In operation of the printer, the cylinder turns at a selected rotation rate per unit time, and the lead screw systematically advances the laser head over the printer cylinder surface in a barber pole fashion. The lasers within the laser head can be individually energized for precise pattern control. As the laser head is guided over the surface of the printer cylinder, laser light is directed in a precise desired pattern corresponding to the desired high or low conductivity inducing polymer layers or regions on the insulating substrate. Typically, the precise pattern is computer generated. In the precise areas where laser energy enters the donor film, the high or low conductivity inducing polymer is transferred onto the insulating substrate. Fine accuracy of the printing pattern is needed. This accuracy can be achieved by proper programming of the computer guidance systems. Further advantageous embodiments of laser induced thermal transfer imaging processes that can be employed in practice of various aspects of the present invention are disclosed in U.S. patent application Ser. No. ______, filed concurrently herewith, entitled “Method for Increasing Mobility of Vapor Deposited Pentacene.” This patent application is assigned to E. I. du Pont de Nemours and Company, docket No. CL-2447, and is hereby incorporated by reference herein in its entirety.
Processes and apparatus relating to laser induced thermal transfer imaging are disclosed in the following patent documents for example, which are hereby incorporated by reference herein in their entirety: Blanchet-Fincher U.S. Pat. No. 5,192,580; Blanchet-Fincher U.S. Pat. No. 5,288,528; Blanchet-Fincher U.S. Pat. No. 5,523,192; Blanchet-Fincher U.S. Pat. No. 5,563,019; Blanchet-Fincher U.S. Pat. No. 5,766,819; Blanchet-Fincher U.S. Pat. No. 5,840,463; Blancher-Fincher U.S. Pat. No. 6,143,451; Blancher-Fincher et al. U.S. Pat. No. 6,146,792; Blanchet-Fincher U.S. Patent Pub. No. 2002/0149315; Blanchet-Fincher et al. PCT published application WO 01/87634 A2 dated Nov. 22, 2001; Blanchet-Fincher et al. PCT published application WO 02/08801 A1 dated Jan. 31, 2002; and Blanchet-Fincher et al. PCT published application WO 02/092352 A1 dated Nov. 21, 2002. Apparatus for carrying out laser induced thermal transfer imaging are commercially available from Creo Inc., 3700 Gilmore Way, Burnaby, British Columbia, V5G 4M1, Canada. For example, Creo Trendsetter thermal platesetters can be employed.
Optimization of laser induced thermal transfer imaging processes includes proper formulation of the polymer materials to be transfer printed. Variables to be addressed regarding the polymers include glass transition temperature (Tg) and melt viscosity. For example, polymers having a glass transition temperature within a range between about −20° C. and about 100° C. may provide adequate film stability while avoiding excessive film brittleness. Further for example, polymers having a melt viscosity within a range between about 10 centipoise (CPS) and about 100 CPS may provide adequate workability for coating onto a donor film while having adequate body to form a layer of a desired thickness. Polymer melt viscosity generally increases with increasing average molecular weight. A desired film thickness may be about one micron, for example. Variables to be addressed regarding solvent systems for the polymers include substrate wetting, drying performance, and polymer dispersion or dissolution. Suitable solvent systems are able to wet the substrate surfaces of both donor and receiver films, are able to evaporate to yield a uniform dry film having a surface area of at least about a square foot within a reasonable working time, allow preparation of a polymer layer of a desired thickness, and either disperse or dissolve the polymer. A given solvent wets a substrate if the contact angle of a drop of solvent on the substrate is less than about 90°. Organic solvents or surfactants added to aqueous systems are generally needed in order to wet the sputtered metal surface of the donor film. Variables to be addressed regarding the resulting polymer films include strain to break, surface energy and cohesive energy. In order for the polymer coating to be discontinuously transferred from the donor film to the recipient film, the polymer coating must be able to cleanly break without stretching or distortion at the boundaries defined by the laser head as it is indexed over the cylinder surface. Various materials can be added to the polymers and solvent system of a composition to be used in furtherance of such optimization. For example, copolymers, coating aids, plasticizers, flow additives, slip agents, anti-halation agents, antistatic agents, surfactants, dispersants, and other agents known to be used in the formulation of coatings may be added.
A series of test transistors were fabricated in accordance with the present invention and tested for conductivity. For the preparation of each test transistor, a silicon dioxide glass slide was provided to serve as the insulating substrate. A surface of the silicon dioxide slide was thoroughly cleaned in the same manner as discussed above in connection with
The test results in Table 1 showed in series 1-5 that the conductivity for a pentacene semiconductor layer was relatively high when deposited over poly(4-vinylpyridine) or poly(para-vinylphenol), and relatively low when deposited over poly(vinylidene difluoride-co-methyl-vinyl-ether) or poly(n-butyl methacrylate). The test results further showed in series 6-8 that the conductivity for a 5,5′-bis-(4-ethylphenyl)2,2′-bithiophene semiconductor layer was relatively high when deposited over poly(4-vinylpyridine), and relatively low when deposited over poly(vinylidene difluoride-co-methyl-vinyl-ether) or poly(n-butyl methacrylate). The test results also showed in series 9-11 that the conductivity for a 5,5′-bis-(8-hexylfluorine-1-yl)2,2′-bithiophene semiconductor layer was relatively high when deposited over poly(4-vinylpyridine), and relatively low when deposited over poly(vinylidene difluoride-co-methyl-vinyl-ether) or poly(n-butyl methacrylate).
While the present invention has been disclosed in the context of various aspects of presently preferred embodiments, it will be recognized that the invention may be suitably applied to other environments consistent with the claims which follow.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owners to license others on reasonable terms as provided for by the terms of Advanced Technology Program Cooperative Agreement No. 70NAN1B2H3032 awarded by the National Institute of Standards and Technology.
