Abstract:
A method for forming a semiconductor structure. The method includes providing a semiconductor structure including a semiconductor substrate. The semiconductor substrate includes (i) a top substrate surface which defines a reference direction perpendicular to the top substrate surface and (ii) a semiconductor body region. The method further includes implanting an adjustment dose of dopants of a first doping polarity into the semiconductor body region by an adjustment implantation process. Ion bombardment of the adjustment implantation process is in the reference direction. The method further includes (i) patterning the semiconductor substrate resulting in side walls of the semiconductor body region being exposed to a surrounding ambient and then (ii) implanting a base dose of dopants of a second doping polarity into the semiconductor body region by a base implantation process. Ion bombardment of the base implantation process is in a direction which makes a non-zero angle with the reference direction.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to methods for changing threshold voltages of transistors and more particularly to methods for changing threshold voltages of transistors by ion implantation. 
     BACKGROUND OF THE INVENTION 
     In a conventional process for changing threshold voltage of a transistor on a chip, the semiconductor body of the transistor can be doped with dopants by ion implantation. Control of the value of the threshold voltage is critical to good performance and low power consumption of the circuits employing the transistors. In Multi-Gate FETs (MuGFETs), such as FinFETs, Tri-Gate, and other related structures, doping the MuGFET body is best performed with angled implants at some point following fin (body) formation, as this allows total dose delivery to the body to be minimally affected by incidental variations in fin dimensions. When multiple threshold-voltages are required for the same polarity (e.g. n-type) FET, it is desirable that the spacing between two FETs with differing threshold voltages can be as close, physically, as two such FETs having the same threshold voltage; this provides for high circuit density and low manufacturing cost. Unfortunately, this spacing is such that the blocking resist used to allow such threshold-adjusting implants into one FET and not an adjacent one, prohibits the use of tilted, or angled, implants that are needed to dope the fins in a uniform manner. Therefore, there is always a need for a method of changing the threshold voltage of a transistor that is better than that of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a semiconductor structure fabrication method, comprising providing a semiconductor structure which includes a semiconductor substrate, wherein the semiconductor substrate includes a top substrate surface which defines a reference direction perpendicular to the top substrate surface and pointing from outside to inside of the semiconductor substrate, and wherein the semiconductor substrate further includes a semiconductor body region; then implanting an adjustment dose of dopants of a first doping polarity into the semiconductor body region by an adjustment implantation process, wherein ion bombardment of the adjustment implantation process is in the reference direction; then patterning the semiconductor substrate resulting in side walls of the semiconductor body region being exposed to a surrounding ambient; and then implanting a base dose of dopants of a second doping polarity into the semiconductor body region by a base implantation process, wherein ion bombardment of the base implantation process is in a direction which makes a non-zero angle with the reference direction. 
     The present invention provides a method of changing the threshold voltage of a transistor that is better than that of the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1M  show cross-section views used to illustrate a fabrication process of a semiconductor structure, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A-1M  show cross-section views used to illustrate a fabrication process of a semiconductor structure  100 , in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 1A , the fabrication process of the semiconductor structure  100  starts with an SOI (Silicon On Insulator) substrate  110 + 120 + 130 . The SOI substrate  110 + 120 + 130  comprises a silicon substrate  110 , a BOX (buried oxide) layer  120  on top of the silicon substrate  110 , and an active silicon layer  130  on top of the BOX layer  120 . The SOI substrate  110 + 120 + 130  can be formed by a conventional method. Alternatively, the substrate can comprise a conventional (bulk) silicon wafer, and furthermore, may further comprise an isolation layer comprising either doped silicon or silicon-germanium in lieu of the BOX layer  120 . 
