Abstract:
A manufacturing method of carbon nanotube transistors is disclosed. The steps include: forming an insulating layer on a substrate; forming a first oxide layer on the insulating layer using a solution with cobalt ion catalyst by spin-on-glass (SOG); forming a second oxide layer on the first oxide layer using a solution without the catalyst; forming a blind hole on the second oxide layer using photolithographic and etching processes, the blind hole exposing the first oxide layer, the sidewall of the second oxide layer, and the insulating layer; forming a single wall carbon nanotube (SWNT) connecting the first oxide layer separated by the blind hole and parallel to the substrate; and forming a source and a drain connecting to both ends of the SWNT, respectively.

Description:
BACKGROUND OF THE INVENTION 
     This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 092122853 filed in Taiwan on Aug. 20, 2003, the entire contents of which are hereby incorporated by reference. 
     1. Field of Invention 
     The invention relates to a manufacturing method of transistors and, in particular, to a manufacturing method of carbon nanotube transistors. 
     2. Related Art 
     In the trend of miniaturization, the manufacturing processes of the integrated circuit (IC) based upon silicon wafers are facing bottleneck problems in optics and physics and pressures from research investments. People have started trying various kinds of nanotransistors made from nanomolecules, so that hundreds of times more transistors than the prior art can be put into a same area. A nanometer is one-billionth meter. In the development of all sorts of nanotransistors, the technique that uses carbon nanotubes as the basic building blocks is the fastest. It is expected to be the best material for nano-grade computer products in the next generation. 
     The carbon nanotube was discovered by Japan NEC researcher in 1991 when he was studying carbon family chemicals. It is a cylindrical carbon material with a diameter between 1 and 30 nanometers. The carbon nanotube is known to be the thinnest tube discovered in Nature. It is thermally conductive, electrically conductive, robust, chemically stable, and soft. It is mainly comprised of one or several layers of unsaturated graphene layer. These little tubes are actually elliptical micro molecules. They are formed under high temperatures in the water vapor generated by carbon arc and laser. The central portion of the carbon nanotube graphene layer completely consists of six-cite rings. Both ends of the turning points have five- or seven-cite rings. Each carbon atom has the SP2 structure. Basically, the structure and chemical properties of the graphene layer on the carbon nanotube are similar to carbon sixty (C60). The carbon nanotubes can be semiconductors or conductors. Because of this special property, the carbon nanotube plays an important role in electronic circuits. 
     A necessary condition for using carbon nanotubes in future circuits is that they can be used to make transistors. The semiconductor carbon nanotube can be used as the gate in a field effect transistor (FET). Imposing a voltage can increase its conductivity to be 106 times that of the silicon semiconductor. The operating frequency can reach 1012 Hz, which is 1000 times the frequency that can reached by current CMOS. IBM has successfully used individual single wall or multi wall carbon nanotube as the channel of FET&#39;s to obtain carbon nanotube transistors for test. The single wall carbon nanotubes (SWNT&#39;s) consist of a single shell of carbon atoms. The so-called CNT is a macro carbon molecule with many properties. There are single wall CNT (SWCNT) and multiple wall CNT (MWCNT). There are three kinds of carbon nanotube preparation methods. The first is called the plasma discharging method; the second is called the laser ablation method; and the third is called the metal catalyst thermal chemical vapor deposition method, in which the carbon nanotubes are formed by using iron, cobalt, and nickel metal particles to thermally decompose acetylene or methane in a high-temperature furnace. 
     Using the reactions in the third type carbon nanotube production method, the disclosed manufacturing method of carbon nanotube FET&#39;s does not require the use of highly pollutant alkaline metals. The processes involved are very simple and compatible with existing IC processes. 
     SUMMARY OF THE INVENTION 
     An objective of the invention is to provide a manufacturing method of carbon nanotube transistors to solve the foregoing problems and difficulties in the prior art. 
     Another objective of the invention is to provide a manufacturing method of carbon nanotube transistors to simplify the conventional production processes. With currently available equipment, the production and research costs can be greatly reduced. 
