Abstract:
A method of fabricating a semiconductor device includes first providing an insulation substrate. A patterned conductive layer is formed over the insulation substrate, and the patterned conductive layer includes a channel region and a number of protruding regions. A gate structure layer is formed over the insulation substrate. The gate structure layer covers a part of the patterned conductive layer, and each of the protruding regions has an exposed region. A doping process is performed to dope at least the exposed region of the patterned conductive layer to form a number of S/D regions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and claims priority benefit of an application Ser. No. 11/470,627, filed on Sep. 7, 2006, now allowed as U.S. Pat. No. 7,504,694, which claims the priority benefit of Taiwan application serial no. 95117253, filed on May 16, 2006. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a semiconductor device. More particularly, the present invention relates to a semiconductor transistor for testing, which is compatible with fabrication process of common semiconductor devices and can be achieved without additional processing steps, and thus the deterioration of various internal portions of the device under electrical stress can be conveniently detected. 
     2. Description of Related Art 
     In recent years, since flat panel displays have been gradually popularized and have replaced traditional CRT screens, thin-film transistor liquid crystal display (TFT-LCD) related industries are developing rapidly. Among others, the thin-film transistor (TFT), a crucial component, turns out to be one of the most important subjects currently. 
     It is generally known that the active region of a TFT comprises polysilicon silicon or amorphous silicon rather than single-crystal silicon, thus having a number of defects which lead to many problems, such as reliability. Therefore, it is urgent to improve the reliability of TFTs. 
     The active region of a TFT, also known as the channel region, is the region below the gate structure. Conventionally, in order to detect the deterioration of the channel region under persistent operating voltage, a set of test operating voltages is applied to electrodes of the gate, source, and drain, so as to generate an electrical stress on the transistor. After applying the electrical stress for a period of time, a characteristic curve of the transistor is tested to detect the change of the characteristics of the transistor caused by applying the electrical stress. 
     However, in addition to the fact that additional processes are required to fabricate the transistor for testing, the conventional detecting method can only test the overall structure of the transistor, and cannot detect various portions of the channel region due to the structure of the transistor for testing. Therefore, the conventional detecting method may cause wrong judgment. One of the possible situations is that, for example, a part of the channel region has been deteriorated. However, as for the overall testing, only a slight deviation occurs, and cannot indicate the existence of deteriorated portion. Thus, the conventional detecting result may lead to a wrong judgment. 
     SUMMARY OF THE INVENTION 
     The present invention provides a structure of semiconductor device and a method of fabricating the same. The semiconductor device structure comprises a special S/D electrode structure, which can be used to completely detect the deterioration of the device with simple analysis, thus facilitating the research and development of improving the reliability of the device. 
     The present invention provides a structure of semiconductor device, which comprises an insulation substrate. A channel layer is disposed on the insulation substrate. A plurality of doped layers is disposed on the insulation substrate, and protrudes from the channel layer. The doped layers form at least two S/D electrode pairs, and each of the S/D electrode pairs has a different extension direction with respect to the channel layer. A gate dielectric layer is disposed on the channel layer. A gate layer is disposed on the gate dielectric layer. 
     In the aforementioned device structure according to an embodiment of the present invention, for example, the extension direction of at least one of the S/D electrode pairs is a first direction, and the extension direction of at least another one of the S/D electrode pairs is a second direction. For example, the first direction is perpendicular to the second direction. 
     In the aforementioned device structure according to an embodiment, for example, the first direction is a longitudinal direction of the channel layer, and the second direction is a transverse direction of the channel layer, and at least two S/D electrode pairs are arranged in the second direction. 
     In the aforementioned device structure according to an embodiment of the present invention, for example, the extension direction of each of the S/D electrode pairs comprises at least three directions. 
     The present invention further provides a method of fabricating the semiconductor device, which comprises providing an insulation substrate. A patterned conductive layer is formed over the insulation substrate, wherein the patterned conductive layer comprises a channel region and a plurality of protruding regions. A gate structure layer is formed above the insulation substrate, and covers a part of the patterned conductive layer, wherein each of the protruding regions has an exposed region. A doping process is performed to dope at least the exposed region of the patterned conductive layer to form a plurality of S/D regions. 
     In the aforementioned method of fabricating the device structure according to an embodiment of the present invention, for example, the extension direction of at least one of the S/D electrode pairs is a first direction, and the extension direction of at least another one of the S/D electrode pairs is a second direction. For example, the first direction is perpendicular to the second direction. 
     In order to make the aforementioned and other features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic sectional view of the structure of semiconductor device according to an embodiment of the present invention. 
         FIG. 1B  is a schematic top view of the structure of semiconductor device according to an embodiment of the present invention. 
         FIG. 2  is a schematic top view of the structure of semiconductor device according to another embodiment of the present invention. 