     Consider the case where FETs (field effect transistors) are to be formed on the semiconductor structure  100  of  FIG. 1A . More specifically, consider the case where a first FET (not shown), a second FET (not shown), and a third FET (not shown) having threshold voltages Vt 1 , Vt 2 , and Vt 3  are to be formed on body regions  131 ,  132 , and  133 , respectively. Assume further that Vt 1 , Vt 2 , and Vt 3  are achieved by, among other things, individually doping the body regions  131 ,  132 , and  133 , respectively. Assume further that if the body regions  131 ,  132 , and  133  are not individually doped, then the threshold voltages of the first, second, and third FETs would be Vt 01 , Vt 02 , and Vt 03 , respectively. Therefore, the individual doping of the body regions  131 ,  132 , and  133  results in the changes ΔVt 1 , ΔVt 2 , and ΔVt 3  in the threshold voltages of the first, second, and third FETs, respectively, wherein:
 
Δ Vt 1= Vt 1− Vt 01  (1)
 
Δ Vt 2= Vt 2− Vt 02  (2)
 
Δ Vt 3= Vt 3− Vt 03  (3)
 
     It is well known that implantation of p-type dopants in the body region of an FET results in an increase in the threshold voltage of the FET. In contrast, implantation of n-type dopants in the body region of an FET results in a decrease in the threshold voltage of the FET. Assume that ΔVt 1 &gt;0, ΔVt 2 &gt;0, and ΔVt 3 &gt;0. Therefore, ΔVt 1 , ΔVt 2 , and ΔVt 3  can be achieved by implanting p-type dopants in the body regions  131 ,  132 , and  133 , respectively. 
     Let Dp 1 , Dp 2 , and Dp 3  be the three doses of p-type dopants that need to be implanted into the body regions  131 ,  132 , and  133  to achieve ΔVt 1 , ΔVt 2 , and ΔVt 3 , respectively. It should be noted that a dose of dopants in a body region is the number of dopant atoms that are implanted into the body region. Also, let Db be a first base dose of p-type dopants, wherein Db&gt;0, Db&lt;Dp 1 , Db&lt;Dp 2 , and Db&lt;Dp 3 . 
     As a result, in one embodiment, Dp 1  can be implanted into the body region  131  by (i) implanting a first adjustment dose of p-type dopants Da 1  and then (ii) implanting the first base dose Db into the body region  131 , wherein Da 1 +Db=Dp 1 . Similarly, Dp 2  can be implanted into the body region  132  by (i) implanting a second adjustment dose of p-type dopants Da 2  and then (ii) implanting the first base dose Db into the body region  132 , wherein Da 2 +Db=Dp 2 . Similarly, Dp 3  can be implanted into the body region  133  by (i) implanting a third adjustment dose of p-type dopants Da 3  and then (ii) implanting the first base dose Db into the body region  133 , wherein Da 3 +Db=Dp 3 . Note that in practice, one can choose the first base dose, Db to directly achieve one of the desired values of Vt, in which case no adjustment implant (i.e. an adjustment dose of value zero) is employed. In one embodiment, Da 1 , Da 2 , and Da 3  are implanted into the body regions  131 ,  132 , and  133  by individual first, second, and third adjustment Vt implantation processes, respectively, whereas Db is implanted into each of the body regions  131 ,  132 , and  133  by a same first base Vt implantation process. 
     Assume that a fourth FET (not shown), a fifth FET (not shown), and a sixth FET (not shown) having threshold voltages Vt 4 , Vt 5 , and Vt 6  are to be formed on body regions  134 ,  135 , and  136  of the active silicon layer  130 , respectively. Assume further that Vt 4 , Vt 5 , and Vt 6  are achieved by, among other things, individually doping the body regions  134 ,  135 , and  136 , respectively. Assume further that if the body regions  134 ,  135 , and  136  are not individually doped, then the threshold voltages of the fourth, fifth, and sixth FETs would be Vt 04 , Vt 05 , and Vt 06 , respectively. Therefore, the individual doping of the body regions  134 ,  135 , and  136  results in the changes ΔVt 4 , ΔVt 5 , and ΔVt 6  in the threshold voltages of the fourth, fifth, and sixth FETs, respectively, wherein:
 
Δ Vt 4= Vt 4− Vt 04  (4)
 
Δ Vt 5= Vt 5− Vt 05  (5)
 
Δ Vt 6= Vt 6− Vt 06  (6)
 
     Assume that ΔVt 4 &gt;0, ΔVt 5 &gt;0, and ΔVt 6 &gt;0. Therefore, ΔVt 4 , ΔVt 5 , and ΔVt 6  can be achieved by implanting p-type dopants in the body regions  134 ,  135 , and  136 , respectively. 
