Abstract:
A thin-film switching device includes an active region including noncrystalline silicon, e.g., hydrogenated amorphous silicon, which includes chlorine distributed in a manner which produces a predetermined photoconductivity and a predetermined field-effect mobility in the active region. Preferably, the active region includes a plurality of hydrogenated amorphous silicon layers, at least one of which includes chlorine. In one embodiment, the plurality of hydrogenated amorphous silicon layers includes a hydrogenated amorphous silicon layer including between 0.1 ppm and 106 ppm chlorine. In another embodiment, the plurality of hydrogenated amorphous silicon layers includes a first hydrogenated amorphous silicon layer having a first chlorine concentration and a second hydrogenated amorphous silicon layer having a second chlorine concentration less than the first chlorine concentration. The first hydrogenated amorphous silicon layer includes 1 ppm to 105 ppm chlorine, and the second hydrogenated amorphous silicon layer includes less than 104 ppm chlorine. Related fabrication methods are also discussed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to microelectronic switching devices and methods of fabrication therefor, more particularly, to thin-film switching devices and methods of fabrication therefor. 
     BACKGROUND OF THE INVENTION 
     Thin film transistors (TFTs) are commonly used to drive pixel electrodes of liquid crystal elements of liquid crystal displays (LCDs). A TFT typically includes an active region comprising amorphous silicon, polysilicon or CdSe. Active regions formed of hydrogenated amorphous silicon (a-Si:H) have particular advantages for mass production and production of large-screen displays. Typical goals in the design of an a-Si:H TFT include increasing the field-effect mobility and reducing off current under backlight illumination. However, since a-Si:H typically has a high photo-conductivity, high levels of leakage current may occur under backlight conditions. The leakage current can be especially large in a projection display using high-intensity backlighting. Thus, it is particularly desirable to decrease photo leakage current in a-Si:H TFT in order to obtain a high quality TFT-LCD. 
     A conventional method for reducing the photo leakage current by reducing the thickness of an undoped a-Si:H layer is described in an article entitled &#34;A 33 cm-Diagonal High-Resolution Multi-color TFT-LCD with Fully Self-Aligned a-Si:HTFTs,&#34; by N.Hirano et al, IDRC 94, pp. 369-371 (1994). Though photo leakage current can be slightly lowered by this method, this approach may also decrease field-effect mobility. 
     SUMMARY OF THE INVENTION 
     In light of the foregoing, it is an object of the present invention to provided thin-film switching devices and methods of fabrication therefor which provide reduced photoconductivity and improved field-effect mobility characteristics. 
     This and other objects, features and advantages are provided according to the present invention by thin-film switching devices and methods of fabrication therefor in which a thin-film switching device is formed having an active region including noncrystalline silicon, e.g., hydrogenated amorphous silicon, which includes chlorine distributed in a manner which produces a predetermined photoconductivity and field-effect mobility. More preferably, the active region includes a plurality of hydrogenated amorphous silicon layers, at least one of which includes chlorine. In an embodiment according to the invention, an active region of a thin-film switching device includes a hydrogenated amorphous silicon layer including a concentration of chlorine which reduces photoconductivity combined with another hydrogenated amorphous silicon layer which has a lower concentration of chlorine which can improve field-effect mobility. 
     Including chlorine in the active region can reduce the photoconductivity of the region, thus potentially reducing leakage current under backlit conditions. By properly distributing the chlorine, reduced photoconductivity characteristics can be achieved without requiring significant reduction in field effect mobility. When switching devices according to the present invention are used to control liquid crystal display elements of a liquid crystal display, improved operation under backlit conditions can be achieved. 
     In particular, according to the present invention, a thin-film switching device includes an active region including noncrystalline silicon which includes chlorine distributed in a manner which produces a predetermined photoconductivity and a predetermined field-effect mobility in the active region. 
