Thin film transistor, method of fabricating thin film transistor and pixel structure

This application is a divisional application of and claims the priority benefit of U.S. application Ser. No. 12/881,178, filed on Sep. 14, 2010, now pending, which claims the priority benefit of Taiwan application serial no. 99127301, filed on Aug. 16, 2010. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of specification.

1. Field of the Invention

The invention relates to a thin film transistor (TFT), a method of fabricating a TFT, and a pixel structure.

2. Description of Related Art

The increasing progress of display technologies brings about great conveniences to people's daily lives. As such, flat panel displays (FPDs) featuring compactness become the main stream display products at present. Among various types of FPDs, liquid crystal displays (LCDs) have superb characteristics, such as high space utilization efficiency, low power consumption, no radiation, and low electromagnetic interference, so that the LCDs have been prevailing. TFTs are often applied to the displays, and therefore structures of the TFTs make a direct impact on performance of the products. Generally, a TFT at least includes a gate, a source, a drain, and a channel layer. Conductivity of the channel layer is determined by changing the voltage applied to the gate. Specifically, the source and the drain are electrically conducted with each other when the channel layer is turned on, and the source and the drain are electrically insulated from each other when the channel layer is turned off.

FIG. 1 is a schematic top view illustrating a conventional TFT. With reference to FIG. 1 , the TFT normally has a gate G, a channel CH located above the gate G, and a source S and a drain D that are located above the channel CH. In such a TFT, the drain D extends from the source S. It can be learned from FIG. 1 that the drain D located above the channel CH/ the gate G (as marked by the reference number “ 10 ”) has a line width different from a line width of the drain D extending away from the channel CH (as marked by the reference number “ 20 ”). This is mainly because the film profile affects critical dimension (CD) of photoresist patterns in an exposing process during fabrication of the TFT. That is to say, although the device patterns in the photo mask have the same line width, the photoresist layer is not exposed to the same degree in the exposing process, given the film layers below the photoresist layer have different heights and thus the thickness of the photoresist layer is changed. Thereby, the CD of the photoresist patterns may differ after the photoresist layer is patterned. Said difference leads to the issue of the drain D having different line widths as shown in FIG. 1 .

If the CD of the photoresist patterns is excessively large, cross-talk effects are likely to occur in the devices. By contrast, if the CD of the photoresist patterns is overly small, the devices may encounter issues of excessive resistance value, broken lines, or short circuit. Accordingly, research and development are mainly geared towards the way to improve CD uniformity in the fabricating process of the TFT.

The invention is directed to a method of fabricating a TFT. The method is conducive to improvement of CD uniformity in the process of fabricating the TFT.

The invention is further directed to a TFT and a pixel structure having the TFT, and the TFT of the invention has a structure capable of preventing short circuit occurring in elements of the TFT.

The invention provides a method of fabricating a TFT. In the method, a substrate having a gate, an insulating layer, and a channel layer is provided. A conductive layer is formed on the substrate to cover the channel layer and the insulating layer. A photoresist layer is formed on the conductive layer. A photo mask is placed above the photoresist layer. The photo mask has a data line pattern, a source pattern, and a drain pattern. A first width (W 1 ) is between the source pattern and the drain pattern, and the data line pattern has a second width (W 2 ). The first width (W 1 ) and the second width (W 2 ) satisfy the following: if W 1 −1(um), then W 2 +a(um), and 0.3<a<0.7. An exposing process is performed with use of the photo mask, and a development process is performed, so as to pattern the photoresist layer. The conductive layer is patterned with use of the photoresist layer as an etching mask to form a source, a drain, and a data line.

The invention further provides a TFT electrically connected to a scan line. The TFT includes a gate, a channel layer, a source, and a drain. The source and the drain extend away from the scan line. A first distance exists between the source and the drain and corresponds to an area above the channel layer. A second distance exists between the source and the drain and corresponds to an area above the scan line. Here, the second distance is greater than the first distance, such that the source and the drain respectively have a deflection portion. The scan line and the deflection portions of the source and the drain are overlapped by about 0 um˜about 1 um.

