Abstract:
A technique for and structures for camouflaging an integrated circuit structure. The technique includes the use of a light density dopant (LDD) region of opposite type from the active regions resulting in a transistor that is always off when standard voltages are applied to the device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to U.S. patent application Ser. No. ______ filed on the same date as this application and entitled “Symmetric Non-Intrusive and Covert Technique to Render a Transistor Permanently Non-Operable” (attorney docket 620296-9), the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The technology disclosed herein relates to integrated circuits (ICs) and semiconductor devices in general and their methods of manufacture wherein the integrated circuits and semiconductor devices employ camouflaging techniques which make it difficult for the reverse engineer to discern how the semiconductor device functions.  
       RELATED TECHNOLOGY  
       [0003]     The presently disclosed technology is related to the following US patents: 
        (1) U.S. Pat. Nos. 5,866,933; 5,783,375 and 6,294,816 teach transistors in a CMOS circuit that are connected by implanted (and therefore hidden and buried) lines between the transistors by modifying the p+ and n+ source/drain masks. These implanted interconnections form 3-input AND or OR circuits that look substantially identical to the reverse engineer. Also, buried interconnects force the reverse engineer to examine the IC in greater depth to try to figure out the connectivity between transistors and hence their function.     (2) U.S. Pat. Nos. 5,783,846; 5,930,663 and 6,064,110 teach a further modification in the implant masks so that the implanted connecting lines between transistors have a gap inserted, with approximately the length of the minimum feature size of the CMOS technology being used. If this gap is “filled” with one kind of implant, the line conducts; but if it is “filled” with another kind of implant, the line does not conduct. The intentional gaps are called channel blocks. The reverse engineer is forced to determine connectivity on the basis of resolving the implant type at the minimum feature size of the CMOS process being used.     (3) U.S. Pat. No. 6,117,762 teaches method and apparatus for protecting semiconductor integrated circuits from reverse engineering. Semiconductor active regions are formed on a substrate and a silicide layer is formed both over at least one active region of the semiconductor active regions and over a selected substrate area for interconnecting the at least one active region with another region through the silicide formed on the selected substrate area. This connection, as affected by the silicide layer, is substantially invisible to the reverse engineer unless imaged via cross-sectional techniques, which are prohibitively costly and time consuming.        
 
       BACKGROUND INFORMATION  
       [0007]     The creation of complex integrated circuits and semiconductor devices can be a very expensive undertaking given the large number of hours of sophisticated engineering talent involved in designing such devices. Additionally, integrated circuits can include read only memories and/or EEPROMs into which software, in the form of firmware, is encoded. Additionally, integrated circuits are often used in applications involving the encryption of information. Therefore, in order to keep such information confidential (i.e. design, critical information and encryption), it is desirable to keep such devices from being reverse engineered. Thus, there are a variety of reasons for protecting integrated circuits and other semiconductor devices from being reverse engineered.  
         [0008]     In order to keep the reverse engineer at bay, different techniques are known in the art to make integrated circuits more difficult to reverse engineer. One technique is to alter the composition or structures of the transistors in the circuit in such a way that the alteration is not easily apparent, forcing the reverse engineer to carefully analyze each transistor (in particular, each CMOS transistor pair for CMOS devices), and thwarting attempts to use automatic circuit and pattern recognition techniques in order to reverse engineer an integrated circuit. Since integrated circuits can have hundreds of thousands or even millions of transistors, forcing the reverse engineer to carefully analyze each transistor in a device can effectively frustrate the reverse engineer&#39;s ability to reverse engineer the device successfully.  
         [0009]     A conductive layer, such as silicide, is often used during the manufacturing of semiconductor devices. In modern CMOS processing, especially with a feature size below 0.5 μm, a silicide layer is utilized to improve the conductivity of gate, source and drain contacts. In accordance with general design rules, any active region providing a source or drain is silicided. This silicide layer is very thin and difficult for the reverse engineer to see. Hence, if there are ways to modify the transistor through the modification of the silicide layer so as to change the transistor functionality then the modification would be difficult to determine.  