     We disclose a general embodiment to demonstrate the invention can achieve the above objectives. The detailed steps include: forming an insulating layer on a substrate; forming a first oxide layer on the insulating layer using a solution with cobalt ion catalyst by spin-on-glass (SOG); forming a second oxide layer on the first oxide layer using a solution without the catalyst; forming a blind hole on the second oxide layer using photolithographic and etching processes, the blind hole exposing the first oxide layer, the sidewall of the second oxide layer, and the insulating layer; forming a single wall carbon nanotube (SWNT) connecting the first oxide layer separated by the blind hole and parallel to the substrate; and forming a source and a drain connecting to both ends of the SWNT, respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIGS. 1A through 1F show cross-sectional views of the production steps in the first embodiment of the invention; 
     FIGS. 2A through 2F show cross-sectional views of the production steps in the second embodiment of the invention; 
     FIGS. 3A through 3E show cross-sectional views of the production steps in the third embodiment of the invention; and 
     FIGS. 4A through 4I show cross-sectional views of the production steps in the fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A through 1F show the production steps of the carbon nanotube transistors according to a first embodiment of the invention. 
     As shown in FIG. 1A, an insulating layer  101  is formed on a substrate  100 . The insulating layer  101  can be made of SiO 2  or Si X N Y  using the chemical vapor deposition (CVD) method. 
     With reference to FIG. 1B, a first oxide layer  102  containing a catalyst is formed on the insulating layer  101 . First, a coating solution is prepared. The coating solution is applied on the insulating layer  101  by the SOG method. Finally, the coating solution layer (not shown) on the insulating layer is dried in two steps. The coating solution consists of at least a solution containing TEOS, pure alcohol and catalyst ions. One can also add an ammonia solution (NH 4 OH+alcohol). The catalyst ion can be cobalt, nickel, or iron ion. The two-step drying includes drying at the temperature of 100˜120° C. for one hour and then drying at the temperature of 350˜500° C. for another hour. 
     As shown in FIG. 1C, a second oxide layer without the catalyst is formed on the first oxide layer  102 . First, a coating solution is prepared and applied on the first oxide layer  102  by the SOG method. Afterwards, the coating solution layer (not shown) is dried. The coating solution consists at least a TEOS solution. 
     As shown in FIG. 1D, after exposure and developing using a mask, a blind hole  104  is formed by dry or wet etching. The blind hole  104  exposes part of the insulating layer  101 , the sidewall  105  of the first oxide layer  102 , and the sidewall  106  of the second oxide layer  103 . 
     As shown in FIG. 1E, a carbon nanotube  107  is formed. Both ends of the carbon nanotube  107  are connected to the sidewall  105  of the first oxide layer  102 . The alcohol (C 2 H 5 OH) inside the first oxide layer  102  is the reactant for the carbon nanotube  107 . It reacts with the catalyst inside the first oxide layer  102  under the temperature of 850° C. The reason it does not form the carbon nanotube between the sidewall  106  of the second oxide layer is that there is no reactant and catalyst in the second oxide layer  103 . Thus, the carbon nanotube  107  can be fixed between the sidewall  105  of the first oxide layer  102 . 
     As shown in FIG. 1F, a source  108   a  and a drain  108   b  are connected to both ends of the carbon nanotube  107 , respectively. The source  108   a  and the drain  108   b  can be formed using electron-beam (E-beam) photolithography along with a lift-off means. 
     Please refer to FIGS. 2A through 2F for the production steps in a second embodiment of the invention. 
     As shown in FIG. 2A, a first insulating layer  201  is formed on a substrate  200 . The insulating layer  201  can be made of SiO 2  or Si X N Y  using the chemical vapor deposition (CVD) method. 
     With reference to FIG. 2B, a source  208   a  and a drain  208   b  are formed on the first insulating layer  201 . The detailed steps include using metal sputtering to form a metal layer (not shown) on the first insulating layer  201  and using photolithography and etching to form the source  208   a  and the drain  208   b . They are separated by a gap  204 . The metal can be titanium. 