     Let Dp 4 , Dp 5 , and Dp 6  be the three doses of p-type dopants that need to be implanted into the body regions  134 ,  135 , and  136  to achieve ΔVt 4 , ΔVt 5 , and ΔVt 6 , respectively. Also, let Db′ be a second base dose of p-type dopants, wherein Db′&gt;0, Db′&lt;Dp 4 , Db′&lt;Dp 5 , and Db′&lt;Dp 6 . As a result, in one embodiment, Dp 4  can be implanted into the body region  134  by (i) implanting a fourth adjustment dose of p-type dopants Da 4  and then (ii) implanting the second base dose Db′ into the body region  134 , wherein Da 4 +Db′=Dp 4 . Similarly, Dp 5  can be implanted into the body region  135  by (i) implanting a fifth adjustment dose of p-type dopants Da 5  and then (ii) implanting the second base dose Db′into the body region  135 , wherein Da 5 +Db′=Dp 5 . Similarly, Dp 6  can be implanted into the body region  136  by (i) implanting a sixth adjustment dose of p-type dopants Da 6  and then (ii) implanting the second base dose Db′ into the body region  136 , wherein Da 6 +Db′=Dp 6 . In one embodiment, Da 4 , Da 5 , and Da 6  are implanted into the body regions  134 ,  135 , and  136  by individual fourth, fifth, and sixth adjustment Vt implantation processes, respectively, whereas Db′ is implanted into each of the body regions  134 ,  135 , and  136  by a same second base Vt implantation process. 
     In one embodiment, the implantation processes described above can be carried out in detail as follows. 
     With reference to  FIG. 1B , in one embodiment, a pad film  140  is formed on top of the active silicon layer  130 . The pad film  140  can comprise silicon dioxide. The pad film  140  can be formed by (i) CVD (Chemical Vapor Deposition) of silicon dioxide on top of the active silicon layer  130  or (ii) thermally oxidizing the top surface of the active silicon layer  130  resulting in the pad film  140  on top of the active silicon layer  130 . 
     Next, with reference to  FIG. 1C , in one embodiment, a photoresist layer  150   a  is formed on top of the pad film  140 . The photoresist layer  150   a  can be formed by a spin-on process followed by baking. 
     Next, in one embodiment, the photoresist layer  150   a  is patterned resulting in a photoresist trench  151  in the photoresist layer  150   a  such that the entire body region  131  overlaps the photoresist trench  151  in a direction defined by an arrow  151 ′ (also called a direction  151 ′) which is perpendicular to the top surface  137  of the active silicon layer  130 . Moreover, the other body regions  132 ,  133 ,  134 ,  135 , and  136  ( FIG. 1A ) do not overlap the photoresist trench  151  in the direction  151 ′ (i.e., the entire body regions  132 ,  133 ,  134 ,  135 , and  136  overlap the patterned photoresist layer  150   a  in the direction  151 ′). 
     Next, in one embodiment, the body region  131  is doped with p-type dopants by the first adjustment Vt implantation process such that Da 1  is implanted into the body region  131 . More specifically, the body region  131  is doped by implanting p-type dopants into the body region  131  with the patterned photoresist layer  150   a  as a blocking mask. In one embodiment, the bombarding direction of the first adjustment Vt implantation process is in the direction  151 ′. The first adjustment Vt implantation process hereafter is referred to as the first adjustment Vt implantation process  151 ′. Hereafter, an ion implantation process and the arrow representing the direction of the ion bombardment of the ion implantation process have the same reference numeral for simplicity. After the first adjustment Vt implantation process  151 ′ is performed, the photoresist layer  150   a  is removed by wet etching. 
     In summary, the body region  131  is doped with Da 1  by performing the first adjustment Vt implantation process  151 ′ through the photoresist trench  151  using the patterned photoresist layer  150   a  as a blocking mask. 
     Next, with reference to  FIG. 1D , in one embodiment, the body region  132  is doped with Da 2 . The body region  132  is doped with Da 2  in a manner similar to the manner in which the body region  131  is doped with Da 1  in  FIG. 1C . More specifically, the body region  132  is doped with Da 2  by performing the second adjustment Vt implantation process  152 ′ through a photoresist trench  152  using a patterned photoresist layer  150   b  as a blocking mask. 