         FIGS. 3A-3C  are schematic sectional views of the process for fabricating the semiconductor device of  FIG. 2  according to an embodiment of the present invention. 
         FIG. 4  is a schematic top view of the structure of semiconductor device according to another embodiment of the present invention. 
         FIGS. 5A-5D  show the observed changes of the transfer characteristic curves after applying an electrical stress according to an embodiment of the present invention. 
         FIGS. 6A-6D  show the observed changes of the transfer characteristic curves after applying an electrical stress according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present invention provides a structure of semiconductor device and a method of fabricating the same. The semiconductor device structure comprises a special S/D electrode structure, which can be used to completely detect the deterioration of the device with simple analysis, thus facilitating the research and development of improving the reliability of the device. 
     In the present invention, a plurality of S/D electrodes is disposed on the periphery of the channel region. Any two of the S/D electrodes form an S/D electrode pair, and the S/D electrode pair and a gate form a TFT, so as to detect or apply electrical stress to the channel region in the extension direction of the two S/D electrodes. Several embodiments are described below to illustrate the present invention. However, the present invention is not limited to the embodiments given below. 
       FIG. 1A  is a schematic sectional view of the structure of semiconductor device according to an embodiment of the present invention.  FIG. 1B  is a schematic top view of the structure of semiconductor device according to an embodiment of the present invention. Referring to  FIGS. 1A and 1B , the structure of the semiconductor device mainly comprises a channel layer  102  disposed on a substrate  100 . The substrate  100  is an insulation substrate, for example, a silicon oxide insulation substrate. The channel layer  102  is, for example, a polysilicon layer. A protruding portion  104  is disposed on the periphery of the channel layer  102 , and the protruding portion  104  is, for example, two doped S/D electrodes  104  which form an S/D electrode pair. It should be noted that the channel layer  102  and the S/D electrodes  104  are preferably a single structural layer, and can be defined in the same photolithographic and etching processes, which can be known from the description of the process below. A gate structure layer, comprising a gate dielectric layer  106  and a gate layer  108 , is formed over the substrate  100  and covers the channel layer  102 . The gate layer  108  for example can be a polysilicon layer or a metal layer. For example, the gate layer  108  and the S/D electrodes  104  can be doped with the dopant such as an N-type dopant or a P-type dopant in the same doping process. 
     Only two S/D electrodes  104  are taken as an example to illustrate the structure as shown in  FIG. 1B , and thus there is only one detecting direction with respect to the channel layer  102 , i.e., the extension direction of the two S/D electrodes  104 , which is also known as a longitudinal direction. However, different regions of the channel layer  102  along the longitudinal direction cannot be detected. 
     Therefore, for the structure as shown in  FIG. 1B , the present invention further provides an improved S/D electrode structure.  FIG. 2  is a schematic top view of the structure of semiconductor device according to another embodiment of the present invention. Referring to  FIG. 2 , a sectional structure along an S/D electrode pair of  FIG. 2  is similar to that of  FIG. 1A . The structure has a channel layer  202  disposed on the substrate  200 . A plurality of protruding regions is also disposed on the periphery of the channel layer  202 . The protruding regions are doped layers serving as a plurality of S/D electrodes. Preferably, doped layers  206   a ,  206   b  form an S/D electrode pair. Doped layers  208   a ,  208   b  and doped layers  210   a ,  210   b  respectively form another S/D electrode pair. Moreover, doped layers  212   a ,  212   b  also form another S/D electrode pair. Then, a gate structure layer  204 , comprising a gate dielectric layer and a gate layer, is formed over the substrate  200  and covers the channel layer  202 . 
     Definitely, the constitution of the S/D electrode pairs can be different from that described above, and any two of the S/D electrodes can be selected to form an S/D electrode pair. In general, for example, an S/D electrode  206   a  and S/D electrode  210   a  can also form an S/D electrode pair, and meanwhile, S/D electrode  206   a  and S/D electrode  210   b  can also form another S/D electrode pair as well. However, the S/D electrode pair can also be arranged in two directions in practical design. For example, the extension direction of the S/D electrodes  206   a  and  206   b  is called the longitudinal direction, and the extension direction of the S/D electrodes  208   a  and  208   b  is called the transverse direction. Moreover, for example, the channel layer  202  is divided into several regions along the longitudinal direction. The S/D electrodes  210   a  and  210   b , and S/D electrodes  212   a  and  212   b  are respectively added in the transverse direction to detect the deterioration of various regions correspondingly. Definitely, another S/D electrode pair can be arranged in the longitudinal direction, which is the variation of the present invention in practical design and the details will not be illustrated herein again. In other words, for example, the direction of the S/D electrode pairs can also comprise three or more directions. 