     As shown in FIG. 2C, a first oxide layer  202  with a catalyst and a second oxide layer  203  with no catalyst are formed on the substrate  200  that has the source  208   a , the drain  208   b , and the first insulating layer  201 . To form the first oxide layer, one first prepares a coating solution and applies the coating solution on the source  208   a  and the drain  208   b  by the SOG method, filling the gap  204 . Afterwards, the coating solution layer (not shown) covering the source  208   a , the drain  208   b , and the gap  204  is dried. The coating solution for the first oxide layer  202  consists of at least a solution containing TEOS, pure alcohol and catalyst ions. One can also add an ammonia solution (NH 4 OH+alcohol). The catalyst ion can be cobalt, nickel, or iron ion. To form the second oxide layer  203 , one first prepares a coating solution and applies the coating solution on the first oxide layer  202  by the SOG method. Afterwards, the coating solution (not shown) on the first oxide layer is dried. The coating solution here consists of at least a TEOS solution. 
     As shown in FIG. 2D, after exposure and developing using a mask, a blind hole  209  is formed by dry or wet etching. The blind hole  209  exposes part of the insulating layer  201 , the sidewall  205  of the first oxide layer  202 , the sidewall  206  of the second oxide layer  203 , and the sidewall  210  of the source  208   a  and the drain  208   b.    
     As shown in FIG. 2E, a carbon nanotube  207  is formed. Both ends of the carbon nanotube  207  are connected to the sidewall  205  of the first oxide layer  202 . The alcohol (C 2 H 5 OH) inside the first oxide layer  202  is the reactant for the carbon nanotube  207 . It reacts with the catalyst inside the first oxide layer  202  under the temperature of 850° C. The reason it does not form the carbon nanotube between the sidewall  206  of the second oxide layer is that there is no reactant and catalyst in the second oxide layer  203 . Thus, the carbon nanotube  207  can be fixed between the sidewall  205  of the first oxide layer  202 . 
     As shown in FIG. 2F, a second insulating layer  211  is formed on the second oxide layer  203  that contains the blind hole  209 . The forming method can be the CVD method. Once the second insulating layer  211  fills the blind hole  209 , it pushes down the carbon nanotube  207  in the blind hole  209 . The carbon nanotube  207  thus has a concave shape and touches the sidewall  210  of the source  208   a , the drain  208   b  and part of the first insulating layer  201 . Therefore, the carbon nanotube  207  connects the source  208   a  and the drain  208   b . The second insulating layer consists of SiO 2  or Si x N y . 
     Please refer to FIGS. 3A through 3F for the production steps in a third embodiment of the invention. 
     As shown in FIG. 3A, a first insulating layer  301  is formed on a substrate  300 . The insulating layer  301  can be made of SiO 2  or Si X N Y  using the chemical vapor deposition (CVD) method. 
     With reference to FIG. 3B, a source  308   a  and a drain  308   b  are formed on the first insulating layer  301 . The detailed steps include using metal sputtering to form a metal layer (not shown) on the first insulating layer  301  and using photolithography and etching to form the source  308   a  and the drain  308   b . They are separated by a gap  304 . The metal can be titanium. 
     As shown in FIG. 3C, a first oxide layer  302  with a catalyst and a second oxide layer  303  with no catalyst are formed on the substrate  300  that has the source  308   a , the drain  308   b , and the first insulating layer  301 . To form the first oxide layer, one first prepares a coating solution and applies the coating solution on the source  308   a  and the drain  308   b  by the SOG method, filling the gap  304 . Afterwards, the coating solution layer (not shown) covering the source  308   a , the drain  308   b , and the gap  304  is dried. The coating solution for the first oxide layer  302  consists of at least a solution containing TEOS, pure alcohol and catalyst ions. One can also add an ammonia solution (NH 4 OH+alcohol). The catalyst ion can be cobalt, nickel, or iron ion. To form the second oxide layer  303 , one first prepares a coating solution and applies the coating solution on the first oxide layer  302  by the SOG method. Afterwards, the coating solution (not shown) on the first oxide layer is dried. The coating solution here consists of at least a TEOS solution. 
     As shown in FIG. 3D, after exposure and developing using a mask, a blind hole  309  is formed by dry or wet etching. The blind hole  309  exposes part of the insulating layer  301 , the sidewall  305  of the first oxide layer  302 , the sidewall  306  of the second oxide layer  303 , and some surface and the sidewall  312  of the source  308   a  and the drain  308   b . The sidewall  312  of the source  308   a  and the drain  308   b  protrudes from the sidewall  305  of the first oxide layer  302  and the sidewall  306  of the second oxide layer  303 . 