The invention further provides a TFT electrically connected to a scan line. The TFT includes a gate, a channel layer, a source, and a drain. The source and the drain extend away from the scan line. A first distance exists between the source and the drain and corresponds to an area above the channel layer. A second distance exists between the source and the drain and corresponds to an area above the scan line. Here, the second distance is greater than the first distance, such that the source and the drain respectively have a deflection portion. The deflection portions of the source and the drain have an extending direction. The scan line has an extending direction. Here, an acute angle (α) is between the extending direction of the deflection portions of the source and the drain and the extending direction of the scan line.

The invention further provides a pixel structure including a data line, a first scan line, a first TFT, a second TFT, a second scan line, a third TFT, a main pixel electrode, and a sub-pixel electrode. The first TFT and the second TFT are disposed on the first scan line, and the first TFT and the second TFT are electrically connected to the first scan line and the data line. The first TFT has a first gate, a first channel layer, a first source, and a first drain. The second TFT has a second gate, a second channel layer, a second source, and a second drain. The first source is electrically connected to the data line, and the second drain is connected to the first source. The second scan line is parallel to the first scan line. The third TFT is disposed on the second scan line and electrically connected to the second scan line. The third TFT has a third gate, a third channel layer, a third source, and a third drain. The third source is connected to the second source. The main pixel electrode is electrically connected to the first drain of the first TFT. The sub-pixel electrode is electrically connected to the third source of the third TFT. Particularly, the third source and the third drain extend away from the second scan line. A first distance exists between the third source and the third drain and corresponds to an area above the third channel layer. A second distance exists between the third source and the third drain and corresponds to an area above the second scan line. Here, the second distance is greater than the first distance, such that the third source and the third drain respectively have a deflection portion. The second scan line and the deflection portions of the third source and the third drain are overlapped by about 0 um˜about 1 um.

The invention further provides a pixel structure including a data line, a first scan line, a first TFT, a second TFT, a second scan line, a third TFT, a main pixel electrode, and a sub-pixel electrode. The first scan line is not parallel to the data line. The first TFT and the second TFT are disposed on the first scan line, and the first TFT and the second TFT are electrically connected to the first scan line and the data line. The first TFT has a first gate, a first channel layer, a first source, and a first drain. The second TFT has a second gate, a second channel layer, a second source, and a second drain. The first source is electrically connected to the data line, and the second drain is connected to the first source. The second scan line is parallel to the first scan line. The third TFT is disposed on the second scan line and electrically connected to the second scan line. The third TFT has a third gate, a third channel layer, a third source, and a third drain. The third source is connected to the second source. The main pixel electrode is electrically connected to the first drain of the first TFT. The sub-pixel electrode is electrically connected to the third source of the third TFT. Particularly, the third source and the third drain extend away from the second scan line. A first distance exists between the third source and the third drain and corresponds to an area above the third channel layer. A second distance exists between the third source and the third drain and corresponds to an area above the second scan line. Here, the second distance is greater than the first distance, such that the third source and the third drain respectively have a deflection portion. The deflection portions of the third source and the third drain have an extending direction. The second scan line has an extending direction. Here, an acute angle (α) is between the extending direction of the deflection portions of the third source and the third drain and the extending direction of the second scan line.

In the invention as described above, the photo mask used for patterning the conductive layer is characterized in that the first width (W 1 ) is between the source pattern and the drain pattern, and the data line pattern has a second width (W 2 ). Here, the first width (W 1 ) and the second width (W 2 ) satisfy the following: if W 1 −1(um), then W 2 +a(um), and 0.3<a<0.7. The photoresist patterns formed by using the photo mask serve as the etching mask for defining the source, the drain, and the data line, such that the source, the drain, and the data line can have uniform CD.

In addition, the second scan line and the deflection portions of the source and the drain are overlapped by about 0 um˜about 1 um, or an acute angle (α) is formed between the extending direction of the deflection portions of the source and the drain and the extending direction of the second scan line. Thereby, short circuit between the source and the drain can be precluded.

It is to be understood that both the foregoing general descriptions and the following detailed embodiments are exemplary and are, together with the accompanying drawings, intended to provide further explanation of technical features and advantages of the invention.

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a schematic top view illustrating a conventional TFT.

FIG. 2A to FIG. 2C are schematic cross-sectional views illustrating a process of fabricating a TFT according to an embodiment of the invention.

FIG. 3 is a schematic top view illustrating the TFT depicted in FIG. 2C .

FIG. 4 is a schematic top view illustrating a pixel structure according to an embodiment of the invention.