     Preferably, the active region includes a plurality of hydrogenated amorphous silicon layers, at least one of which includes chlorine. In one embodiment, the plurality of hydrogenated amorphous silicon layers includes a hydrogenated amorphous silicon layer including between 0.1 ppm and 106 ppm chlorine. In another embodiment, the plurality of hydrogenated amorphous silicon layers includes a first hydrogenated amorphous silicon layer having a first chlorine concentration and a second hydrogenated amorphous silicon layer having a second chlorine concentration less than the first chlorine concentration. The first hydrogenated amorphous silicon layer includes 1 ppm to 105 ppm chlorine, and the second hydrogenated amorphous silicon layer includes less than 104 ppm chlorine. 
     According to another embodiment, a thin-film switching device includes a substrate, an insulated gate electrode on the substrate, an active region on the insulated gate electrode, the active region including noncrystalline silicon including chlorine, and spaced-apart source/drain electrodes on the active region. The insulated gate may include a first conductive region on the substrate and an insulation region covering the first conductive region. The active region may include an active region on the insulation region, overlying the first conductive region. The spaced apart source/drain electrodes may include spaced apart first and second ohmic contact regions on the active region, disposed on opposite sides of the conductive region, and spaced apart second and third conductive regions on respective ones of the spaced apart first and second ohmic contact regions. Preferably, the active region includes chlorine distributed in a manner which produces a predetermined photoconductivity and a predetermined fieldeffect mobility in the active region. More preferably, the active layer includes a plurality of hydrogenated amorphous silicon layers, at least one of which includes chlorine. 
     According to method aspects, a noncrystalline silicon active region is formed for a thin-film switching device, the active region including chlorine distributed in a manner which produces a predetermined photoconductivity and a predetermined field-effect mobility in the active region. Preferably, the active region is formed by forming a plurality of hydrogenated amorphous silicon layers, at least one of which includes chlorine. 
     The plurality of hydrogenated amorphous silicon layers may be formed by exposing a substrate to a silicon gas to form a hydrogenated amorphous silicon layer, and exposing the substrate to a mixture including a silicon gas and a chlorinated silicon gas to form a hydrogenated amorphous silicon layer including chlorine, both steps preferably using a plasma enhanced chemical vapor deposition process. Exposing the substrate to a mixture including a silicon gas and a chlorinated silicon gas may include exposing the substrate to a chlorinated silicon gas selected from the group consisting of SiCl 4 , SiH 2  Cl 2 , and SiHCl 3 . Exposing the substrate to a mixture including a silicon gas and a chlorinated silicon gas may include exposing the substrate to a silicon gas selected from the group consisting of SiH 4 , SiH 4  /H 2 , Si 2  H 6 , Si 2  H 6  /He, and Si 2  H 6  /H 2 . Exposing the substrate to a mixture including a silicon gas and a chlorinated silicon gas includes the step of exposing the substrate to a mixture including a chlorinated silicon gas selected from the group consisting of SiCl 4 , SiH 2  Cl 2 , and SiHCl 3  and a silicon gas selected from the group consisting of SiH 4 , SiH 4  /H 2 , Si 2  H 6 , Si 2  H 6  /He, and Si 2  H 6  /H 2 . The chlorinated silicon gas and the silicon gas may be provided to the substrate at respective first and second mass flow rates such that the ratio of the first mass flow rate to the second mass flow rate is between 0.0001 and 0.6. 
     According to other method aspects, a plurality of hydrogenated amorphous silicon layers may be formed by exposing a substrate to a mixture of a silicon gas and a chlorinated silicon gas to form a first hydrogenated amorphous silicon layer including less than 104 ppm chlorine, and exposing the substrate to a mixture including a silicon gas and a chlorinated silicon gas to form a second hydrogenated amorphous silicon layer including 1-105 ppm chlorine. The first hydrogenated amorphous silicon layer may be formed by providing the chlorinated silicon gas and the silicon gas to the substrate at respective first and second mass flow rates such that the ratio of the first mass flow rate to the second mass flow rate is less than 0.4. The second hydrogenated amorphous silicon layer may be formed by providing the chlorinated silicon gas and the silicon gas to the substrate at respective third and fourth mass flow rates such that the ratio of the third mass flow rate to the fourth mass flow rate is between 0.001 and 0.5. The mixture may include a chlorinated silicon gas selected from the group consisting of SiCl 4 , SiH 2  Cl 2 , and SiHCl 3  and a silicon gas selected from the group consisting of SiH 4 , SiH 4  /H 2 , Si 2  H 6 , Si 2  H 6  /He, and Si 2  H 6  /H 2 . 
