Method of forming poly-silicon thin film transistors

A method of forming poly-silicon thin film transistors is described. An amorphous silicon thin film transistor is formed on a substrate, and then the Infrared (IR) heating process is used. A gate metal and source/drain metal are heated rapidly, and conduct heat energy to an amorphous silicon layer. Next, crystallization occurs in the amorphous silicon layer to form poly-silicon. Therefore a poly-silicon thin film transistor is produced.

FIELD OF THE INVENTION

The present invention relates to a method of forming a poly-silicon thin film transistor, and more particularly, to a method of changing an amorphous silicon thin film transistor directly into a poly-silicon thin film transistor.

BACKGROUND OF THE INVENTION

Thin film transistors (TFT) have generally been used as devices for active matrix liquid crystal displays (AMLCD). The thin film transistors usually have two types of amorphous silicon (a-Si) and poly-silicon (poly-Si) for different silicon films. The mobility of an a-Si TFT is only about 0.5–1 cm2/V-s, but a poly-Si TFT has much higher mobility for better crystal character and fewer crystal defects in poly-Si. Therefore, displays fabricated from poly-Si TFT have advantages of high response speed, high resolution, and integrated driver circuits.

There are some drawbacks such as lower product yield, complex processes, and high process cost existing in the poly-Si TFT fabrication. In contrast, a-Si TFTs are mainly fabricated for displays with a lower process cost and well-developed techniques.

Owing to the bottleneck of crystallization techniques, the poly-Si TFT is still not the market mainstream; the conventional methods for fabricating poly-Si films are solid phase crystallization (SPC), excimer laser annealing (ELA), and metal induced lateral crystallization (MILC). SPC is not applicable to flat panel display fabrication because the upper limit temperature of a glass substrate is 650° C., while ELA has drawbacks of the high cost for laser light, process stability, and poor crystal uniformity. Besides, MILC may have problems in metal diffusion. Therefore, present poly-Si forming methods have technical problems and higher production cost for more complex processes. So the poly-Si TFT is still unable to compete with the a-Si TFT.

Further, because the technical problems in poly-Si films fabrication have not been overcome, some manufacturing difficulties exist if the poly-Si TFT fabrication present is utilized to develop large-size displays.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a method of forming poly-Si TFT for improving the flat panel display performance. The poly-Si TFT with high mobility can be produced and the superiority in a-Si TFT fabrication also can be kept at the same time. Each film material has different absorption of infrared rays (IR); a metal film with good absorption of IR and thermal stability is used as a hot plate, and the metal film serving as the hot plate absorbs heat energy from the IR by IR heating after producing an a-Si TFT. Then, the metal film transfers the heat energy to the a-Si layer, and the a-Si layer crystallizes to become a poly-Si layer, thus forming a poly-Si TFT.

According to one preferred embodiment of this invention, a bottom-gate a-Si TFT with back channel etch (BCE) structural type is produced on a substrate, and then IR heating is performed. A gate metal and a source/drain (S/D) metal in the a-Si TFT absorb heat energy from the IR rapidly and transfer the heat energy to the a-Si layer; the a-Si layer is therefore induced to crystallize and become a poly-Si layer. Thus, a BCE-type a-Si TFT is transformed into a BCE-type poly-Si TFT.

According to another preferred embodiment of this invention, a bottom-gate a-Si TFT with channel protect (CHP) structural type is produced on a substrate, and then IR heating is performed. A gate metal and a S/D metal in the a-Si TFT absorb heat energy from the IR rapidly and transfer the heat energy to the a-Si layer; the a-Si layer is therefore induced to crystallize and become a poly-Si layer. Thus, a CHP-type a-Si TFT is transformed into a CHP-type poly-Si TFT.

According to still another preferred embodiment of this invention, a top-gate a-Si TFT is produced on a substrate, and then IR heating is performed. In the same way, a gate metal and a S/D metal in the a-Si TFT absorb heat energy from the IR rapidly and transfer the heat energy to the a-Si layer; the a-Si layer is therefore induced to crystallize and become a poly-Si layer. Thus, a top-gate a-Si TFT is transformed into a top-gate poly-Si TFT.

With the application of the present invention, in addition to preserving the superiority of a-Si TFT fabrication, the poly-Si TFT with good electrical performance is also formed. Thus, the driver circuits can be integrated on a panel to reduce the cost of adding driver circuits to the a-Si TFT.