         [0010]      FIG. 1  depicts a prior art modern CMOS device. In this example, the substrate  20  is a p-type substrate. Referring to the NMOS device, active regions  4 ,  6  disposed in the substrate  20  have n-type conductivity. The light density dopant (LDD) regions  14  have the same conductivity type as active regions  4 ,  6 , but with a much lower dose than active regions  4 ,  6 . The gate comprises a gate oxide layer  8 , and a self-aligned polysilicon gate  10 . Oxide sidewall spacers  16  form the differentiation between the active regions  4 ,  6  and the LDD regions  14 . Field oxide  2  provides separation between transistors. Referring to the PMOS device, a well  21  of n-type conductivity is disposed in the substrate  20 . Active regions  23 ,  25  having p-type conductivity are disposed within n-type well  21 . LDD regions  15  have the same conductivity type as active regions  23 ,  25 , but with a much lower dose than active regions  23 ,  25 . The gate comprises a gate oxide layer  8 , and a self-aligned polysilicon gate  10 . Oxide sidewall spacers  16  form the differentiation between the active regions  23 ,  25  and the LDD regions  15 . The silicide layer  12 , is deposited and sintered over the active regions  4 ,  6 ,  23 ,  25  to make better contact. The silicide layer  12 , is optionally deposited over the poly gates  10  as well. For the prior art CMOS device of  FIG. 1 , the NMOS or PMOS transistors normally turn “ON” when a voltage is applied to V 1    51  or V 2    50 , respectively.  
         [0011]     Many prior art techniques for discouraging or preventing reverse engineering of a circuit cause the IC to look different from a standard IC. Techniques are needed in which the transistors, and thus the circuits, are constructed to look essentially the same as conventional circuits, but where the functionality of selected transistors, and hence their circuits, is varied. The minor differences between the conventional circuit and the modified circuit should be difficult to detect by reverse engineering processes. In addition, the techniques should strive to modify only a vendor&#39;s library design instead of forming a completely new and differently appearing library. Requiring only modification to an existing library results in a simpler path to implementation.  
       SUMMARY OF THE PRESENTLY DISCLOSED TECHNOLOGY  
       [0012]     It is an object of the presently disclosed technology to make reverse engineering even more difficult and, in particular, to offset or shift the active regions away from the gate and to use LDD regions of a conductivity type opposite to that of the active regions resulting in a transistor that is always off when standard voltages are applied to the device. It is believed that this will make the reverse engineer&#39;s efforts all the more difficult in terms of making it very time consuming and perhaps exceedingly impractical, if not impossible, to reverse engineer a chip employing the presently disclosed technology.  
         [0013]     The Inventors named herein have previously filed patent applications and have received patents in this general area of technology, that is, relating to the camouflage of integrated circuit devices in order to make it more difficult to reverse engineer them. The technology disclosed herein can be used harmoniously with the techniques disclosed above in the prior United States patents to further confuse a reverse engineer.  
         [0014]     Note that the presently disclosed technology might only be used one in a thousand instances of transistor structures occurring on the chip in question, but the reverse engineer will have to look very carefully at each transistor structure or connection knowing full well that for each transistor structure or connection that he or she sees, there is a very low likelihood that it has been modified by the presently disclosed technology. The reverse engineer will be faced with having to find the proverbial needle in a haystack.  
         [0015]     The presently disclosed technology includes a method of manufacturing a semiconductor device in which some selected non-operable transistors look the same as the operable transistors, but which have a modified LDD implant which renders them inoperable. The modified LDD implant is of an opposite conductivity type than the conductivity of the active regions of the transistor, and hence these implants will result in a transistor that will not turn on when normally biased.  
         [0016]     In another aspect, depending on the design rules of the fabrication process, the present invention will offset the silicide on one side of the transistor so as to ensure that the LDD region is isolated from the silicide in order to prevent leakage that might cause the transistor to turn on.  
         [0017]     In another aspect, the present invention provides a camouflaged circuit structure, comprising: a substrate; a plurality of active regions having a first conductivity type disposed in said substrate; and at least one light density dopant region having a second conductivity type, said at least one light density dopant region being adjacent to at least one of said plurality of active regions, wherein said camouflaged circuit structure is non-operational for a nominal voltage applied to said camouflaged circuit structure.  
         [0018]     In another aspect, the present invention provides a method of camouflaging a circuit comprising the steps of: providing at least one light density dopant region having an opposite conductivity type of an adjacent active region; and modifying an active region mask such that said at least one light density dopant region is sufficiently large to avoid punch through for nominal voltages applied to said camouflaged circuit, wherein said circuit remains OFF when any nominal voltage is applied to said circuit.  
