Abstract:
A method with a mechanically strained silicon for enhancing the speeds of integrated circuits or devices is disclosed. The method with a mechanically strained silicon for enhancing the speeds of integrated circuits or devices includes the following steps: (a) providing a substrate, (b) fixing the substrate, (c) applying a stress upon the substrate, and (d) inducing a strain in one of a device and a circuit by stressing the substrate.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for enhancing the speed of integrated circuits, and more particularly to a method with a mechanically strained silicon for enhancing the speed of integrated circuits or devices. 
   BACKGROUND OF THE INVENTION 
   In the field of the integrated circuit technology, the mobility of the electronic component is limited by the physical property of the carrier. In the prior art, it has disclosed that the strained silicon is applied on the metal oxide-semiconductor field-effect transistors (MOSFETs) in order to modify the physical property of the devices and increase the speed of MOSFETs. The silicon is typically formed on the relaxed silicon-germanium buffer layers. Because of the different lattice constants of silicon and germanium, wherein the lattice constant of germanium is 4% larger than that of silicon, the silicon would be strained by the tensile stress provided from the silicon-germanium buffer layers. The silicon-germanium buffer can be formed on the substrate of silicon-on-insulator (SOI) or on the traditional substrate of silicon. No matter the silicon-germanium buffer is formed on the substrate of SOI or on the traditional substrate of silicon, the speeds of P-type MOSFETs and N-type MOSFETs will be improved and increased. 
   If the strained silicon is directly formed on the traditional silicon wafer, the relaxed silicon-germanium buffer of the substrate has to be formed on the traditional silicon wafer first. Some defects would be performed due to the large difference between the lattice constant of the relaxed silicon-germanium buffer and that of silicon wafer. In order to minimize such kind of defects, the graded relaxed silicon-germanium buffer with increasing concentration of the germanium, where the thickness of the relaxed silicon-germanium buffer is around 0.5 to 2 μm, will be grown on silicon substrate first beneath the relaxed silicon-germanium buffer with a constant concentration of germanium, then the thin layer of the strained silicon layer is formed on the top of the buffer layer by the epitaxy growth. In all kinds of the methods applied for forming the blanket strained silicon in the prior art, the relaxed silicon-germanium buffer has to be formed on the substrate first. The steps of forming the relaxed silicon-germanium buffer on the substrate will induce the generateon of the dislocation at the interface of the strained silicon layer and the silicon-germanium buffer, and the generateon of the dislocation will decrease the quality of the strained silicon. 
   The mechanical method for providing the strain on the substrate is the four point bending method disclosed by Jeffery C. Shuling (as shown in Jeffery C. Shuling et al, IEEE Sensors Journal, Vol. 1, No. 1, pp. 14-30). The first step of the four point bending method is to fix the whole wafer with two points of the substrate, and then the strain on the wafer is induced by applying the stress with the other two points of the substrate. Generally speaking, the method is applied for the adjustment of the stress on the material of piezoresistance. Hitachi company in Japan and the research groups of MIT have used this method to process the strains of the components in the applications of MOSEFTs. Although the four point bending method is convenient and easy to be processed on the strained substrate, the strained substrate can&#39;t sustain the strain for a long time. 
   Therefore, the present invention provides a method for maintain the strained silicon layer for a long time, and the method with a mechanically strained silicon is capable of enhancing the speed of integrated circuits or devices so as to overcome the disadvantages of the prior art described above. 
   SUMMARY OF THE INVENTION 
   It is an aspect of the present invention to provide a method for enhancing the speed of integrated circuits or devices on the substrate. According to the present invention, the substrate is strained when it is applied by the stress. After the substrate is strained, the effective mass of the carrier on the silicon channel is decreased and carrier mobility of the channel increased. Therefore, due to the enhancement of the carrier mobility, the operating speed of the devices on the substrate is substantially increased. 
   In accordance with an aspect of the present invention, a method for providing a strain in the device with one of a device and circuit on the substrate is provided. The method includes the following steps: (a) providing a substrate, (b) fixing the substrate, (c) applying a stress upon the substrate, and (d) inducing a strain in one of a device and a circuit by stressing the substrate. 
   Preferably, the substrate is made of one selected from a group consisting of a silicon, polysilicon, amorphous silicon, silicon germanium, compound substrate containing the elements of groups III, IV, and V, plastics, and metal sheet. 
