Abstract:
A simplified tri-layer process for forming a thin film transistor matrix for a liquid crystal display is disclosed. By using a backside exposure technique twice, two masking steps for patterning an etch stopper layer, and an upper doped and a lower intrinsic semiconductor layers, respectively, can be omitted. Further, owing to the back-exposing energy for patterning the semiconductor layers is less than that for patterning the etch stopper layer, the resulting etch stopper layer is enclosed with the resulting semiconductor layers, and the contact of the two semiconductor layers can be achieved.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for forming a thin film transistor (TFT) matrix for a liquid crystal display (LCD), and more particularly to a simplified tri-layer process for forming the TFT matrix with reduced masking steps. 
     BACKGROUND OF THE INVENTION 
     For conventional manufacturing processes of a TFTLCD, a tri-layer process and a back channel etch (BCE) process are main streams for forming the TFT matrix. Compared to a BCE structure, a tri-layer structure additionally includes an top nitride over the semiconductor layer as a etch stopper so that the etching step for defining a source/drain and channel region can be well controlled. Accordingly, the thickness of the active layer can be made to be thinner in the tri-layer structure than in the BCE structure, which is advantageous for the stability of resulting devices and performance in mass production. However, the provision of the additional etch stopper layer needs an additional masking step, thereby making the tri-layer process relatively complicated. 
     Conventionally, six to nine masking steps are required for either a BCE process or a tri-layer process. After the formation of the TFT matrix, a step of providing a black matrix around each pixel electrode region is generally required to improve the performance of the LCD. The provision of the black matrix after the process, however, will have difficulty in alignment. 
     On the other hand, the count of photo-masking and lithography steps directly affects not only the production cost but also the manufacturing time. Moreover, for each photo-masking and lithography step, the risks of mis-alignment and contamination may be involved so as to affect the production yield. Therefore, many efforts have been made to improve the conventional processes to reduce masking steps. 
     For example, for a BCE structure, U.S. Pat. Nos. 5,346,833 and 5,478,766 issued to Wu and Park et al., respectively, disclose 3 and/or 4 -mask processes for making a TFTLCD, which are incorporated herein for reference. By the way, it is to be noted that the 3-mask process for each of Wu and Park et al. does not include the step of forming and patterning of a passivation layer. If a passivation layer is required to assure of satisfactory reliability, the count of photo-masking and lithography steps should be four. Further, Wu and Park et al. use an ITO layer, which is integrally formed with the ITO pixel electrode, as the connection line between the TFT unit and the data line so that the area of the TFTLCD is limited due to the high resistivity of ITO. 
     As for the tri-layer structure, a conventional 6-mask process is illustrated as follows with reference to FIGS.  1 A˜ 1 G which are cross-sectional views of intermediate structures at different stages. The conventional process includes steps of: 
     i) applying a first conductive layer onto a glass substrate  10 , and using a first photo-masking and lithography procedure to pattern and etch the first conductive layer to form an active region  11  consisting of a scan line and a gate electrode of a TFT unit, as shown in FIG. 1A; 
     ii) sequentially forming tri-layers including an insulation layer  121 , a semiconductor layer  122  and an etch stopper layer  123 , and a photoresist  124  on the resulting structure of FIG. 1A, as shown in FIG.  1 B. 
     iii) using a second photo-masking and lithography procedure to pattern and etch the etch stopper layer  123  to form an etch stopper  13  which have a shape similar to the shape of the gate electrode, as shown in FIG. 1C; 
     iv) using a third photo-masking and lithography procedure to pattern and etch the semiconductor layer  122  to form a channel structure  14 , as shown in FIG. 1D; 
     v) sequentially applying a doped semiconductor layer and a second conductive layer on the resulting structure of FIG. 1D, and using a fourth photo-masking and lithography procedure to pattern and etch them to form source/drain regions  15  and data and connection lines  16 , as shown in FIG. 1E; 
     vi) applying a passivation layer  17  on the resulting structure of FIG. 1E, and using a fifth photo-masking and lithography procedure to pattern and etch the passivation layer  17  to create tape automated bonding (TAB) openings (not shown), and create a contact window  18 , as shown in FIG. 1F; and 
     vii) applying a transparent electrode layer on the resulting structure of FIG. 1F, and using a sixth photo-masking and lithography procedure to pattern and etch the transparent electrode layer to form a pixel electrode  19 , as shown in FIG.  1 G. 