         [0019]     In another aspect, the present invention provides a non-operable transistor comprising: a substrate; a first active region having a first conductivity type disposed in said substrate; a second active region having a second conductivity type disposed in said substrate; a first light density dopant region adjacent said first active region, said first light density dopant region having a second conductivity type; a second light density dopant region adjacent said second active region, said second light density dopant region having a first conductivity type; and a gate structure disposed adjacent said first light density dopant region and said second light density dopant region. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  depicts prior art cross-section of a CMOS device with LDD regions;  
         [0021]      FIGS. 2   a  through  2   g  depict various steps in the manufacturing of a camouflaged integrated circuit structure in accordance with the present invention; and  
         [0022]      FIG. 3  depicts a cross-section of another embodiment of a CMOS device manufactured in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Semiconductor device manufacturing employs many techniques, process steps, and technologies that are well known. These techniques, process steps and technologies vary with feature size, material composition and other specific device attributes. The following is a general discussion of modifications that may be made to the masks used in manufacturing a CMOS device. The discussion below is provided as an example only.  
         [0024]      FIG. 2   a  depicts a substrate  20 , for purposes of this discussion the substrate  20  is a p-type substrate; however the substrate could alternatively be a n-type substrate. A mask layer  27  is disposed over substrate  20  and photolithographically patterned to act as a mask for subsequent implantation. The substrate  20  is then exposed to ions  31 . Ions  31  are chosen such that the ions  31 , when implanted in substrate  20 , will result in a well of opposite conductivity type to that of substrate  20  (e.g. a n-type well  21  for the case of a p-type substrate  20 ). The mask layer  27  is removed and another mask (not shown) is disposed over substrate  20  and photolithographically patterned to act as a mask for subsequent thermal oxide growth. The substrate  20  is heated and field oxide  2  is grown as shown in  FIG. 2   b . The second mask is then removed.  
         [0025]     In  FIG. 2   b , the field oxide  2  acts to separate the transistors. The left side of the substrate will become an inoperable NMOS device in this embodiment, while the right side of the substrate will become an inoperable PMOS device in this embodiment. Then, a gate oxide layer  8  and a polysilicon layer  10  are preferably disposed over the substrate  20 . The polysilicon layer  10  and gate oxide layer  8  are etched to form poly gates for the inoperable NMOS and PMOS devices. Another mask  26  is photolithographically patterned as shown over the CMOS device leaving openings over one side of the NMOS portion of the CMOS device and over one side of the PMOS portion of the CMOS device. The substrate  20  is then exposed to ions  32 . This results in light density dopant (LDD) regions  14   a ,  15   a , as shown in  FIG. 2   c , with one LDD region being associated with the one of the PMOS and NMOS transistors and the other LLD regions being associated with the other one of the PMOS and NMOS transistors. The ions  32  are chosen such that the LDD regions  14   a ,  15   a  are of the same conductivity type as substrate  20  (e.g. p-type in the case of a p-type substrate  20 ). One skilled in the art will appreciate that mask  26  does not need to be a separate step, but instead, when the polysilicon layer  10  and gate oxide layer  8  are etched, they may be initially partially etched to provide an opening for ions  32  and then etched to fully define the poly gates.  
         [0026]     In  FIG. 2   c , the mask  26 , if used, is removed. Alternatively, the polysilicon layer  10  and gate oxide layer  8  are etched to form the poly gate. A mask  28  is then disposed over the CMOS device and photolithographically patterned as shown covering the previously formed LDD regions  14   a ,  15   a . The substrate  20  is exposed to ions  33 . This results in light density dopant (LDD) regions  14   b ,  15   b , as shown in  FIG. 2   d . In this example, the ions  33  are chosen such that the LDD regions  14   b ,  15   b  are of an opposite conductivity type to substrate  20  (e.g. n-type in the case of a p-type substrate  20 ).  