Further, the poly-Si TFT devices formed by the present invention are useful for improving the liquid crystal display (LCD) performance and even employed to drive the organic light-emitting display (OLED).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention utilizes the different IR absorption of various film materials; the IR heating process is performed on an a-Si TFT product, and the film material with higher IR absorption and good thermal stability absorbs heat energy from the IR and transfers the heat energy to the a-Si layer. The a-Si layer consequently crystallizes to become a poly-Si layer. Hot plate crystallization (HPC) technique is utilized, the film material with higher IR absorption and good thermal stability in the TFT is used as a hot plate, and then the hot plate transfers heat to the a-Si layer to induce crystallization in the a-Si layer. The a-Si layer is thus changed into the poly-Si layer. Therefore, the IR heating process is merely added after the a-Si TFT fabrication processes without changing the general processes and process conditions in the a-Si TFT fabrication, and then the poly-Si TFT is directly obtained from the a-Si TFT.

The metal film is the film material with higher IR absorption and good thermal stability apparently in the TFT devices, and therefore a gate metal and a source/drain (S/D) metal all can be used as the hot plates for absorbing heat energy from the IR. Then, the a-Si layer is crystallized by obtaining the heat energy transferred from the metal films; the a-Si layer is thus effectively changed into the poly-Si layer, and the poly-Si TFT is formed.

The poly-Si TFT is formed by combining the present a-Si TFT fabrication processes with the present invention. According to differences in the TFT structure, there are two kinds of bottom-gate and top-gate structures. The bottom-gate structure further comprises two types of BCE and CHP structure generally used in the a-Si TFT fabrication. The method of forming the poly-Si TFT in accordance with the present invention is combined with the foregoing TFT structures respectively to produce the poly-Si TFT.

The present invention discloses a method of forming the poly-Si TFT combined with the bottom-gate BCE structural type. Referring toFIG. 1A, a gate metal102is first formed on a substrate100by, for example, physical vapor deposition (PVD), and is then patterned by, for example, photolithography and etching. The substrate100may be a glass substrate, and the gate metal102is a material with good electric conductivity such as chromium (Cr), molybdenum (Mo) or moly-tungsten (MoW), and the gate metal102is also a material with good IR absorption and thermal stability.

A gate insulator104, an amorphous silicon (a-Si) layer106, and a doped a-Si layer108are formed in turn on the gate metal102and the substrate100simultaneously by, for example, plasma enhanced chemical vapor deposition (PECVD), and the preferred material of the gate insulator104is silicon nitride (SiNx) or silicon oxide (SiOx). Then, the a-Si layer106and the doped a-Si layer108are patterned partially by, for example, photolithography and etching to form an active layer region (not shown).

Next, referring toFIG. 1B, a S/D metal110is formed by, for example, PVD, and is patterned by, for example, photolithography and etching to form a data-line. The S/D metal110is a material with good electric conductivity such as Cr, Mo or MoW, and the S/D metal110is also a material with good IR absorption and thermal stability. Then, with the S/D metal110serving as a hard mask, the doped a-Si layer108between the S/D metal110is etched to form an opening112exposing a portion of the a-Si layer106; the a-Si layer106in the opening112represents a channel region.

The a-Si TFT is finished through the aforementioned processes; then, referring toFIG. 1C, the heating process with an IR114is utilized, and the a-Si TFT is directly changed into the poly-Si TFT.

The heating process with the IR114can utilize any IR supply method; the preferable method in the present invention is a pulsed rapid thermal processing (PRTP) technology. Because of the film materials with different IR absorption, the TFT is heated selectively by the IR114. The gate metal102and the S/D metal110have higher IR absorption and so absorb heat energy from the IR114rapidly. Therefore, the gate metal102and the S/D metal110are heated selectively to serve as the hot plates for transferring the heat energy to the doped a-Si layer108and the a-Si layer106, and then the doped a-Si layer108and the a-Si layer106are induced to crystallize to become a doped poly-Si layer109and a poly-Si layer107. The poly-Si TFT is thus formed.

The highest corresponding output temperature of the heating process is preferably about 900° C. The proportion occupied by metal films in the TFT is sufficient to transfer the heat energy absorbed by the doped a-Si layer108and the a-Si layer106for forming a poly-Si. The heating process employed is a pulsed heating process, not a continuous heating, and heats the film materials in the TFT selectively. Further, a glass substrate is unable to absorb the IR114effectively, so the device property is not affected and there are no glass substrate deformation problems associated with the process temperature.

The present invention discloses another method of forming the poly-Si TFT combined with the bottom-gate CHP structural type. Referring toFIG. 2A, a gate metal202is first formed on a substrate200by, for example, PVD, and is then patterned by, for example, photolithography and etching. The substrate200may be a glass substrate, and the gate metal202is a material with good electric conductivity such as Cr, Mo or MoW; the gate metal202is also a material with good IR absorption and thermal stability.

A gate insulator204, an a-Si layer206, and a protective layer208are formed in turn on the gate metal202and the substrate200simultaneously by, for example, PECVD, and the preferred material of the gate insulator204and the protective layer208is SiNxor SiOx.

