Use of silicon block process step to camouflage a false transistor

[0001] This patent application is a divisional application of U.S. application Ser. No. 10/637,848, filed on Aug. 7, 2003 now U.S. Pat. No. 6,979,606, which application claims the benefit of U.S. Provisional Application 60/428,634 filed Nov. 22, 2002, the contents of which are incorporated herein by reference.

[0002] This application is related to co-pending U.S. patent application Ser. No. 09/758,792 entitled “Circuit Protection Implemented Using a Double Polysilicon Layer CMOS Process” filed on Jan. 11, 2001 by J. P. Baukus, Lap Wai Chow and W. C. Clark.

[0003] The present invention 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.

The present invention is related to the following U.S. patents by some of the same inventors as the present inventors:

• • [0004] (1) U.S. Pat. Nos. 5,866,933; 5,783,375 and 6,294,816 teach connecting transistors in a CMOS circuit by implanted (and therefore hidden and buried) lines between the transistors. The implanted lines are formed by modifying the p+ and n+ source/drain masks. These implanted interconnections are used to make 3-input AND or OR circuits 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.
  • [0005] (2) U.S. Pat. Nos. 5,783,846; 5,930,663 and 6,064,110 teach modifying the source/drain implant masks to provide a gap in the implanted connecting lines between transistors. The length of the gap being approximately 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.
  • [0006] (3) U.S. Pat. No. 6,117,762 teaches a method and an apparatus for protecting semiconductor integrated circuits from reverse engineering. Semiconductor active areas are formed on a substrate and a silicide layer is formed over at least one active area of the semiconductor active areas and over a selected substrate area. The silicide layer connecting the at least one active area with another active area.

[0007] The creation of complex integrated circuits and semiconductor devices can be an expensive undertaking because of 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. Further, integrated circuits are often used in applications involving the encryption of information. In order to keep the encrypted information confidential, devices should be protected from being reverse engineered. Thus, there can be a variety of reasons for protecting integrated circuits and other semiconductor devices from being reversed 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 make the connections between transistors difficult to determine forcing the reverse engineer to perform a careful analysis of 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 analyze each transistor carefully in a device can effectively frustrate the reverse engineer's ability to reverse engineer the device successfully.

[0009] A conductive layer, such as silicide, is often used during the manufacture of semiconductor devices. In modern CMOS processing, especially with a minimum feature size below 0.5 μm, a silicide layer is utilized to improve the conductivity of gate, source and drain contacts. In accordance with typical design rules, any active region resulting in a source/drain region is silicided.

[0010] One reverse engineering technique involves de-layering the completed IC by means of chemical mechanical polishing (CMP) or other etching processes. The etching processes may, under some conditions, reveal the regions between where the silicide was formed on the substrate, and where it was not, i.e. the regions defined by the silicide block mask step and by regions where structures, such as a polysilicon gate, prevent the silicide layer from being deposited on the substrate. These regions may be revealed because, under some kinds of etches, there is an observable difference in topology due to different etching rates for silicided vs. pure silicon. The reverse engineer, by noting the silicided areas vs. non-silicided areas, may make reasonable assumptions as to the function of the device. This information can then be stored into a database for automatic classification of other similar devices.

[0011] Some methods of protecting against reverse engineering may be susceptible to discovery under some reverse engineering techniques, such as chemical-mechanical polishing (CMP) or other etching techniques. For example, FIG. 1a depicts a possible top-down view of a false transistor made in accordance with U.S. patent application Ser. No. 09/758,792 after etching. During the manufacturing of the false transistor, and in accordance with normal design rules, the silicide block mask allows for a silicide layer 15, see FIG. 1b, to be placed completely over the active regions 12, 16, and optionally over gate layer 14. Gate layer 14 may be a polysilicon layer. During the CMP process, the gate layer 14 would be removed, thereby resulting in the top-down view as shown in FIG. 1a. As shown, the silicide layer edge 18 aligns with the gate edge 11, 13, thus the reverse engineer only sees one line along the gate edge 11, 13.

[0012] As will be described below, the top-down view of the false transistor is different from a top-down view of a true transistor and as such, the difference may be a signature that the transistor is not a true transistor.

[0013] For functional or true transistors, as shown in FIGS. 2a and 2b, the silicide layer edge 18′ is offset from the polysilicon gate layer 14 due to the presence of sidewall spacers 19 that are formed adjacent to gate layer 14. A light doped density (LDD) implant 10 is typically formed after the formation of the gate layer 14 and before the formation of the sidewall spacers. After sidewall spacers 19 are formed, active areas 12, 16 are typically formed in the substrate. The formation of active areas 12, 16 saturate most of the LDD implant, so that only the portion of the LDD implant 10 that is under the sidewall spacers 19 effectively remains. A conductive layer, such as silicide, is typically placed over the active areas 12, 16 and the gate layer 14. The gate layer 14 and sidewall spacers 19, prevent the silicide from being deposited upon the substrate in those areas. Thus, the artifact edge 18′ is spaced from and lies mostly parallel with the edges 11, 13 of the gate layer 14 for a true transistor. Thus, from the examination of the top-down view the reverse engineer may be able to determine that a structure originally placed in the area was in fact a false transistor meant to confuse the reverse engineer due to the absence of artifact edges 18′ lying spaced from and mostly parallel with edges 11, 13 of the polysilicon gate 14. A reverse engineer could then program computer software to recognize the absence of artifact edges 18′ of the silicide layers lying separate from and being mostly parallel with the edges 11, 13 of the gate layer 14 as indications of false transistors. One skilled in the art will appreciate that although FIG. 1b depicts active regions 12, 16 adjacent to the gate region 14 and FIG. 2b depicts LDD implants 10 adjacent to the gate region 14, it is extremely difficult, if not impossible, for the reverse engineer to determine the different doping levels of the LDD implant 10 and the active regions 12, 16.