     Six masking steps, however, are still too complicated. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a reduced mask process for forming a thin film transistor (TFT) matrix for a liquid crystal display (LCD), in which the count of photo-masking and lithography steps can be reduced to three. 
     Another object of the present invention is to provide a tri-layer process for forming a thin film transistor (TFT) matrix for a liquid crystal display (LCD), in which the connection line between a TFT unit and a data line has a relatively low resistivity compared to the ITO connection line so as to be suitable for a large-area TFTLCD. 
     According to a first aspect of the present invention, a process for forming a thin film transistor (TFT) matrix for a liquid crystal display (LCD) includes steps of providing a substrate made of an insulating material; forming a first conductive layer on a first side of the substrate, and using a first masking and patterning procedure to remove a portion of the first conductive layer to define a scan line and a gate electrode of a TFT unit; successively forming an insulation layer, a semiconductor layer, an etch stopper layer, and a first photoresist layer on the substrate with the scan line and the gate electrode; providing a first exposing source from a second side of the substrate opposite to the first side by using the scan line and the gate electrode as shields to obtain a first exposed area and a first unexposed area; removing the first photoresist layer, and the etch stopper layer of the first exposed area so that the remaining portion of the etch stopper layer in the first unexposed area has a specific shape similar to the shape of the scan line together with the gate electrode; forming a doped semiconductor layer and a second photoresist layer on the substrate with the etch stopper layer of the specific shape; providing a second exposing source from a second side of the substrate opposite to the first side by using the scan line and the gate electrode as shields to obtain a second exposed area and a second unexposed area; removing the second photoresist layer, the doped semiconductor layer of the second exposed area, and the semiconductor layer of the second exposed area so that shapes of remaining portions of the doped semiconductor layer and the semiconductor layer in the second unexposed area are similar to that of the etch stopper layer of the specific shape, but have areas larger than that of the etch stopper layer of the specific shape, thereby making the doped semiconductor layer in contact with the semiconductor layer; successively forming a transparent conductive layer and a second conductive layer on the substrate, and using a second masking and patterning procedure to remove a portion of the transparent conductive layer and a portion of the second conductive layer to define a pixel electrode region and data and connection lines, respectively; removing another portion of the doped semiconductor layer with a remaining portion of the second conductive layer as shields to define source/drain regions; forming a passivation layer on the substrate, and using a third masking and patterning procedure to remove a portion of the passivation layer; and removing another portion of the second conductive layer with the patterned passivation layer as shields to expose the pixel electrode region. 
     When the exposing sources are light radiation, the insulating material is a light-transmitting material such as glass. 
     In an embodiment, the second exposing source has an intensity less than an intensity of the first exposing source. For example, the intensity of the first exposing source is ranged between 1000-1500 mJ/cm 2 , preferably about 1250 mJ/cm 2 , and the intensity of the second exposing source is ranged between 400-600 mJ/cm 2 , preferably about 500 mJ/cm 2 . 
     In an alternative embodiment, the second exposing source is provided within a period of time less than a period of time within which the first exposing source is provided. For example, when the first and second exposing sources are of the same intensity, the period of time for providing the first exposing source is ranged between 400-600 ms, preferably about 500 ms, and the period of time for providing the second exposing source is ranged between 150-250 ms, preferably about 200 ms. 
     Of course, the second exposing source can be provided both weaker and shorter, or stronger but even shorter, or weaker but longer that the first exposing source is, as long as the purposes that the shapes of the remaining portions of the doped semiconductor layer and the semiconductor layer in the second unexposed area are similar to that of the etch stopper layer of the specific shape, but have areas larger than that of the etch stopper layer of the specific shape, and the doped semiconductor layer is in contact with the semiconductor layer are achieved. 
     Preferably, the first conductive layer is formed of chromium, molybdenum, tantalum, tantalum molybdenum, tungsten molybdenum, aluminum, aluminum silicide, copper, or a combination thereof. 
     Preferably, the insulation layer is formed of silicon nitride, silicon oxide, silicon oxynitride, tantalum oxide, aluminum oxide or a combination thereof. 
     Preferably, the etch stopper layer is formed of silicon nitride, silicon oxide, or silicon oxynitride. 