     Next, with reference to  FIG. 1E , in one embodiment, the body region  133  is doped with Da 3 . The body region  133  is doped with Da 3  in a manner similar to the manner in which the body region  131  is doped with Da 1  in  FIG. 1C . More specifically, the body region  133  is doped with Da 3  by performing the third adjustment Vt implantation process  153 ′ through a photoresist trench  153  using a patterned photoresist layer  150   c  as a blocking mask. 
     Next, with reference to  FIG. 1F , in one embodiment, the body region  134  is doped with Da 4 . The body region  134  is doped with Da 4  in a manner similar to the manner in which the body region  131  is doped with Da 1  in  FIG. 1C . More specifically, the body region  134  is doped with Da 4  by performing the fourth adjustment Vt implantation process  154 ′ through a photoresist trench  154  using a patterned photoresist layer  150   d  as a blocking mask. 
     Next, with reference to  FIG. 1G , in one embodiment, the body region  135  is doped with Da 5 . The body region  135  is doped with Da 5  in a manner similar to the manner in which the body region  131  is doped with Da 1  in  FIG. 1C . More specifically, the body region  135  is doped with Da 5  by performing the fifth adjustment Vt implantation process  155 ′ through a photoresist trench  155  using a patterned photoresist layer  150   e  as a blocking mask. 
     Next, with reference to  FIG. 1H , in one embodiment, the body region  136  is doped with Da 6 . The body region  136  is doped with Da 6  in a manner similar to the manner in which the body region  131  is doped with Da 1  in  FIG. 1C . More specifically, the body region  136  is doped with Da 6  by performing the sixth adjustment Vt implantation process  156 ′ through a photoresist trench  156  using a patterned photoresist layer  150   f  as a blocking mask. 
     Next, in one embodiment, after the removal of the patterned photoresist layer  150   f  ( FIG. 1H ) resulting in structure  100  of  FIG. 1I , the pad film  140  and the active silicon layer  130  are patterned resulting in the body regions  131 ,  132 ,  133 ,  134 ,  135 , and  136  of  FIG. 1J . Hereafter, the body regions  131 ,  132 ,  133 ,  134 ,  135 , and  136  are called the fin regions  131 ,  132 ,  133 ,  134 ,  135 , and  136 , respectively. 
     Next, with reference to  FIG. 1K , in one embodiment, a photoresist region  160  is formed on top of the fin regions  134 ,  135 , and  136  such that the fin regions  134 ,  135 , and  136  are not exposed to the surrounding ambient and such that the fin regions  131 ,  132 , and  133  are exposed to the surrounding ambient. The photoresist region  160  can be formed by a lithographic process. 
     Next, in one embodiment, the fin regions  131 ,  132 , and  133  are doped with p-type dopants by the first base Vt implantation process such that Db is implanted into each of the fin regions  131 ,  132 , and  133 . More specifically, the fin regions  131 ,  132 , and  133  are doped by implanting p-type dopants into the fin regions  131 ,  132 , and  133  with the photoresist region  160  as a blocking mask. In one embodiment, the ion bombardment of the first base Vt implantation process is performed in two bombarding directions defined by arrows  170   a  and  170   b  (also called directions  170   a  and  170   b ). The directions  170   a  and  170   b  are not perpendicular to the top surface  137  of the active silicon layer  130  ( FIG. 1C ). Alternatively, the ion bombardment of the first base Vt implantation process is performed in one of two directions  170   a  and  170   b . In one embodiment, the energies of ions incident on the fin regions  131 ,  132 , and  133  are sufficiently small such that the first base dose Db in each of the fin regions  131 ,  132 , and  133  created by the first base Vt implantation process essentially do not depend on the thicknesses  131 ′,  132 ′, and  133 ′ of the fin regions  131 ,  132 , and  133 , respectively. 