FIG. 5 is a schematic enlarged view of an area marked as “ 300 ” in FIG. 4 .

First Embodiment

FIG. 2A to FIG. 2C are schematic cross-sectional views illustrating a process of fabricating a TFT according to an embodiment of the invention. With reference to FIG. 2A , in a method of fabricating the TFT according to this embodiment, a substrate 100 is provided, and a gate G, an insulating layer 102 , and a channel layer CH are already formed on the substrate 100 . According to an embodiment of the invention, an ohmic contact layer OM is further formed on the channel layer CH.

The substrate 100 can be made of glass, quartz, organic polymer, a non-light-transmissive/reflective material (such as a conductive material, wafer, ceramics, or the like), or other suitable materials.

The gate G is formed on the substrate 100 . The gate G is formed by depositing a conductive material layer on the substrate 100 and patterning the conductive material layer by performing a photolithography and etching process, for instance. In consideration of conductivity, the gate G is typically made of a metal material. However, the invention is not limited herein. According to other embodiments of the invention, the gate G can be made of other conductive materials (such as an alloy, a metal nitride material, a metal oxide material, a metal oxynitride material, or other suitable materials) or a layer in which the metal material and other conductive materials are stacked together.

The insulating layer 102 is foamed on the substrate 100 and covers the gate G. Here, the insulating layer 102 can be referred to as the gate insulating layer. The insulating layer 102 is, for example, formed by performing a chemical vapor deposition process, a physical vapor deposition process, or other known deposition processes. A material of the insulating layer 102 includes silicon oxide, silicon nitride, silicon oxynitride, other appropriate materials, or a layer in which at least two of the aforesaid materials are stacked.

A material of the channel layer CH is, for example, amorphous silicon, polysilicon, metal oxide semiconductor, or other appropriate semiconductor materials. A material of the ohmic contact layer OM can be doped polysilicon or other materials capable of enhancing electrical contact between metal and silicon. According to an embodiment of the invention, the channel layer CH and the ohmic contact layer OM are formed by performing a deposition process (e.g., a chemical vapor deposition process, a physical vapor deposition process, or other known deposition processes) to sequentially deposit a channel material layer and an ohmic contact material layer and then patterning the channel material layer and the ohmic contact material layer.

A conductive layer 104 is formed on the substrate 100 to cover the channel layer CH and the insulating layer 102 . The conductive layer 104 is, for example, formed by performing a chemical vapor deposition process, a physical vapor deposition process, or other known deposition processes. In this embodiment, the conductive layer 104 covers the ohmic contact layer OM and the insulating layer 102 . A material of the conductive layer 104 can be a metal material, an alloy, a metal nitride material, a metal oxide material, a metal oxynitride material, other suitable materials, or a layer in which the metal material and other conductive materials are stacked together.

A photoresist layer 106 is formed on the conductive layer 104 . The photoresist layer 106 is made of an organic photo-sensitive material, for instance.

A photo mask 110 is then placed above the photoresist layer 106 . In particular, the photo mask 110 has a data line pattern 110 a , a source pattern 110 b , and a drain pattern 110 c . A first width (W 1 ) is between the source pattern 110 b and the drain pattern 110 c , and the data line pattern 110 a has a second width (W 2 ). The first width (W 1 ) and the second width (W 2 ) satisfy the following: if W 1 −1(um), then W 2 +a(um), and 0.3<a<0.7.

An exposing process 112 is then performed with use of the photo mask 110 , and a development process is carried out, so as to pattern the photoresist layer 106 and form the structure shown in FIG. 2B . Specifically, after the photoresist layer 106 is patterned, the patterned photoresist layer has a photoresist pattern 106 a corresponding to the data line pattern 110 a of the photo mask 110 , a photoresist pattern 106 b corresponding to the source pattern 110 b of the photo mask 110 , and a photoresist pattern 106 c corresponding to the drain pattern 110 c of the photo mask 110 . Energy of the exposing process 112 is approximately 23 mj/cm2˜26 mj/cm 2 .