   Preferably, the substrate is one of a silicon-on-insulator (SOI) and a silicon-germanium-on-insulator (SGOI). 
   Preferably, the substrate is one selected from a group consisting of a substrate with a fabricated device, a substrate with a fabricated circuit, a substrate having a surface attached to a device, and a substrate having a surface attached to a circuit. 
   Preferably, the substrate is propped up by a transverse rod when the edge of the substrate is fixed, such that a tensile strain is mainly generated on a side propped up by the rod, and a compressive strain is mainly generated on the other side. 
   Preferably, at least two points on one side of the substrate are propped up by the rod when the edge of the substrate is fixed, such that two kinds of strain are formed on either side of the substrate. 
   Preferably, the substrate has one of a device and a circuit on either side of the substrate by one of an integrated fabrication and a glue. 
   Preferably, one of said device and said circuit is attached on said substrate by wafer bonding. 
   Preferably, the strain is one selected from a group consisting of a tensile strain, a compressive strain, and a combination thereof, and is provided on either side of the substrate. 
   Preferably, the method is employed by a set of mechanical modules. 
   Preferably, the mechanical modules include a device for clipping and hooking and a movable shaft device. 
   Preferably, a coagulative fluid which is melt at high temperature is injected into the mechanical modules, and then the strained substrate is fixed by the coagulated solid at room temperature. 
   It is another aspect of the present invention to provide a method with a mechanically strained silicon for enhancing a speed of one of an integrated circuit and a devices thereof. The method includes the following steps: (a) providing a first substrate, (b) forming plural holes on a surface of the first substrate, (c) filling the plural holes of the first substrate with a volume-changeable substance, (d) providing a second plane, (e) covering the second substrate onto the first substrate, (f) changing a volume of the substance, and (g) inducing a strain on the second substrate by the volume-changed substance. 
   Preferably, the volume changed in the step (f) is achieved by one of providing different thermal expansion coefficients between the volume changeable substance and substrate and performing a chemical reaction within the substrate. 
   Preferably, one of a device and a circuit is formed on the second substrate by one of an integrated formation and an attachment. 
   Preferably, the second substrate is one of a silicon-on-insulator (SOI) and a silicon-germanium-on-insulator (SGOI). 
   Preferably, an integrated circuit component and a photoelectrical component are formed on the second substrate before binding the first substrate and the second substrate. 
   Preferably, a surface of the second substrate is flattened for fabricating the integrated circuit and the photoelectrical component thereon after the strain is formed on the second substrate. 
   Preferably, the strain is one selected from a group of a tensile strain, a compressive strain, and a combination thereof. 
   The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed descriptions and accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1(   a ),  1 ( b ), and  1 ( c ) are the schematical views showing the substrate strained by a stress according to the preferred embodiment of the present invention; 
       FIGS. 2(   a ),  2 ( b ), and  2 ( c ) are the schematical views showing the substrate strained by a stress according to the preferred embodiment of the present invention; 
       FIGS. 3(   a ) and  3 ( b ) are the schematical views showing the complex substrate strained by a stress according to the preferred embodiment of the present invention; 
       FIGS. 4(   a ),  4 ( b ) and  4 ( c ) are the schematical views showing the module for providing a stress according to the preferred embodiment of the present invention; 
       FIGS. 5(   a ) and  5 ( b ) are the schematical views showing the strained substrate contained in the coagulative fluid according to the preferred embodiment of the present invention; 
       FIGS. 6(   a ),  6 ( b ),  6 ( c ) and  6 ( d ) are the schematical views showing the method of applying a strain to the substrate by changing the volume of the volume-changeable substance in the tank according to the preferred embodiment of the present invention; 
       FIG. 7  is an analysis graph of the strained substrate obtained by analyzing the strain thereof with a simulation software; and 
       FIG. 8  is a graph illustrating the relationships between the drain currents and the drain voltages of the transistors on the unstrained substrate and on the strained substrate. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for the purpose of illustration and description only; it is not intended to be, exhaustive or to be limited to the precise form disclosed. 
   The present invention provides a method for providing a mechanical stress on the substrate to bend the substrate and form a strained substrate. Once the substrate is strained by the stress, the effective masses of the electrons on the strained channel and those of the holes on the strained channel are decreased, and the mobility of the electrons on the strained channel and those of the holes on the strained channel are increased. After the physical property of the strained substrate has modified into a better situation, the operation speed of the devices on the strained substrate increase due to the increasing mobility of the electrons on the strained channel and those of the holes on the strained channel. Therefore, the properties of the electronic devices on the strained substrate, such as MOSFETs, are improved and the mobility of the electronic devices on the strained substrate are increased. 