         [0027]     In  FIG. 2   d , a layer of oxide  29  is disposed over the substrate  20 . The oxide layer  29  is then etched, by a timed etch, such that the oxide is removed from atop the source and drain regions  14   a ,  14   b ,  15   a  and  15   b  leaving behind oxide shoulders  29   a ,  29   b ,  29   c  and  29   d . The shoulders provide sidewall spacers which help provide a mask for standard source and drain implants as described above with reference to  FIG. 1 . In the case of a conventional working CMOS device, two source/drain implant masks, one for a p-implant and one for a n-implant, would each be patterned to have large openings whereby the entire n or p, as the case may be, active areas are exposed and implanted. The poly gates  10  and their associated sidewall spacers and the field oxide regions  2  provide pattern definition so that when implantation occurs, the gate  10  is self-aligned in the case of a conventional working CMOS device. In order to render at least one of the CMOS devices on a chip inoperative, the conventionally used source/drain implant masks are modified so that instead of the poly gates  10  and their associated sidewall spacers providing pattern definition, the mask is modified to shift the location of the edge of the implant away from the gate  10 . This shifting can occur on one or both sides of a given gate  10 . This shifting will now be described in greater detail with reference to  FIGS. 2   e  and  2   f.    
         [0028]     In  FIG. 2   e , the N++ source/drain implant  34  for the NMOS side of the inoperative device is defined by a patterned mask  37 . It should be noted that this mask preferably has an additional opaque region (compared to the mask used for a working CMOS device), which is labeled  37 ′ and which shifts an edge of the resulting N++ doped region  34  (see  FIG. 2   f ) away from the edge of sidewall spacer  29   a  (where the implant would normally occur in self-aligned devices). This shift also moves the doped region  34  away from gate  10 . The size of the shift must be large enough to block current flow from the source to the drain under normal bias conditions and also to account for anticipated mask alignment errors and other process parameters.  
         [0029]     Similarly, the PMOS side of the device receives the P++ source/drain implant  35  for the PMOS side of the inoperative device is defined by a patterned mask  38  as shown in  FIG. 2   f . It should be noted that this mask has an additional opaque region (compared to the mask used for a working device), which is labeled  38 ′ and which shifts an edge of resulting the P++ doped region  25  (see  FIG. 2   g ) away from the edge of sidewall spacer  29   d  (where the implant would normally occur in self-aligned devices). This shift also moves the doped region  25  away from gate  10 . Again, the size of the shift must be large enough to block current flow from the source to the drain under normal bias conditions and also to account for anticipated mask alignment errors and other process parameters.  
         [0030]     The ions  34  are selected such that the doped regions  4 , 6  are of an opposite conductivity type to that of LDD region  14   a , but are of the same conductivity type to that of LDD region  14   b  (n-type in this embodiment). The ions  35  are selected such that the doped regions  23 ,  25  are of an opposite conductivity type to that of LDD region  15   b , but are of the same conductivity type to that of LDD region  15   a  (p-type in this embodiment).  
         [0031]     The sidewall spacers  30   a ,  30   b ,  29   c  and  29   d  used for this inoperable device have the same dimensions as conventional sidewall spacers for an operable device. Thus, the reverse engineer would have no indication about the functionality of the device by the sizes of the sidewall spacers  30   a ,  30   b ,  29   c ,  29   d . While the side wall spacers have a normal size, the underlying LLD regions can be larger than normal. Note that LDD region  14   a  has a width which is about twice (or more) as wide as spacer  30   a  and that LDD region  15   b  has a width which is about twice (or more) as wide as spacer  29   d , but the actual sizes will depend on the amount of shift which is needed, as is explained above.  
         [0032]     The resulting inoperable CMOS device is shown in  FIG. 2   g . Also, in  FIG. 2   g , an optional silicide layer  12  is disposed over the NMOS and PMOS devices. For the NMOS device, the silicide layer  12  allows for an electrical conductive path from V 1    51  to substrate  20  through LDD region  14   a , while the doped LDD region  14   a  also prevents an electrical path from active region  4  to active region  6 . Thus, the NMOS device formed will be OFF for any standard voltage applied to V 1    51 . For the PMOS device, the silicide layer  12  allows for an electrical conductive path from V 2    50  to n-well  21  through LDD region  15   b , while the doped LDD region  15   b  also prevents an electrical path from active region  25  to active region  23 . Thus, the device formed will be OFF for any standard voltage applied to V 2    50 .  