     According to other method aspects, a thin-film switching device is fabricated by forming an insulated gate electrode on a substrate, forming an active region on the insulated gate electrode, the active region including noncrystalline silicon which includes chlorine, and forming spaced-apart source/drain electrodes on the active region. The active region preferably is formed by forming a hydrogenated amorphous silicon region including chlorine distributed in a manner which produces a predetermined photoconductivity and a predetermined field-effect mobility in the active region, more preferably, by forming a plurality of hydrogenated amorphous silicon layers, at least one of which includes chlorine. Thin-film switching devices which offer improved photoconductivity characteristics may thereby be provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some of the objects and advantages of the present invention having been stated, others will be more fully understood from the detailed description that follows and by reference to the accompanying drawings in which: 
     FIG. 1 is a sectional view of an inverted staggered thin film transistor (TFT) according to the present invention; 
     FIG. 2 is a graph illustrating a relationship between of flow-rate ratio of SiH 2  Cl 2  gas to SiH 4  gas and chlorine content of an amorphous silicon layer; 
     FIGS. 3 and 4 are graphs illustrating drain current vs. gate voltage characteristics of a control groups of TFTs; 
     FIGS. 5 and 6 are graphs illustrating drain current vs. gate voltage characteristics of experimental groups of TFTs; 
     FIGS. 7 and 8 are graphs illustrating field-effect mobility characteristics of control groups of TFTs; 
     FIGS. 9 and 10 are graphs illustrating field-effect mobility characteristics of experimental groups of TFTs; 
     FIGS. 11 and 12 are graphs illustrating output characteristics of control roups of TFTs; 
     FIGS. 13 and 14 are graphs illustrating output characteristics of experimental groups of TFTs; 
     FIG. 15 is a graph illustrating relationships between flow-rate ratio of SiH 2  Cl 2  gas to SiH 4  gas and dark-conductivity and photo-conductivity of an morphous silicon layer; 
     FIG. 16 is a graph illustrating transfer characteristics of control groups of TFTs; and 
     FIG. 17 is a graph illustrating transfer characteristics of experimental groups of TFTs. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as 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. It will also be understood that when a layer is referred to as being &#34;on&#34; a substrate, it can be directly on the layer or substrate, or intervening layers may also be present. &#34;Switching device&#34; as used refers to devices which conduct electric current through an active region under control of one or more electrodes, e.g., thin-film transistors (TFTs) and the like. In the drawings, the thickness of layers and regions are exaggerated for clarity, and like numbers refer to like elements throughout. 
     A Thin Film Switching Device 
     FIG. 1 illustrates an inverted staggered thin film switching device, e.g., a thin-film transistor (TFT), according to the present invention. Referring to FIG. 1, a gate electrode 11 and an insulation layer 12 are stacked on an insulation substrate 10. An active region 17 with a double layer structure including a lower active layer 13 and an upper active layer 14, is formed on the insulation layer 12. Spaced apart source/drain electrodes 18, including conductive regions 16 and ohmic contact regions 15, are formed on a predetermined area of the upper active layer 14. In embodiments according to the present invention, the active region 17 has a two-layer structure including a hydrogenated amorphous silicon (a-Si:H) layer and a 0.1 ppm to 106 ppm chlorine-containing hydrogenated amorphous silicon layer (a-Si:H(:Cl)). The chlorine-containing hydrogenated amorphous silicon layer (a-Si:H(:Cl)) may be either the lower layer 13 or the upper layer 14. 