   Please refer to  FIGS. 1(   a ) to  1 ( c ) which show are the basic embodiment of the present invention.  FIG. 1(   a ) is a top view of the round substrate  10 . There are devices/integrated circuits  13 ,  14 ,  15 , and  16  attached to the substrate or fabricated on either side of the round substrate  10 . The edge of the round substrate  10  is fixed by the clipping devices  12  as shown in  FIG. 1(   b ). When the round substrate  10  is applied by a upward mechanical stress  20 , the round substrate  10  would be bended as the strained substrate  11  as shown in  FIG. 1(   c ). The devices/integrated circuits  13  and  15  on the top of the round substrate  10  is under a tensile strain, and the devices/integrated circuits  14  and  16  on the bottom of the round substrate  10  is under a compressive strain. 
   The method of applying the stress on the square substrate  17  and the strain on the square substrate induced thereby could be found in  FIGS. 2 . Please refer to  FIGS. 2(   a ) to  2 ( c ), which illustrate the examples of the square substrate  17  and the strained square substrate  18 .  FIG. 2(   a ) is a top view of the square substrate  17 . Before the square substrate  18  is stressed, the clipping devices  12  are applied to fix the edge of the square substrate  17  first. When the edge of the square substrate  17  is fixed, the square substrate  17  is pressed by a upward mechanical stress  20  from the symmetrical axis of the square substrate  17  in order to induce the symmetrical strain on the square substrate  17 . The symmetrical strain is obtained after the square substrate  17  is pressed by the mechanical stress  20 . A simpler and easier embodiment of providing a mechanical stress on the symmetrical axis of the square substrate is shown in  FIG. 2(   b ). The first step of the method is to put a transverse rod  21  under the bottom of the square substrate  17 , wherein the transverse rod  21  is along the direction of the symmetrical axis of the square substrate  17 . Then the transverse rod  21  is raised up to the square substrate  17  to provide the mechanical stress  20  on the square substrate  17 . After the mechanical stress  20  is applied on the square substrate  17 , it is bended and became the strained square substrate  18 . Another method for forming the strained square substrate is illustrated in  FIG. 2(   c ). Several vertical supporting rods  22  are put equally under the bottom of the square substrate  17 , wherein the vertical supporting rods  22  are along the symmetrical axis of the square substrate  17 . Each distance between the adjacent supporting rods is the same. Two adjacent vertical supporting rods  22  are taken as a sample and shown in  FIG. 2(   c ). The even and equal stresses are applied from various vertical supporting rods  22 , and the square substrate  17  is pressed by the sum of the mechanical stresses  20 , which are from the whole vertical supporting rods  22 . After that, the square substrate  17  is bended as the strained square substrate  18  by the sum of the mechanical stresses  20 . 
   The substrate could be the substrate whose surface has not been processed, the semiconductor substrate whose surface has been processed for the integrated circuits already, or the complex substrates formed by the substrates  31  and  32  in  FIGS. 3(   a ) and  3 ( b ). In addition, the substrates  31  and  32  could be processed by the method of a mechanical grind, an chemical etching or a smart-cut in order to control the thickness of the substrates  31  and  32  when the substrates are bound or bonded with other substrate or when the substrates  31  and  32  are still not be strained. The strained substrates  311  and  321  would be formed by the substrates  31  and  32  after the mechanical stress  20  is applied. 
   Except the above methods of applying the stress for the strain on the substrate, there are some other methods to achieve this goal, such as the specific mechanical modules or appliances to induce the substrate strained. When the specific mechanical module in the present invention is applied to strain the substrate, the steps of the method are simplified. Such result is shown in  FIGS. 4(   a ) to  4 ( c ), the mechanical module  40  for providing the mechanical stress contains the clipping devices  41  and the movable shaft device  42 . The mechanical module  40  is suitable for the substrates with various shapes when the substrate  43  ( FIG. 4(   a )) could be fixed on the mechanical module  40  by the clipping devices  41 , wherein the clipping devices  41  are on the two sides of the mechanical module  40 . When the substrate  43  is fixed on the mechanical module  40 , the movable shaft  42  raises the substrate  43  from the bottom of the substrate  43  and applies the stress on the substrate  43 . Accordingly, and the substrate  43  is strained as the strained substrate  44  shown in  FIG. 4  ( b ). The device with one movable shaft  42  is shown in  FIG. 4(   b ), and the device with multiple symmetrical movable shafts  45  is shown in  FIG. 4(   c ). Please refer to  FIGS. 4(   a ) to  4 ( c ), the mechanical module  40  provides the symmetrical stress on the substrates  43  and the symmetrically strained substrates  44  are formed accordingly. 