     The longitudinal direction refers to, for example, the direction of applying the electrical stress. The electrical stress can be generated by applying a voltage difference Vgs between the gate and the source, and meanwhile applying a voltage difference Vds between the drain and the source. After a period of time, the applied voltages are removed. Then, any two S/D electrodes are selected so as to measure the characteristics of the corresponding portions. Some of the results of actual measurement are shown in  FIGS. 5A-5D  and  FIGS. 6A-6D , which will be illustrated later. An embodiment of the process of fabricating the aforementioned semiconductor structure is described below. 
       FIGS. 3A-3C  are schematic sectional views of the process of fabricating the semiconductor device of  FIG. 2  according to an embodiment of the present invention. Referring to  FIG. 3A , an insulation substrate  200  is provided, and serves as a substrate layer. The insulation substrate  200  is, for example, a silicon oxide substrate. A conductive layer  201 , for example a polysilicon layer, is formed over the substrate  200 . The process for forming the conductive layer  201  is, for example, chemical vapor deposition (CVD). Then, the structure with a plurality of protruding regions  206   a ,  206   b ,  208   a ,  208   b ,  210   a ,  210   b ,  212   a , and  212   b  on the periphery as shown in  FIG. 2  can be defined in the conductive layer  201  through the photolithographic and etching process. 
     Referring to  FIG. 3B , a gate dielectric layer and a gate layer are sequentially formed above the substrate  200 , and are defined as a gate dielectric layer  300  and a gate layer  302  through the photolithographic and etching process. The gate dielectric layer  300  and the gate layer  302  form the gate structure layer  204  on the conductive layer  201 . The gate structure layer  204  covers a part of the conductive layer  201 , and the uncovered portion is predetermined to be a plurality of S/D electrodes which are formed subsequently. 
     Referring to  FIG. 3C , a doping process  304  is performed to dope the gate layer  302  and the exposed portion of the conductive layer  201  with the dopants of the desired conductive type, so as to form the S/D electrodes  206   a ,  206   b . Moreover, the covered region of the conductive layer  201  functions as the channel region  202 , i.e., the active region of the transistor. 
     The structure of semiconductor device fabricated according to the above method can be used to conveniently detect the deterioration of the device. However, it should be noted that the fabricating method is compatible with the process of fabricating other common devices. Therefore, no additional process is required to fabricate the structure of semiconductor device of the present invention for testing, so the present invention basically does not cause an increase in the manufacturing cost. 
     According to the same fabricating process, S/D electrodes different from the above arrangement can be fabricated.  FIG. 4  is a schematic top view of the structure of semiconductor device according to another embodiment of the present invention. As show in  FIG. 4 , the channel region below the gate structure  400  is, for example, an active region formed by polysilicon. A plurality of S/D electrodes  402  is disposed on the periphery of the channel region. Any two of the S/D electrodes can be selected to achieve an operation of the transistor, so as to measure the conducting currents of various portions or to test other parameters. 
     The special device structure provided by the present invention is applicable to analyzing the deterioration of various portions in the active region of the device being subjected to the electrical stress, and detecting the implied information that can not be observed by conventional reliability measurement. Thus, not only can the wrong judgment resulting from the conventional measurement be avoided, but also a structure design for improving the reliability of the device can be provided as reference. 
     Several examples of detecting the deterioration of various portions in the active region are given below, which show that the present invention provides more effective reliability analysis. 
     First, the applied conditions are described. Referring to  FIG. 2  again, for example, the S/D electrodes  206   a ,  206   b  are taken as terminals being subjected to the electrical stress. A voltage Vg=10V is applied to the gate layer  302 , and meanwhile a voltage Vd=20V is applied to the drain  206   b , and the source  206   a  is connected to ground. Moreover, the aforementioned voltages are maintained for 1000 seconds. Then, the S/D electrodes  206   a ,  206   b  are divided, for example, into a source end portion, a middle portion, and a drain end portion along the longitudinal extension direction for analysis and test. The source end portion can be measured by the S/D electrodes  208   a  and  208   b , the middle portion can be measured by the S/D electrodes  210   a  and  210   b , and the drain end portion can be measured by the S/D electrodes  212   a  and  212   b . In this embodiment, only one S/D electrode pair is taken as the example to illustrate the S/D electrodes in the longitudinal direction; however, it is not the only way. In the aspect of the common fabricating dimension, the transverse width of the channel region  202  is represented by Ws, which for example is 5 μm. Furthermore, the longitudinal length of the channel region  202  is represented by L, which for example is 10 μm. The width of the gate is represented by W G , which for example is 7 μm. The lengths of the source end portion and the drain end portion in the longitudinal direction are, for example, 1 μm. The length of the middle portion is, for example, 7 μm. 