     According to a theory of operation which does not limit the scope of the invention, the presence of chlorine in the a-Si:H(:Cl) layer may lead to breaking of Si-H bond and Si-Si bonds having relatively low bond energies. This may shift the Fermi level of the layer in direction which lowers the photo-conductivity of the layer. The characteristics of the thin film switching device may be improved by controlling the concentration of chlorine to provide different levels in the various active layers. For example, the upper active layer 14 may include a concentration of chlorine which lowers off-current, the highly incorporated chlorine decreasing the photo-conductivity of the upper active layer 14. The chlorine concentration of the lower active layer 13 may then be controlled to produce a lower chlorine concentration which improves field-effect mobility and threshold voltage. According to one embodiment, one of the active layers may be formed of a-Si:H(:Cl) containing 0 ppm to 104 ppm chlorine, and the other active layer may be formed of a-Si:H(:Cl) containing 1 ppm to 105 ppm chlorine. 
     Those skilled in the art will appreciate that although a two-layer active region is illustrated, other structures may be used within the scope of the invention. For example, structure including more than two active layers may be used. In addition to structures including discrete active layers with respective chlorine concentrations, a single hydrogenated amorphous silicon layer may be formed which has a graduated chlorine concentration profile. Similarly, structures other than the inverted staggered structure illustrated in FIG. 1 may be utilized with the invention, including, for example, planar or inverted planar structures. 
     Fabrication Methods for TFTs 
     Referring now to FIG. 1, a first conductive layer, e.g., a chromium (Cr) or aluminum (Al) layer, is formed on an insulation substrate 10 and then patterned to form a conductive region 11. An insulation layer 12 is then formed on the resultant structure using, for example, a monolayer structure of silicon nitride or a stacked structure such as a silicon oxide (SiO 2 )/silicon nitride (SiN x ) structure or an aluminum oxide (Al 2  O 3 )/silicon nitride (SiN x ) structure. 
     A hydrogenated amorphous silicon (a-Si:H) layer 13 is then formed on the insulation layer, preferably to a thickness of 100A to 200 Å, by exposing the structure to a gas selected from the group consisting of SiH 4 , SiH 4  /H 2 , Si 2  H 6  and Si 2  H 6  /He gases at a mass flow rate of 0.1 sccm to 10 sccm, at a temperature of 150° C. to 350° C. A 0.1 ppm to 106 ppm chlorine-containing hydrogenated amorphous silicon layer 14 (a-Si:H(:Cl) layer) is then formed on the a-Si:H layer 13, preferably to a thickness of 500 Åto 2,500 Å, by exposing the structure to a gas mixture including a silicon gas selected from the group consisting of SiH 4 , SiH 4  /H 2 , Si 2  H 6 , Si 2  H 6  /He and Si 2  H 6  /H 2 , and a chlorinated silicon gas selected from the group consisting of SiCl 4 , SiH 2  Cl 2  and SiHCl 3 . The silicon source gas preferably is flowed at 0.1 sccm to 10 sccm, and the chlorinated silicon gas is preferably flowed at 0.00001 to 6 sccm. Subsequently, an n +  -amorphous silicon layer or n +  -microcrystalline silicon layer may be formed on the a-Si:H(:Cl) layer 14 and patterned to form ohmic contact regions 15. A conductive layer, e.g., aluminum, chrome or the like, may then be formed on the ohmic contact regions 15, and patterned to form spaced apart conductive regions 16, thereby forming a thin-film transistor 5. 