     Preferably, the semiconductor layer is formed of intrinsic amorphous silicon, micro-crystalline silicon or polysilicon, and the doped semiconductor layer is formed of highly doped amorphous silicon, highly doped micro-crystalline silicon or highly doped polysilicon. 
     Preferably, the transparent conductive layer is formed of indium tin oxide, indium zinc oxide or indium lead oxide. 
     Preferably, the second conductive layer is a chromium/aluminum or a molybdenum/aluminum/molybdenum composite layers. 
     Preferably, the passivation layer is formed of silicon nitride or silicon oxynitride. 
     Preferably, the third masking and patterning procedure additionally removes portions of the etching stopper layer and the semiconductor layer to define an isolation window. 
     Preferably, the third masking and patterning procedure additionally defines a plurality of pad regions around the TFT matrix. 
     After the third masking and patterning procedure, it is preferred that a portion of the second conductive layer surrounding the pixel electrode remains as a black matrix. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention may best be understood through the following description with reference to the accompanying drawings, in which: 
     FIGS.  1 A˜ 1 G are cross-sectional views of intermediate structures of a conventional TFTLCD, which schematically show the formation of the TFT matrix; 
     FIGS.  2 A˜ 2 L are cross-sectional views of intermediate structures of a TFTLCD according to the present invention, which schematically show a preferred embodiment of a process for forming the TFT matrix; 
     FIGS.  3 A˜ 3 E are partial top plane views corresponding to the structures of FIGS. 2B,  2 F,  2 I,  2 K and  2 L, respectively; 
     FIG. 4 is a partial top plane view schematically showing pad regions around the TFT matrix of a TFTLCD; and 
     FIG. 5 is a photo bias vs. exposing energy plot for a certain exposing source. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following description of the preferred embodiment of this invention is presented herein for purpose of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed. 
     A preferred embodiment of a process for forming a TFT matrix of a TFTLCD according to the present invention directs to a three-mask process, and it is illustrated with reference to FIGS.  2 A˜ 2 L. It is to be noted that the division of steps and the provision of serial numbers as below are for corresponding to the drawings, and for easy illustration and understanding, instead of critically indicating the separation or the integration of steps. The preferred embodiment of the process includes steps of: 
     i) applying a Cr layer  21  onto a front side  201  of a glass substrate  20  as a first conductive layer, as shown in FIG. 2A; 
     ii) using a first photo-masking and lithography procedure to pattern and etch the Cr layer  21  to form an active region  31  consisting of a scan line  311  and a gate electrode  312  of a TFT unit, as shown in FIGS. 2B and 3A wherein FIG. 2B is a cross-sectional view taken along the A—A line of FIG. 3A; 
     iii) consecutively and sequentially forming a silicon nitride layer  221  as an insulation layer, an intrinsic amorphous silicon (i-a-Si) layer  222  as a semiconductor layer, a top silicon nitride layer  223  as an etch stopper layer, and a photoresist  224  on the resulting structure of FIG. 2B, as shown in FIG. 2C, and exposing the resulting structure from the back side  202  of the substrate  20 , as indicated by arrows, wherein a portion of the photoresist  224  above the region  31  is shielded by the region  31  from exposure so as to exhibit a self-aligned effect; 
     iv) etching off the exposed photoresist  224 , a portion of the top silicon nitride layer  223  thereunder, and the remaining photoresist so that the remaining etch stopper structure  23  has a shape similar to the region  31 , as shown in FIG.  2 D and with reference to FIG. 3A; 
     v) applying an n +  amorphous silicon layer  225  on the resulting structure of FIG. 2D as a highly doped semiconductor layer, and then another photoresist layer  226 , as shown in FIG. 2E, and exposing the resulting structure from the back side  202  of the substrate  20  with less energy than the exposure energy in step iii), as indicated by arrows, wherein a portion of the photoresist  226  above the region  31  is shielded by the region  31  from exposure so as to exhibit a self-aligned effect; 
     vi) etching off the exposed photoresist  226 , a portion of the n +  amorphous silicon layer  225 , a portion of the intrinsic amorphous silicon layer  222  and the remaining photoresist so that the remaining semiconductor layers  222  and  225  have shapes similar to the shape of the region  31  or the etch stopper structure  223 , but have relatively large areas, as shown in FIG.  2 F and with reference to FIG. 3B wherein FIG. 2F is a cross-sectional view taken along the B—B line of FIG. 3B, thereby defining a channel region  22  and achieving a contact  40  of the layers  222  and  225 ; 