       FIGS. 5A-5D  show the observed changes of the characteristic curves after applying an electrical stress according to an embodiment of the present invention.  FIG. 5A  shows the characteristic curve of conducting current I D  to the gate voltage V G  of the drain in the longitudinal direction measured by the S/D electrodes  206   a ,  206   b , wherein V G  indicates the gate voltage. The dashed line indicates the characteristic curve of the original semiconductor testing device before applying the electrical stress, and the solid line indicates the characteristic curve of the semiconductor testing device after applying the electrical stress. Moreover, the upper curve is under the condition that the voltage difference V D  between the drain and the source is 3 V, and the lower curve is under the condition that V D  is 0.1 V. As known from the characteristic curve of I D  to V G , when V D =0.1 V, the deterioration is quite obvious, and when V D =3 V, no distinct change can be observed. 
     Then, the changes of the characteristic curves of I D  to V G  in other regions are tested subsequently.  FIG. 5B  is the measurement result of the source end portion.  FIG. 5C  is the measurement result of the middle portion.  FIG. 5D  is the measurement result of the drain end portion. Obviously, among the characteristics of the three regions, the middle portion and the drain end portion have distinct deterioration. It should be noted that, with referring to  FIG. 5D  and  FIG. 5A , the drain end portion has serious deterioration no matter in V D =0.1V or V D =3V. However, in  FIG. 5A , the deterioration is not obvious, and especially when V D =3V, the data of  FIG. 5A  cannot show the deterioration of the drain end portion. But, as the present invention provides concurrent tests to several regions, the deteriorated regions can be precisely detected. 
     The reasons for the deterioration can be analyzed. The region near the drain electrode end  206   b , also known as the B 1 D region, is measured by the S/D electrodes  212   a  and  212   b . As shown in  FIG. 5D , for example, since the sub-threshold and above threshold currents decrease significantly, two defects, namely the deep state and the tail state, occur in the polysilicon of the drain end region. The defect of deep state causes an increase in current leakage, which is also one of the main reasons of the increase of the current leakage, and thus the increase of the current leakage is observed. Then, the middle region, also known as the B 1 M region, is measured by the S/D electrodes  210   a  and  210   b . It can be seen from the characteristics as shown in  FIG. 5C  that only the current of the sub-threshold region increases significantly. Thus, it can be concluded that only the deep state occurs in the middle region. The region near the source end  206   a , also known as the B 1 S region, is measured by the S/D electrodes  208   a  and  208   b  and has the characteristics as shown in  FIG. 5B  that the sub-threshold current increases and the current leakage decreases. Therefore, it can be concluded that the hole trapping occurs near the source electrode end. The deterioration of the polysilicon active region of a device can be fully understood through detailed analysis of the three regions. Thus, the reliability of the device can be improved according to the above information. 
     Furthermore, the precision that the device structure of the present invention determines the deterioration of the device can be understood from another measurement condition. The condition of the electrical stress is change to Vg=6V, Vd=12V, and Vs=0V, and the electrical stress is applied for 1000 seconds. Then, the longitudinal and transverse conductivity characteristics are measured, and the results are as shown in  FIGS. 6A-6D .  FIG. 6A  is the conductive current along the longitudinal direction of the S/D electrodes  206   a  and  206   b .  FIGS. 6B-6D  are similar to  FIGS. 5B-5D , indicating the transverse conductive currents measured in three regions along the longitudinal direction. Comparing with  FIG. 6A , no obvious change of sub-threshold, above threshold, and leakage currents occurs. 
     However, from the transverse conductivity characteristics of the three regions (B 1 S, B 1 M, B 1 D), it can be known that the polysilicon actually has the deterioration as shown in  FIGS. 6B-6D . The characteristics of the corresponding source end (B 1 S) and the middle region (B 1 M) of  FIGS. 6B and 6C  are not deteriorated. However, as for the characteristics of the drain end (B 1 D) as shown in  FIG. 6D , the sub-threshold and the saturated currents decrease, which shows that the polysilicon of the drain end has certain defects. As the defect cannot be effectively observed from the result as shown in  FIG. 6A , the wrong judgment that the device is not subjected to the stress and has no deterioration may be made. After prolonged operation, the driving current of the device will be lower than the expected value, which leads to the abnormal operation when it is applied to the driver or the amplifier of the display. 
     Therefore, when the reliability test is preformed, the aforementioned structure provided by the present invention can be used to measure various portions to achieve precise analysis, which effectively improves the reliability of the product. Moreover, other tests and applications are also possible. 
     In addition, the measured parameters of  FIGS. 5A-5D  and  FIGS. 6A-6D  are only examples for illustrating the application of testing. The parameters to be measured can be changed according to the actual requirement. In other words, the semiconductor device provided by the present invention can be applied to various tests, and is not limited to the application described above. 
     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 present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.