         [0033]     One skilled in the art will appreciate that the shorting of the NMOS device to the substrate would not be preferred if the voltage applied to the substrate  20  was not the same as the voltage applied to V 1    51 . Many NMOS devices are connected such that the substrate  20  and V 1    51  are connected to Vss. However, if the voltage applied to substrate  20  was not the same as the voltage applied to V 1    51 , then a silicide block mask would be used to provide a silicide gap that prevents the silicide layer  12  from extending over LDD region  14   a , as shown in  FIG. 3 . Therefore, the silicide layer would be unable to provide an electrical path from V 1    51  to substrate  20  through LDD region  14   a . However, the presence of the LDD region  14   a  being oppositely doped from active regions  4 ,  6  would prevent the transistor from turning ON when standard voltages are applied to V 1    51 . A silicide block mask could also be used to prevent the silicide from extending over LDD implant  15   b , also shown in  FIG. 3 .  
         [0034]     While the examples given in  FIGS. 2   a - 2   f  and  FIG. 3  depict the active regions  4 ,  25  being offset and thus slightly smaller, one skilled in the art will appreciate that instead of offsetting active regions  4 ,  25 , active region  6  or active region  23  could be offset. Of course, this would require the doping of LDD region  14   b  or LDD region  15   a  to be of an opposite type than active region  6 , or active region  23 .  
         [0035]     One skilled in the art will appreciate that there are many different types of CMOS manufacturing process with different feature sizes. The present invention may be applied to any CMOS manufacturing process. For purposes of further clarification, typical dimensions will be supplied for a 0.35 μm process.  
         [0036]     For both the PMOS and NMOS devices, the dimensions of the field oxide portions  29   a ,  29   b ,  29   c ,  29   d , determine the size of the LDD regions  14   a ,  14   b ,  15   a  and  15   b . The LDD regions  14   a ,  15   b , and thus the field oxide portions  29   a , and  29   d , are preferably chosen to be sufficiently large to avoid punch through for standard voltages applied to V 2    50 , or V 1    51  and as small as possible in order to avoid detection. In an embodiment utilizing 0.35 μm process the standard LDD regions  14   b ,  15   a  are approximately 0.1 micrometers in width. As previously discussed, the modified LDD regions  14   a ,  15   b  are larger than the standard (or conventional) LDD regions  14   b ,  15   a . In order to prevent punch through, the LDD regions  14   b  and  15   a  are preferably, approximately 0.2 micrometers wide when a voltage of approximately 3.5 Volts is applied to V 1    50  or V 2    51 . Increasing the size of the LDD regions  14   a ,  15   b  results in a slight decrease in the size of the active regions  4 ,  25  normally found in a CMOS device.  
         [0037]     For the NMOS device, the dimensions of the optional silicide gap are preferably chosen such that the optional silicide gap is ensured to be over at least the LDD region  14   a  or  15   b , taking into account the alignment tolerances for the process, thus preventing V 1    51  from shorting to the substrate  20 . The dimensions of the optional silicide gap is dependent upon the mask alignment error for the process used. Typically, the optional silicide gap is less than 0.3 micrometers.  
         [0038]     The presently disclosed technology provides an IC that is difficult to reverse engineer given that the size difference in the active regions is small, and the conductivity type of implant for a LDD region is very difficult to determine given the small dosage levels used in forming LDD regions. Additionally, the silicide layer is difficult to detect. As a result, the false transistor formed in accordance with the present invention will look operational to the reverse engineer. In a sea of millions of other transistors, these two features will be difficult to detect easily, thus forcing the reverse engineer to examine every transistor.  
         [0039]     Additionally, the presently disclosed technology is preferably used not to completely disable a multiple transistor circuit in which the invention is used, but rather to cause the circuit to function in an unexpected or non-intuitive manner. For example, what appears to be an OR gate to the reverse engineer might really function as an AND gate. Or what appears as an inverting input might really be non-inverting. The possibilities are almost endless and are almost sure to cause the reverse engineer so much grief that he or she gives up as opposed to pressing forward to discover how to reverse engineer the integrated circuit device on which these techniques are utilized. This the term “inoperable” as used herein in intended to cover possibilities where the resulting circuit is either inoperable or operates in an unexpected or non-intuitive manner.  
         [0040]     In terms of making these devices, the foregoing description has been presented in terms implanting ions in order to form doped or implanted regions. Those skilled in the art will appreciate the fact that the doped regions can be formed by other techniques, such as diffusion doping.  
         [0041]     Having described the presently disclosed technology in connection with certain preferred embodiments thereof, modification will now certainly suggest itself to those skilled in the art. As such, the presently disclosed technology is not to be limited to the disclosed embodiments, except as is specifically required by the appended claims.