Note that a thickness H 2 of the photoresist layer 106 corresponding to the data line pattern 110 a of the photo mask 110 (i.e., the photoresist pattern 106 a ) is greater than a thickness H 1 of the photoresist layer 106 corresponding to the source pattern 110 b (i.e., the photoresist pattern 106 b ) and the drain pattern 110 c (i.e., the photoresist pattern 106 c ) of the photo mask 110 . In an embodiment of the invention, the thickness H 2 of the photoresist layer 106 corresponding to the data line pattern 110 a of the photo mask 110 (i.e., the photoresist pattern 106 a ) differs from the thickness H 1 of the photoresist layer 106 corresponding to the source pattern 110 b (i.e., the photoresist pattern 106 b ) and the drain pattern 110 c (i.e., the photoresist pattern 106 c ) of the photo mask 110 by about 4900 angstroms˜about 6200 angstroms. Namely, the thickness H 2 is greater than the thickness H 1 by about 4900 angstroms˜about 6200 angstroms.

Based on the above, as shown in FIG. 2A and FIG. 2B , the film layers that are located below the photoresist layer 106 and correspond to the data line pattern 110 a of the photo mask 110 (i.e., the photoresist pattern 106 a ) have lower heights than heights of the film layers that are located below the photoresist layer 106 and correspond to the source pattern 110 b (i.e., the photoresist pattern 106 b ) and the drain pattern 110 c (i.e., the photoresist pattern 106 c ) of the photo mask 110 . Therefore, the photoresist patterns 106 a , 106 b , 106 c of the photoresist layer 106 have different thicknesses at said two different locations. Namely, the thickness H 2 is greater than the thickness H 1 . Due to the different thicknesses H 1 and H 2 of the photoresist layer 106 , the photoresist layer 106 at the two different locations is exposed to different extent during the exposing process 112 . Thus, the CD of the patterned photoresist layer 106 may be affected. In the photo mask 110 of this embodiment, the first width (W 1 ) between the source pattern 110 b and the drain pattern 110 c and the second width (W 2 ) of the data line pattern 110 a satisfy the following: if W 1 −1(um), then W 2 +a(um), and 0.3<a<0.7. As such, the CD of the photoresist layer 106 is not affected by the different heights of the film layers at said two different locations.

The conductive layer 104 is patterned with use of the photoresist layer 106 as an etching mask, so as to form the source S, the drain D, and the data line DL, as indicated in FIG. 2C . Here, the conductive layer 104 can be patterned by dry etching or wet etching.

FIG. 3 is a top view illustrating the TFT depicted in FIG. 2C , and the TFT includes the gate G, the channel layer CH, the source S, and the drain D. The source S is electrically connected to the data line DL, and the gate G is electrically connected to the scan line SL. Specifically, the distance between the source S and the drain D is approximately the same as the first width (W 1 ) between the source pattern 110 b and the drain pattern 110 c of the photo mask 110 depicted in FIG. 2A , and the width of the data line DL is approximately the same as the second width (W 2 ) of the data line pattern 110 a of the photo mask 110 depicted in FIG. 2A . Hence, the distance (W 1 ) between the source S and the drain D and the width (W 2 ) of the data line DL also satisfy the following: if W 1 −1(um), then W 2 +a(um), and 0.3<a<0.7.

Second Embodiment

FIG. 4 is a schematic top view illustrating a pixel structure according to an embodiment of the invention. FIG. 5 is a schematic enlarged view of an area marked as “ 300 ” in FIG. 4 . With reference to FIG. 4 and FIG. 5 , the pixel structure of this embodiment includes a data line DL, a first scan line SL 1 , a second scan line SL 2 , a first TFT T 1 , a second TFT T 2 , a third TFT T 3 , a main pixel electrode PE 1 , and a sub-pixel electrode PE 2 .

The first scan line SL 1 and the second scan line SL 2 are parallel. The data line DL and the first and second scan lines SL 1 and SL 2 are alternately arranged. An insulating layer is sandwiched between the data line DL and the first and second scan lines SL 1 and SL 2 . In other words, an extending direction of the data line DL is not parallel to extending directions of the first and second scan lines SL 1 and SL 2 . Preferably, the extending direction of the data line DL is perpendicular to the extending directions of the first and second scan lines SL 1 and SL 2 . In addition, the first and second scan lines SL 1 and SL 2 and the data line DL are in different layers. In consideration of conductivity, the first and second scan lines SL 1 and SL 2 and the data line DL are usually made of a metal material. However, the invention is not limited thereto. According to other embodiments of the invention, the first and second scan lines SL 1 and SL 2 and the data line DL can also be made of any other conductive material. The conductive material is, for example, an alloy, a metal nitride material, a metal oxide material, a metal oxynitride material, other appropriate materials, or a layer in which the metal material and other conductive materials are stacked together.