     vii) sequentially applying an ITO layer  227  and a Cr/Al composite layer  228  on the resulting structure of FIG. 2F as a transparent conductive layer and a second conductive layer, respectively, as shown in FIG. 2G; 
     viii) using a second photo-masking and lithography procedure to pattern and etch the ITO layer  227  and the Cr/Al composite layer  228  to define a pixel electrode region  27  and data and connection lines  28 , respectively, as shown in FIG. 2H; 
     ix) using the remaining Cr/Al layer as a shield to etch off a portion of the n + amorphous silicon layer  225  to define source/drain regions  25 , as shown in FIG.  2 I and with reference to FIG. 3C, wherein FIG. 2I is a cross-sectional view taken along the C—C line of FIG. 3C, and where the TFT unit  32  is formed; 
     x) applying a silicon nitride layer  229  on the resulting structure of FIG. 2I as a passivation layer, and using a third photo-masking and lithography procedure to pattern and etch the passivation layer  229  and the etch stopper layer  223  to define an isolation window area  291 , expose a portion of the data and connection lines  28 , as shown in FIG. 2J, and define tape automated bonding (TAB) openings as pad regions  33  around the TFT unit  32 , as shown in FIG. 4; 
     xi) removing the i-a-Si layer  222  in the isolation window area  291  to create an isolation window  29 , as shown in FIG.  2 K and with reference to FIG. 3D wherein FIG. 2K is a cross-sectional view taken along the D—D line of FIG. 3D; 
     xii) removing another portion of the Cr/Al layer  228  in the pixel electrode region  27  with the patterned passivation layer as shields to define a pixel electrode  30 , as shown in FIG.  2 L and with reference to FIG. 3E wherein FIG. 2L is a cross-sectional view taken along the E—E line of FIG.  3 E. 
     In the step i) of the above embodiment, the first conductive layer  21  can be applied by any suitable conventional technique which is not to be redundantly described here. In this embodiment, the substrate  20  is formed of glass which is transparent for allowing light type of exposing source to transmit therethrough. The substrate  20 , however, can also be made of another transparent, translucent or opaque material, depending on the type of the exposing source. On the other hand, the first conductive layer  21  can also be formed of molybdenum, tantalum molybdenum, tungsten molybdenum, tantalum, aluminum, aluminum silicide or copper. 
     In the step ii) of the above embodiment, the photo-masking and lithography procedure can be performed by any suitable conventional technique which is not to be redundantly described here. 
     In the step iii) of the above embodiment, the insulation layer  221 , semiconductor layer  222 , etch stopper layer  223  and photoresist  224  can be applied by any suitable conventional techniques which are not to be redundantly described here. The insulation layer  221  can also be formed of silicon oxide, silicon oxynitride, tantalum oxide or aluminum oxide. The semiconductor layer  222  can also be formed of micro-crystalline silicon or polysilicon. The etch stopper layer  223  can also be formed of silicon oxide or silicon oxynitride. It is to be noted that the there preferably exists a high etching selectivity between the etching stopper layer  223  and the semiconductor layer  222  so that the etching procedure of the etching stopper layer will not damage the semiconductor layer. For example, the etching gas for the semiconductor layer is selected from a group consisting of carbon tetrafluoride (CF 4 ), boron trichloride (BCl 3 ), chlorine (Cl 2 ), sulfur hexafluoride (SF 6 ), and a mixture thereof. On the other hand, the etching gas for the etch stopper layer is selected from a group consisting of carbon tetrafluoride/hydrogen (CF 4 /H 2 ), trifluoromethane (CHF 3 ), sulfur hexafluoride/hydrogen (SF 6 /H 2 ), and a mixture thereof 
     On the other hand, it is preferred that the exposing energy used in the step iii) is strong enough to define the photoresist  224  to obtain a pattern having a shape similar to the active region  31  but having an area slightly smaller than the active region  31 . 
     In the step iv) of the above embodiment, the etching procedure can be performed by any suitable conventional technique which is not to be redundantly described here. 
     In the step v) of the above embodiment, the doped semiconductor layer  225  and photoresist  226  can be applied by any suitable conventional techniques which are not to be redundantly described here. The doped semiconductor layer  225  can also be formed of highly doped micro-crystalline silicon or highly doped polysilicon. It is to be noted that the there preferably exists a high etching selectivity between the semiconductor layer  222  and the insulation layer  221  and so that the etching procedure of the semiconductor layers  225  and  222  will not damage the insulation layer. 