     As a result, the combination of Da 1  and Db in the fin region  131  results in Dp 1  in the fin region  131 . Similarly, the combination of Da 2  and Db in the fin region  132  results in Dp 2  in the fin region  132 . Similarly, the combination of Da 3  and Db in the fin region  133  results in Dp 3  in the fin region  133 . Because the fin regions  134 ,  135 , and  136  are covered by the photoresist region  160 , the fin regions  134 ,  135 , and  136  are not affected by the first base Vt implantation process. After the first base Vt implantation process is performed, the photoresist region  160  is removed by wet etching. In one embodiment, the spacing (i.e., horizontal distance) of the fins  131 ,  132 ,  133  and the spacing of the fins  134 ,  135 ,  136  each are smaller than the spacing of the fins  133  and  134 . 
     Next, with reference to  FIG. 1L , in one embodiment, a photoresist region  180  is formed on top of the fin regions  131 ,  132 , and  133  such that the fin regions  131 ,  132 , and  133  are not exposed to the surrounding ambient and such that the fin regions  134 ,  135 , and  136  are exposed to the surrounding ambient. The photoresist region  180  can be formed by a lithographic process. 
     Next, in one embodiment, the fin regions  134 ,  135 , and  136  are doped with p-type dopants by the second base Vt implantation process such that Db′ is implanted into each of the fin regions  134 ,  135 , and  136 . More specifically, the fin regions  134 ,  135 , and  136  are doped by implanting p-type dopants into the fin regions  134 ,  135 , and  136  with the photoresist region  180  as a blocking mask. In one embodiment, the ion bombardment of the second base Vt implantation process is performed in two bombarding directions defined by arrows  190   a  and  190   b  (also called directions  190   a  and  190   b ). The directions  190   a  and  190   b  are not perpendicular to the top surface  137  of the active silicon layer  130  ( FIG. 1C ). Alternatively, the ion bombardment of the second base Vt implantation process is performed in one of two directions  190   a  and  190   b . In one embodiment, the energies of ions incident on the fin regions  134 ,  135 , and  136  are sufficiently small such that the second base dose Db′ in each of the fin regions  134 ,  135 , and  136  created by the second base Vt implantation process essentially do not depend on the thicknesses  134 ′,  135 ′, and  136 ′ of the fin regions  134 ,  135 , and  136 , respectively. 
     As a result, the combination of Da 4  and Db′ in the fin region  134  results in Dp 4  in the fin region  134 . Similarly, the combination of Da 5  and Db′ in the fin region  135  results in Dp 5  in the fin region  135 . Similarly, the combination of Da 6  and Db′ in the fin region  136  results in Dp 6  in the fin region  136 . Because the fin regions  131 ,  132 , and  133  are covered by the photoresist region  180 , the fin regions  131 ,  132 , and  133  are not affected by the second base Vt implantation process. After the second base Vt implantation process is performed, the photoresist region  180  is removed by wet etching resulting in the structure  100  of  FIG. 1M . 
     Next, in one embodiment, the first, second, third, fourth, fifth, and sixth FETs (not shown) are formed on the fin regions  131 ,  132 ,  133 ,  134 ,  135 , and  136  of the structure  100  of  FIG. 1M , respectively. It should be noted that Dp 1 , Dp 2 , Dp 3 , Dp 4 , Dp 5 , and Dp 6  that have been implanted into the fin regions  131 ,  132 ,  133 ,  134 ,  135 , and  136  help result in the threshold voltages Vt 1 , Vt 2 , Vt 3 , Vt 4 , Vt 5 , and Vt 6  of the first, second, third, fourth, fifth, and sixth FETs, respectively. 
     In summary, the threshold voltages Vt 1 , Vt 2 , and Vt 3  of the first, second, and third FETs are achieved by, among other things, the individual adjustment Vt implantation processes  151 ′,  152 ′, and  153 ′, respectively, followed by the first base Vt implantation process. The threshold voltages Vt 4 , Vt 5 , and Vt 6  of the fourth, fifth, and sixth FETs are achieved by, among other things, the individual adjustment Vt implantation processes  154 ′,  155 ′, and  156 ′, respectively, followed by the second base Vt implantation process. 
     With reference to  FIG. 1C , it should be noted that the adjustment Vt implantation process  151 ′ is performed in the direction  151 ′ which is perpendicular to the top surface  137  of the active silicon layer  130 . Therefore, the advantage of the adjustment Vt implantation process  151 ′ is that ions implanted into the fin region  131  in the direction  151 ′ are not affected (are not obstructed) by the patterned photoresist layer  150   a . It is well known that the dose of dopants in the fin region  131  created by the adjustment Vt implantation process  151 ′ (ion implantation in vertical direction) depends on the thickness  131 ′ of the fin region  131 . This dependency is undesirable. 