The first TFT T 1 is disposed on the first scan line SL 1 , and the first TFT T 1 is electrically connected to the first scan line SL 1 and the data line DL. The first TFT T 1 has a first gate G 1 , a first channel layer CH 1 , a first source S 1 , and a first drain D 1 . Here, the first gate G 1 is electrically connected to the first scan line SL 1 , and the first source S 1 is electrically connected to the data line DL.

The second TFT T 2 is disposed on the first scan line SL 1 , and the second TFT T 2 is electrically connected to the first scan line SL 1 and the data line DL. The second TFT T 2 has a second gate G 2 , a second channel layer CH 2 , a second source S 2 , and a second drain D 2 . Here, the second gate G 2 is electrically connected to the first scan line SL 1 , and the second drain D 2 is connected to the first source S 1 . Hence, the second drain D 2 is electrically connected to the data line DL. According to this embodiment, the first channel layer CH 1 and the second channel layer CH 2 are the same semiconductor pattern.

The third TFT T 3 is disposed on and electrically connected to the second scan line SL 2 . The third TFT T 3 has a third gate G 3 , a third channel layer CH 3 , a third source S 3 , and a third drain D 3 . Here, the third gate G 3 is electrically connected to the second scan line SL 2 , and the third source S 3 is connected to the second source S 2 .

In particular, the third source S 3 and the third drain D 3 in the third TFT T 3 extend away from the second scan line SL 2 , as indicated in FIG. 5 . A first distance L 1 exists between the third source S 3 and the third drain D 3 and corresponds to an area above the third channel layer CH 3 . A second distance L 2 exists between the third source S 3 and the third drain D 3 and corresponds to an area above the second scan line SL 2 . Here, the second distance L 2 is greater than the first distance L 1 , such that the third source S 3 has a deflection portion ST, and that the third drain D 3 has a deflection portion DT. The second scan line SL 2 and the deflection portion ST of the third source S 3 are overlapped by a length A 2 ranging from about 0 um to about 1 um (A 2 =0 um˜1 um), and the second scan line SL 2 and the deflection portion DT of the third drain D 3 are overlapped by a length A 1 ranging from about 0 um to about 1 um (A 1 =0 um˜1 um). Preferably, the second scan line SL 2 and the deflection portion ST of the third source S 3 are overlapped by the length A 2 ranging from about 0 um to about 0.5 um (A 2 =0 um˜0.5 um), and the second scan line SL 2 and the deflection portion DT of the third drain D 3 are overlapped by the length A 1 ranging from about 0 um to about 0.5 um (A 1 =0 um˜0.5 um). Most favorably, the second scan line SL 2 and the deflection portion ST of the third source S 3 are overlapped by the length A 2 ranging from about 0.59 um to about 0.77 um (A 2 =0.59 um˜0.77 um), and the second scan line SL 2 and the deflection portion DT of the third drain D 3 are overlapped by the length A 1 ranging from about 0.59 um to about 0.77 um (A 1 =0.59 um˜0.77 um). The lengths A 1 and A 2 can be the same or different.

That is to say, in this embodiment, the third source S 3 and the third drain D 3 respectively have the deflection portions ST and DT, and therefore the third source S 3 and the third drain D 3 have the patterns diffusing in an outward manner and are gradually separated from each other. It is also likely for the second scan line SL 2 to overlap (0<A 1 ≦1 um, 0<A 2 ≦1 um) or not to overlap (A 1 =0, A 2 =0) the deflection portion ST of the third source S 3 and the deflection portion DT of the third drain D 3 .

Note that the film layers at the overlapping area and the non-overlapping area of the third source S 3 , the third drain D 3 , and the second scan line SL 2 have different heights. The invention aims at preventing broken lines or short circuit caused by non-uniform CD at the overlapping area and at the non-overlapping area of the third source S 3 , the third drain D 3 , and the second scan line SL 2 . In other words, the CD of the photoresist layer may deviate from the original design during the exposing process because the film layers below the drain D have different heights, as indicated in FIG. 1 . To resolve said issues, the third source S 3 and the third drain D 3 are specially designed in this embodiment as discussed above. Thereby, short circuit does not occur between the third source S 3 and the third D 3 , and broken line of the third source S 3 and broken line of the third drain D 3 can be prevented.