     According to another method aspect, a thin film switching device is formed including a lower active layer 13 made of a-Si:H(:Cl) containing 0 ppm to 104 ppm chlorine, and an upper active layer 14 made of a-Si:14(:Cl) containing 1 ppm to 105 ppm chlorine. After sequentially forming the gate electrode 11 and the insulation layer 12 on the insulation substrate 10 in the same manner as described above, a lower active layer 13 containing 0 ppm to 104 ppm chlorine is formed with 100 Åto 700 Åthickness by exposing the structure to a gas mixture including a silicon gas selected from the group consisting of SiH 4 , SiH 4  /H 2 , Si 2  H 6 , Si 2  H 6  /He and Si 2  H 6  /H 2 , and chlorinated silicon gas selected from the group consisting of SiCl 4 , SiH 2  Cl 2  and SiHCl 3 . The silicon gas is flowed at mass flow rate of 0.1 sccm to 10 sccm, and the chlorinated silicon gas is flowed at a mass flow rate of 0 sccm to 4 sccm. After forming the lower active layer 13, the flow rate of the chlorinated silicon gas is changed to 0.0001 sccm to 5 sccm to form the upper active layer 14 containing 1 ppm to 105 ppm chlorine with a 500 Åto 2,500 Åthickness. Ohmic contact regions 15 and conductive regions 16 are then formed on the upper active layer 14 as described above. 
     Those skilled in the art will appreciate that variations of the abovedescribed operations may also be used within the scope of the invention. For example, instead of forming discrete layers with respective chlorine concentrations, a hydrogenated amorphous silicon active layer may be formed with a graduated chlorine concentration by varying the flow rates of the deposition gases in situ during formation of the layer. 
     Control of Chlorine Content 
     Investigation into the effect of flow rate on chlorine content of an Si:H(:Cl) layer was conducted by forming an amorphous silicon layer insulation substrate while varying the flow-rate ratio of SiH 2  Cl 2  /SiH 4  gases, and measuring the chlorine content within the amorphous silicon layer using secondary ion mass spectrometry (SIMS). Results are illustrated in FIG. 2, in which the chlorine content for flow-rate ratios of SiH 2  Cl 2  gas to SiH 4  gas of 0.2, 0.1 and 0 is represented by a dashed line, a dot-and-dashed line and a solid line, respectively. 
     As illustrated in FIG. 2, when the flow-rate ratio of SiH 2  Cl 2  /SiH 4  was 0.1 and 0.2, the content of chlorine was 7×10 18  cm -3  and 4×10 19  cm -3 , respectively, indicating that chlorine content in the amorphous silicon layer increases as the flow-rate ratio of SiH 2  Cl 2  /SiH 4  increases. FIG. 2 also illustrates that chlorine may still be present even when the flow-rate ratio of SiH 2  Cl 2  /SiH 4  is reduced to 0, possibly because of outgassing of chlorine from the reaction chamber wall during the deposition process. 
     Drain Current vs. Gate Voltage Characteristics 
     In order to investigate conductivity in a thin film switching device fabricated according to the present invention, drain current vs. gate voltage characteristics of devices were measured. A thin film switching device having an active layer formed of a-Si:H with a 1,500 Åthickness, and a thin film switching device having an active layer formed of a-Si:H(:Cl) with 1,500 Åthickness were used as control groups 1 and 2, respectively. A thin film switching device having a lower active layer made of a-Si:H(:Cl) with a 500 Åthickness and an upper active layer made of a-Si:H with a 1,000 Åthickness, and a thin film switching device having a lower active layer made of a-Si:H with a 500 Åthickness and an upper active layer made of a-Si:H(:Cl) with a 1,000 Åthickness were used as experimental groups 1 and 2, respectively. The drain current vs. gate voltage characteristics of the control and experimental groups were measured, and are shown in FIGS. 3 through 6. 
     Referring to FIGS. 3 and 4, the off-current of control group 2 appears to be decreased with respect to control group 1 by a factor of about 1/10. As shown in FIGS. 5 and 6, experimental groups 1 and 2 appear to exhibit a similarly low off-current, even though the part of the active layer is only partially formed of a-Si:H(:Cl). As also illustrated by FIGS. 4 and 5, each subthreshold slope of experimental groups 1 and 2 is about 0.45 V/dec and the on/off current ratio is 10 6  or more. 