     As shown in FIG. 3E, a carbon nanotube  307  is formed. Both ends of the carbon nanotube  307  are connected to the sidewall  305  of the first oxide layer  302 . The alcohol (C 2 H 5 OH) inside the first oxide layer  302  is the reactant for the carbon nanotube  307 . It reacts with the catalyst inside the first oxide layer  302  under the temperature of 850° C. The reason it does not form the carbon nanotube between the sidewall  306  of the second oxide layer is that there is no reactant and catalyst in the second oxide layer  303 . Thus, the carbon nanotube  307  can be fixed between the sidewall  305  of the first oxide layer  302 . Both end of the carbon nanotube  307  are connected to the surfaces of the source  308   a  and the drain  308   b.    
     Please refer to FIGS. 4A through 4I for the production steps in a fourth embodiment of the invention. 
     As shown in FIG. 4A, a first insulating layer  401  is formed on a substrate  400 . The insulating layer  401  can be made of SiO 2  or Si X N Y  using the chemical vapor deposition (CVD) method. 
     As shown in FIG. 4B, a first oxide layer  402  with a catalyst is formed on the first insulating layer  401 . First, one prepares a coating solution and applies it on the first insulating layer  401  by the SOG method. Afterwards, the coating solution layer (not shown) on the first insulting layer  401  is dried in two steps. The coating solution consists at least a solution containing TEOS, pure alcohol and catalyst ions. One can further add an ammonia solution (NH 4 OH+alcohol). The catalyst ion can be one of the cobalt, nickel, and iron ions. The two-step drying includes drying under the temperature of 100˜120° C. for one hour and then under the temperature of 350˜500° C. for another hour. 
     As shown in FIG. 4C, a second oxide layer  403  with no catalyst is formed on the first oxide layer  402 . To form the second oxide layer  403 , one first prepares a coating solution and applies it on the first oxide layer  402  by the SOG method. Afterwards, the coating solution layer (not shown) on the first oxide layer  402  is dried. The coating solution here consists at a TEOS solution. 
     As shown in FIG. 4D, after exposure and developing using a mask, a blind hole  404  is formed by dry or wet etching. The blind hole  404  exposes part of the insulating layer  401 , the sidewall  405  of the first oxide layer  402 , and the sidewall  406  of the second oxide layer  403 . 
     As shown in FIG. 4E, a carbon nanotube  407  is formed. Both ends of the carbon nanotube  407  are connected to the sidewall  405  of the first oxide layer  402 . The alcohol (C 2 H 5 OH) inside the first oxide layer  302  is the reactant for the carbon nanotube  307 . It reacts with the catalyst inside the first oxide layer  302  under the temperature of 850° C. The reason it does not form the carbon nanotube between the sidewall  306  of the second oxide layer is that there is no reactant and catalyst in the second oxide layer  403 . Thus, the carbon nanotube  407  can be fixed between the sidewall  405  of the first oxide layer  402 . 
     As shown in FIG. 4F, a second insulating layer  411  is formed on the second oxide layer  403  that contains the blind hole  404 . The second insulating layer  411  deposited in the blind hole  404  covers the carbon nanotube  407  and pushes it down for the carbon nanotube  407  to touch the first insulating layer  401 . 
     As shown in FIG. 4G, a photoresist pattern  413  is formed by photolithography to fill the blind hole  404  and to cover part of the second insulating layer  411  at the blind hole  411 . The photoresist pattern  413  does not cover the second insulating layer  411  outside the blind hole. 
     As shown in FIG. 4H, the area uncovered by the photoresist pattern  413  is removed by wet etching. The removed part includes the first oxide layer  402  and the second oxide layer  403  that are not covered by the photoresist pattern  413 . After the photoresist pattern  413  is removed, one is left with the carbon nanotube  407  on the first insulating layer and the protruding part  412  covering the carbon nanotube  407  and above the second insulating layer  411 . The protruding part  412  of the second insulating layer exposes both ends  407   a ,  407   b  of the carbon nanotube  407 . 
     As shown in FIG. 4I, a source  408   a  and a drain  408   b  are connected to the two ends  407   a ,  407   b  of the carbon nanotube  407 . The forming steps include first depositing a metal layer (not shown) on the first insulating layer  401  that contains the second insulating layer  414 , and then using photolithography and etching processes to form the source  408   a  and the drain  408   b  from the metal layer. 
     Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.