According to another embodiment of the invention, in the third TFT T 3 , as shown in FIG. 5 , an acute angle a 2 is formed between an extending direction of the deflection portion ST of the third source S 3 and an extending direction of the second scan line SL 2 . An acute angle al is formed between an extending direction of the deflection portion DT of the third drain D 3 and the extending direction of the second scan line SL 2 . Preferably, 0°<α1≦45°, and 0<α 2 ≦45°. In addition, the acute angles α 1 and α 2 can be the same or different.

Namely, in this embodiment, the acute angle α 2 is formed between the extending direction of the deflection portion ST of the third source S 3 and the extending direction of the second scan line SL 2 , and the acute angle α 1 is formed between the extending direction of the deflection portion DT of the third drain D 3 and the extending direction of the second scan line SL 2 . Accordingly, the third source S 3 and the third drain D 3 have the patterns diffusing in an outward manner and are gradually separated from each other. To prevent short circuit that is caused by non-uniform CD at the overlapping area and at the non-overlapping area of the third source S 3 , the third drain D 3 , and the second scan line SL 2 occurs between the third source S 3 and the third drain D 3 , the design of the third source S 3 and the third drain D 3 is modified in the invention, as discussed above.

According to another embodiment of the invention, in the third TFT T 3 , as shown in FIG. 5 , the deflection portion ST of the third source S 3 and the second scan line SL 2 are overlapped by the length A 2 ranging from about 0 um to about 1 um (A 2 =0 um˜1 um), and the deflection portion DT of the third drain D 3 and the second scan line SL 2 are overlapped by the length A 1 ranging from about 0 um to about 1 um (A 1 =0 um˜1 um). In addition, the acute angle α 2 is formed between the extending direction of the deflection portion ST of the third source S 3 and the extending direction of the second scan line SL 2 , and the acute angle α 1 is formed between the extending direction of the deflection portion DT of the third drain D 3 and the extending direction of the second scan line SL 2 .

With reference to FIG. 4 again, in this pixel structure, the main pixel electrode PE 1 is electrically connected to the first drain D 1 of the first TFT T 1 . According to this embodiment, the main pixel electrode PE 1 is electrically connected to the first drain D 1 of the first TFT T 1 through a contact via V 1 . Alignment patterns (alignment slits or alignment protrusions) can be further formed in the main pixel electrode PE 1 . The sub-pixel electrode PE 2 is electrically connected to the third source S 3 of the third TFT T 3 . According to this embodiment, the sub-pixel electrode PE 2 is electrically connected to the third source S 3 of the third TFT T 3 through a contact via V 2 . Alignment patterns (alignment slits or alignment protrusions) can be further formed in the sub-pixel electrode PE 2 . The main pixel electrode PE 1 and the sub-pixel electrode PE 2 can be transparent electrode layers, reflective electrode layers, or transflective electrode layers.

Additionally, in the pixel structure of the embodiment, a capacitor electrode CL and an upper electrode UE can be further disposed to form a capacitor of the pixel structure.

In the previous embodiment depicted in FIG. 4 and FIG. 5 , the design of the third source S 3 and the third drain D 3 of the third TFT T 3 (i.e., the design of the lengths A 1 and A 2 and the acute angles α 1 and α 2 ) can be fulfilled by the photo mask that defines the third source S 3 and the third drain D 3 . In other words, the third source S 3 and the third drain D 3 are often formed by forming a conductive layer and forming a photoresist layer on the conductive layer. The photoresist layer is then patterned with use of the photo mask, and the conductive layer is patterned with use of the patterned photoresist layer as the etching mask, so as to form the third source S 3 and the third drain D 3 . As stated above, the third source pattern and the third drain pattern (i.e. the patterns that define the third source S 3 and the third drain D 3 ) in the photo mask can have the design of the lengths A 1 and A 2 and the acute angles α 1 and α 2 . As such, the third source S 3 and the third drain D 3 formed by performing the patterning process with use of the photo mask can have the features of the lengths A 1 and A 2 and the acute angles α 1 and α 2 .

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the invention cover modifications and variations of the invention provided they fall within the scope of the following claims and their equivalents.