   After the substrate is strained by the mechanical stress, it needs to maintain the substrate in the strained situation for a long time in order to improve the increasing speeds of integrated circuits on the strained substrate in a long-term period. For this reason, the strained substrate  44 , the movable shaft device  42  and the mechanical module  40  are put into the tank  50  full of the coagulative fluid  51 .  FIG. 5(   a ) is a view showing the strained substrate  44  and the mechanical module  40  in the tank  50 . After the strained substrate  44  is coagulated by the coagulative fluid  51  in the tank  50 , the strained substrate  44  is separated from the mechanical module  40 , and the situation is shown in  FIG. 5(   b ). After the separation of the strained substrate  44  and the mechanical module  40 , the source of the mechanical stress for the strained substrate  44  is transferred from the mechanical module  40  to the coagulated solid  52 . The mechanical stress provided from the coagulated solid  52  is kept applying on the strained substrate  44  to maintain the strain. When the mechanical module  40  has been separated from the strained substrate  44 , the module  40  for providing the mechanical stress to the substrate can be repeatedly used for several times. The benefit of the repeatable mechanical module  40  is capable of decreasing the, cost for processing the strained substrate and being helpful on the assembling process of the semiconductor industry. 
   In addition,  FIGS. 6(   a ) to  6 ( d ) provide another method for applying the mechanical stress on the substrate. Please refer to  FIG. 6(   a ), there are several fillisters  61  made on the top of the module  60 . Please refer to  FIG. 6(   b ), after the volume-changeable substance  62  is filled into the fillisters  61  of the module  60 , the substrate  43  is covered on the module  60  in order to seal the fillisters  61 . The methods for connecting the substrate  43  with the module  60  tightly so as to seal the fillisters  61  of the module  60  could be performed by binding or bonding. After the fillisters  61  are sealed, the volume of volume-changeable substance  62  is changed by changing the temperature or changing the pressure. If the volume-changeable substance  62  becomes the swelling substance  63 , as shown in  FIG. 6(   c ), the swelling substance  63  is applied to provide the tensile stress on the substance  43 , and the substrate  43  becomes the strained substrate  46  due to the tensile stress provided by the swelling substance  63 . If the volume-changeable substance  62  becomes the condensing substance  64 , the condensing substance  64  provides the compressive stress on the substrate  43 , and the substrate  43  becomes the strained substrate  47  due to the compressive stress provided by the condensing substance  63 . No matter what kind of the strained substrate is, such as the strained substrate  46  caused by the tensile stress or the strained substrate  47  caused by the compressive stress, the flattening process for the surface of the strained substrate could be performed by the grind method or the etching method. 
   In order to understand the effect of the strained substrate clearly, the simulation software is applied to analyze the speed of the strained substrate. The increasing operation speed of the devices on the strained substrate is proved by the experiment.  FIG. 7  is an analysis graph of the strained substrate when the additional stress is applied on it, and the graph is analyzed by the simulation software named “ANSYS”. The analysis graph shows the strain distributions on the substrate when it is pressed by the mechanical stress. According to the graph, it&#39;s known that when the stress is closer to the center of the substrate, the strain of the substrate is larger.  FIG. 8  is a graph illustrating the relationship between the drain currents and the drain voltages on the transistors of the strained substrate made by the method in the present invention. After the substrate is strained, no matter what the value of the gate voltage is, the drain current of the strained substrate is increased when it is compared to the drain current measured on the unstrained substrate. In  FIG. 8 , the drain current of the strained substrate is increased by 6.5%. 
   In conclusion, the present invention provides a method with a mechanically strained silicon for enhancing the speeds of integrated circuits or devices, and the efficiency of the devices on the substrate will increase indeed. In addition, it is possible that the drain current would be increased when a optimal experiment is provided. 
   While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention needs not be limited to the disclosed embodiments. 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.