Referring toFIG. 2B, the protective layer208is patterned by, for example, photolithography and etching to form an etching stop layer209for protecting the channel region. Then, a doped a-Si layer210is formed on the a-Si layer206by, for example, PECVD. The a-Si layer206and the doped a-Si layer210are patterned partially by, for example, photolithography and etching to form an active layer region (not shown).

Next, referring toFIG. 2C, a S/D metal212is formed by, for example, PVD, and is patterned by, for example, photolithography and etching to form a data-line. The S/D metal212is a material with good electric conductivity such as Cr, Mo or MoW, and the S/D metal110is also a material with good IR absorption and thermal stability. Then, with the S/D metal212serving as a hard mask, the doped a-Si layer210between the S/D metal212is etched to form an opening214exposing the etching stop layer209. The a-Si layer206under the etching stop layer209represents the channel region. Because the etching stop layer209protects the a-Si layer206, the etching for the doped a-Si layer210stops at the etching stop layer209and avoids damaging the a-Si layer206under the etching stop layer209.

The a-Si TFT is finished through the aforementioned processes. With reference toFIG. 2D, the heating process with an IR216is utilized, and the a-Si TFT is directly changed into a poly-Si TFT.

The heating process with the IR216in the present invention is also preferably PRTP technology, and the TFT is heated selectively by the rapid heating process. The highest corresponding output temperature of the heating process is preferably about 900° C. With the gate metal202and the S/D metal212serving as hot plates to absorb heat energy from the IR216rapidly and then transfer the heat energy to the doped a-Si layer210and the a-Si layer206, and the doped a-Si layer210and the a-Si layer206are induced to crystallize and become a doped poly-Si layer211and a poly-Si layer207. The poly-Si TFT is thus formed.

The present invention discloses further another method of forming the poly-Si TFT combined with the top-gate structural type. Referring toFIG. 3A, a buffer layer302is first formed on a substrate300by, for example, PECVD. The substrate300may be a glass substrate, and the buffer layer302may be a SiOxlayer. Then, an a-Si layer304is formed on the buffer layer302by, for example, PECVD.

Next, a gate insulator306is formed on the a-Si layer304by, for example, PECVD; the preferred material of the gate insulator306is SiOx. A gate metal308is formed by, for example, PVD, and is then patterned by, for example, photolithography and etching. The gate metal308is a material with good electric conductivity such as Cr, Mo or MoW, and the gate metal308is also a material with good IR absorption and thermal stability.

Then, referring toFIG. 3B, an ion-implantation is performed, with the gate metal308serving as a mask, and the a-Si layer304on two sides of the gate-metal308is implanted with ions to define a S/D region. A dielectric interlayer310is deposited by, for example, PECVD, and is then patterned by, for example, photolithography and etching to form contact holes311which exposes the S/D region. The preferred material of the dielectric interlayer310is SiNxor SiOx.

Finally, a S/D metal312is formed by, for example, PVD, and is then patterned by, for example, photolithography and etching to form a data line. The S/D metal312is on the dielectric interlayer310and in the contact holes311to contact the a-Si layer304in the S/D region. The S/D metal312is a material with good electric conductivity such as Cr, Mo or MoW, and the S/D metal312is also a material with good IR absorption and thermal stability.

The a-Si TFT is finished through the aforementioned processes. With reference toFIG. 3C, the heating process with an IR314is utilized, and the a-Si TFT is directly changed into the poly-Si TFT.

The heating process with the IR314in the present invention is also preferably a PRTP technology, and the TFT is heated selectively by the rapid heating process. The highest corresponding output temperature of the heating proves is preferably about 900° C. With the gate metal308and the S/D metal312serving as hot plates to absorb heat energy from the IR314rapidly and then transfer the heat energy to the a-Si layer304, and the a-Si layer304is induced to crystallize and become a poly-Si layer205. The poly-Si TFT is thus formed.

With the foregoing embodiments, the only addition to the general a-Si TFT fabrication processes is the IR heating. The metal film materials in the TFT then absorb heat energy from the IR and transfer the heat energy to the a-Si layer, and the a-Si layer is induced to crystallize and become a poly-Si layer. Therefore, the a-Si TFT can be changed directly into a poly-Si TFT by employing the IR heating. The poly-Si TFT formed by the present invention can keep the advantages of the a-Si TFT fabrication and have a better electrical performance at the same time.

In addition to the LCD, the present invention also can be employed to fabricate a poly-Si TFT for driving the OLED, and then the product performance is improved greatly.

The present invention is not limited employed in TFT fabrication for flat panel display; other poly-Si TFT devices also can be fabricated by using the present invention to improve product efficiency. While the present invention has been disclosed with reference to the preferred embodiments of the present invention, it should not be considered as limited thereby. Various possible modifications and alterations by one skilled in the art can be included within the spirit and scope of the present invention, the scope of the invention is determined by the claims that follow.