     Field-effect Mobility 
     Field-effect mobility of the control and experimental groups was also measured, and is displayed in FIGS. 7 through 11. The field-effect mobility was calculated using the equation: 
     
         √I.sub.D =[μ.sub.FE (W/L)C.sub.i (V.sub.G -V.sub.TH)V.sub.D ].sup.1/2 
    
     As illustrated in FIG. 7, the field-effect mobility of control group 1 is around 0.48 cm 2  /Vs, while the field-effect mobility of control group 2, illustrated in FIG. 8, is around 0.40 cm 2  /Vs, indicating that incorporated chlorine in a-Si:H(:Cl) can reduce field-effect mobility. Experimental groups 1 and 2 exhibit field-effect mobility of 0.52 cm 2  /Vs and 0.42 cm 2  /Vs, respectively, indicating a improved mobility in comparison to devices including a single active layer formed of only a-Si:H(:Cl). The field-effect mobility level of experimental groups 1 and appear to be similar to that of switching devices of control group 1, even though experimental groups 1 and 2 include a-Si:H(:Cl). 
     Output Characteristics 
     The output characteristics of each group were also measured, producing the results shown in FIGS. 11 through 14. Here, width (W) and length (L) of the active layers of all groups were 60 μm and 30 μm, respectively. 
     In FIG. 11, the drain current of control group 1 is saturated at 1.12×10 -6  A, given a gate voltage of 20 V. The drain current of control group 2 is saturated at 0.76×10 -6  A, given a gate voltage of 20 V, as is illustrated in FIG. 12, indicating that when the active layer is formed of only a-Si:H(:Cl) (control group 2), the saturation level of the drain current may be sharply decreased. 
     In contrast, as can be seen in FIGS. 13 and 14, drain currents of experimental groups 1 and 2 are saturated at 1.30×10 -6  A and 0.96×10 -6  A, respectively, given a gate voltage of 20 V. Thus, according to the present invention, the output characteristics of thin-film switching device may be improved according to the present invention, especially for thin film switching devices including an lower active layer made of a-Si:H(:Cl) and an upper active layer made of a-Si:H (experimental group 1). 
     Effect of Flow-rate Ratio of SiH 2  Cl 2  Gas to SiH 4  Gas on Dark-conductivity and Photo-conductivity 
     In order to observe the relationship between the chlorine content within the active layer and the conductivity under both dark and illuminated conditions, a relationship between mass flow rate and conductivity under both dark and illuminated conditions was measured by varying the flow-rate ratio of SiH 2  Cl 2  /SiH 4  gases in a range from 0 to 0.12. The conductivity under the illumination was measured at 100 mW/cm 2 , producing the results shown in FIG. 15. 
     As shown in FIG. 15, the flow-rate ratio of SiH 2  Cl 2  /SiH 4  appears to bear an inverse relationship to dark-conductivity or photo-conductivity. It may be postulated that the decrease in the dark-conductivity and photo-conductivity is due to a Fermi level shift as the chlorine content increases. 
     Transfer Characteristics 
     Transfer characteristics under the dark and illuminating conditions of each group were also compared, producing the results shown in FIGS. 16 and 17. For purposes of the testing, channels of each thin film switching device were illuminated with 1200L× of light. 
     Referring to FIG. 16, the off-current of control group 2 appears lower than that of control group 1 by about 1/100, under illuminated conditions. As can be seen in FIG. 17, the off-current of experimental group 2 (having the lower active layer made of the a-Si:H and the upper active layer made of the a-Si:H(:Cl)) appears to be decreased by about 1/100 compared to the off current of control group 1,indicating that photo leakage-current can be decreased without significantly decreasing threshold voltage. 
     In sum, an improved thin-film switching device can be achieved when the active layer of the device includes chlorine distributed in a manner which reduces photoconductivity without significantly effecting field-effect mobility and threshold voltage, e.g, when the active layer includes multiple layers including a-Si:H layer and an a-Si:H(:Cl) layer. Accordingly, when such switching devices are used as switching elements for liquid crystal display elements of a liquid crystal display (LCD), leakage current under backlit conditions can be reduced, improving the performance of the LCD. 
     In the drawings and specification, there have been disclosed typical embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.