     With reference to  FIG. 1K , it should be noted that the first base Vt implantation process is performed in the directions  170   a  and  170   b , wherein the directions  170   a  and  170   b  are not perpendicular to the top surface  137  of the active silicon layer  130  ( FIG. 1C ). It should be noted that because the energies of ions incident on the fin regions  131 ,  132 , and  133  are sufficiently small, the doses of dopants in the fin regions  131 ,  132 , and  133  created by the first base Vt implantation process essentially do not depend on the thicknesses  131 ′,  132 ′, and  133 ′ of the fin regions  131 ,  132 , and  133 , respectively. The disadvantage of the first base Vt implantation process is that ions implanted into the fin regions  131 ,  132 , and  133  in the directions  170   a  and  170   b  are affected (i.e., are obstructed) by the photoresist region  160 . This obstruction can be seen clearly in  FIG. 1K  when ions are implanted into the fin regions  131 ,  132 , and  133  in the direction  170   b.    
     In the embodiments described above, Dp 1  in the fin region  131  is achieved by the combination of the adjustment Vt implantation process  151 ′ and the first base Vt implantation process. Therefore, this combination (i) takes the advantage and (ii) reduces the disadvantage of both the adjustment Vt implantation process  151 ′ and the first base Vt implantation process. Therefore, this combination of the present invention is better than the prior art in which Dp 1  in the fin region  131  is achieved by either (a) ion implantation in vertical direction or (b) ion implantation in slant direction. Similarly, the other implantation processes to implant Dp 2 , Dp 3 , Dp 4 , Dp 5 , and Dp 6  into the fin regions  132 ,  133 ,  134 ,  135 , and  136 , respectively, are also better than the prior art. 
     In the description above, it is assumed that ΔVt 1 &gt;0, ΔVt 2 &gt;0, and ΔVt 3 &gt;0. Assume alternatively that ΔVt 1 &lt;0, whereas ΔVt 2 &gt;0 and ΔVt 3 &gt;0. Therefore, in one embodiment, ΔVt 1  can be achieved by (i) implanting the first adjustment dose of n-type dopants Dan 1  and then (ii) implanting the first base dose of p-type dopants Db into the body region  131 , wherein the combination of Dan 1  and Db in the fin region  131  results in ΔVt 1 . 
     In the description above, it is assumed that Db&lt;Dp 1 , Db&lt;Dp 2 , and Db&lt;Dp 3  (wherein Db is the first base dose of p-type dopants). Assume alternatively that Db is selected such that Db&gt;Dp 1 , whereas Db&lt;Dp 2  and Db&lt;Dp 3 . As a result, ΔVt 1  can be achieved by (i) implanting the first adjustment dose of n-type dopants Dan 2  and then (ii) implanting the first base dose of p-type dopants Db into the body region  131 , wherein the combination of Dan 2  and Db in the fin region  131  results in ΔVt 1 . 
     In the description above, it is assumed that Db is the first base dose of p-type dopants. Assume alternatively that Db is selected such that Db is the first base dose of n-type dopants. As a result, ΔVt 1  can be achieved by (i) implanting the first adjustment dose of p-type dopants Da 1 ′ and then (ii) implanting the first base dose of n-type dopants Db into the body region  131 , wherein the combination of Da 1 ′ and Db in the fin region  131  results in ΔVt 1 . Similarly, ΔVt 2  can be achieved by (i) implanting the second adjustment dose of p-type dopants Da 2 ′ and then (ii) implanting the first base dose of n-type dopants Db into the body region  132 , wherein the combination of Da 2 ′ and Db in the fin region  132  results in ΔVt 2 . Similarly, ΔVt 3  can be achieved by (i) implanting the third adjustment dose of p-type dopants Da 3 ′ and then (ii) implanting the first base dose of n-type dopants Db into the body region  133 , wherein the combination of Da 3 ′ and Db in the fin region  133  results in ΔVt 3 . 
     While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.