     On the other hand, the purpose of exposing the resulting structure from the back side  202  of the substrate  20  with less energy than the exposure energy in step iii) can be achieved by providing a weaker exposing source or a shorter exposing time. For example, the exposing intensity in step iii) is 1250 mJ/cm 2 , and the exposing intensity in step v) is 500 mJ/cm 2 . In an alternative example, when both of the exposing intensities are 500 mJ/cm 2 ,the exposing time in step iii) is 500 ms, and the exposing time in step v) is 200 ms. Owing to the back-exposing energy for patterning the semiconductor layers  225  and  222  is less than that for patterning the etch stopper layer  223 , the relatively small etch stopper structure  223  is enclosed with the relatively large semiconductor layers  225  and  222  with a photo bias d (FIG. 2F) of about 0.2-0.7 microns, and preferably 0.4 microns. Accordingly, the contact  40  of the two semiconductor layers  225  and  222  can be achieved. FIG. 5 illustrates the relationship between photo bias in micrometer and exposing energy in arbit unit (a.u.) of a certain exposing source wherein 1 a.u.=2.5 mJ/cm 2 . By adjusting the exposing energy of the exposing source, a proper photo bias is obtained so as to change the critical dimension of the photoresist mask. In other words, the quantity of the exposing energy has a highly effect on the critical dimension of a photoresist mask. Therefore, the top-view area of the patterned photoresist can be controlled by adjusting the exposing energy applied thereon. 
     In the step vi) of the above embodiment, the etching procedure can be performed by any suitable conventional technique which is not to be redundantly described here. 
     In the step vii) of the above embodiment, the transparent conductive layer  227  and the second conductive layer  228  can be applied by any suitable conventional techniques which are not to be redundantly described here. In this embodiment, the transparent conductive layer  227  can also be formed of indium zinc oxide or indium lead oxide. The second conductive layer  228  can also be a Mo/Al/Mo layer. 
     In the step viii) of the above embodiment, the photo-masking and lithography procedure and the etching procedure can be performed by any suitable conventional techniques which are not to be redundantly described here. The term “data and connection lines  28 ” used herein includes a data line  28   a , a connection line  28   b  and an additional portion  28   c , wherein the first connection line  28   b  connects the data line  28   a  and the TFT unit  32 , and the additional portion  28   c  will be further etched in the step xii) to leave a portion surrounding the pixel electrode  30  to serve as a black matrix  301 . 
     In the step ix) of the above embodiment, the etching procedure can be performed by any suitable conventional technique which is not to be redundantly described here. 
     In the step x) of the above embodiment, the passivation layer  229  can be applied by any suitable conventional technique which is not to be redundantly described here. The passivation layer  229  can also be formed of silicon oxynitride. On the other hand, the photo-masking and lithography procedure can be performed by any suitable conventional technique which is not to be redundantly described here. 
     In the step xi) of the above embodiment, the etching procedure can be performed by any suitable conventional technique which is not to be redundantly described here. It is to be noted that the formation of the isolation window  29  and thus the definition of the isolation window area  291  are not essential to the TFT matrix. The high impedance of the channel region  22  may exhibit an isolation effect to some extent. 
     In the step xii) of the above embodiment, the etching procedure can be performed by any suitable conventional technique which is not to be redundantly described here. 
     According to the process mentioned above, the formation of the etch stopper structure  223  is performed by a backside exposure technique as disclosed in the step iii), which uses the existent active region as shields so that one masking step is omitted. The formation of the channel region  22  is performed by another backside exposure technique as disclosed in the step v), which also uses the existent active region as shields so that another masking step is omitted. Further, in this embodiment, the transparent conductive layer is formed before the second conductive layer rather than after the passivation layer. Therefore, an additional masking and patterning procedure for creating the contact via for exposing the data and connection lines can be omitted. In other words, in this embodiment, only three masking and patterning procedures are required. 
     Moreover, there is an additional advantage by having the transparent conductive layer formed before the second conductive layer. After a portion of the second conductive layer is further removed after the third masking and patterning procedure, a remaining portion of the second conductive layer surrounding the pixel electrode can function as black matrix, as shown in FIG.  3 E. 
     While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.