[0014] Therefore, a need exists to provide a semiconductor device and a method of manufacturing semiconductor devices that uses artifact edges to confuse the reverse engineer. Providing artifact edges that are not indicative of the actual device formed will further confuse the reverse engineer and result in incorrect conclusions as to the actual composition, and thus function, of the device.

[0015] One aspect of this invention is to make reverse engineering even more difficult and, in particular, to confuse the reverse engineer's study of the artifacts revealed during the reverse engineering process by providing artifacts that are not indicative of the underlying processing and circuit features. The result is that the reverse engineer is given large reason to doubt the validity of typical conclusions. It is believed that it will not only be time consuming to reverse engineer a chip employing the present invention but perhaps impractical, if not impossible.

[0016] Another aspect of the present invention is that it does not rely upon modifications or additions to the function of the circuitry that is to be protected from reverse engineering, nor does it require any additional processing steps or equipment. Instead, a highly effective deterrent to reverse engineering is accomplished in a streamlined manner that adds neither processing time nor complexity to the basic circuitry.

[0017] 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 present invention can be used harmoniously with the techniques disclosed above in the prior United States Patents to further confuse the reverse engineer.

[0018] The present invention might only be used once in a thousand of instances on the chip in question. Thus, the reverse engineer will have to look very carefully at each transistor or connection. The reverse engineer will be faced with having to find the proverbial needle in a haystack.

[0019] Another aspect of the present invention is a method of manufacturing a semiconductor device in which a conductive layer block mask is modified resulting in reverse engineering artifacts that are misleading and not indicative of the true structure of the device.

[0020] An aspect of the present invention is to provide a camouflaged circuit structure, comprising: a gate layer having a first gate layer edge and a second gate layer edge; a first active area disposed adjacent said first gate layer edge; a second active area disposed adjacent said second gate layer edge; and a conductive layer having a first artifact edge and a second artifact edge, said conductive layer partially formed over said first active area and said second active area; wherein said first artifact edge of said conductive layer is offset from said first gate layer edge, and said second artifact edge of said conductive layer is offset from said second gate layer edge.

[0021] Another aspect of the present invention is a method of confusing a reverse engineer comprising the steps of: providing a false semiconductor device without sidewall spacers having at least one active region; and forming a conductive layer partially over the at least one active region such that an artifact edge of said conductive layer of said false semiconductor device without sidewall spacers mimics an artifact edge of a conductive layer of a true semiconductor device having sidewall spacers.

[0022] Another aspect of the present invention is a method of camouflaging an integrated circuit structure comprising the steps of: forming the integrated circuit structure having a plurality of active areas; and forming a conductive block layer mask to thereby form artifact edges of a conductive layer that are located in a same relative locations for non-operational transistors without sidewall spacers as well as operational transistors with sidewall spacers.

[0023] Another aspect of the present invention is a method of protecting an integrated circuit design comprising the steps of: modifying a silicide block mask used during the manufacture of a false transistor such that edges of a silicide layer for the false transistor are placed in substantially the same relative locations as edges of a silicide layer for a true transistor; and manufacturing said integrated circuit.

[0024] Another aspect of the present invention is a circuit structure comprising: a gate layer having a first gate layer edge and a second gate layer edge; a first active area, said first active area being formed during a single processing step, said first active area having a width, said first active area formed adjacent said first gate layer edge; a second active area, said second active area being formed during a single processing step, said second active area having a width, said second active area formed adjacent said second gate layer edge; a conductive layer having a first artifact edge and a second artifact edge, said conductive layer being formed over said first active area and over said second active area, a width of said conductive layer formed over said first active area being less than said width of said first active area, a width of said conductive layer formed over said second active area being less than said width of said second active area.

[0025] Another aspect of the present invention is a method of hiding a circuit function comprising the steps of: forming at least one active region of a device with a single processing step, said at least one active region having a width; and forming a conductive layer partially over the at least one active region wherein a width of said conductive layer is less than the width of the at least one active region.

[0033] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which an embodiment of the invention is shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiment set forth herein.

[0034] Many methods of manufacturing semiconductor devices are well known in the art. The following discussion focuses on modifying a conductive layer block mask used during the manufacture of semiconductor devices in order to confuse the reverse engineer. The discussion is not intended to provide all of the semiconductor manufacturing details, which are well known in the art.

[0035] In order to confuse the reverse engineer, the placement of an artifact edge of a silicide layer that would be seen when a reverse engineer examines devices manufactured with other reverse-engineering-detection-prevention techniques is changed. In reverse-engineering-detection-prevention techniques, false, or non-operational, transistors are used along with true, or operational, transistors. Some false transistors are manufactured without sidewall spacers, see FIG. 1b, while corresponding true transistors may well have sidewall spacers 19, as shown in FIG. 2b. From a top-down view, and through most reverse engineering techniques, these false transistors look the same as operational transistors. However, under some reverse engineering techniques, such as chemical mechanical polishing (CMP) or other etching processes, the artifact edges of the silicide layer may give away the reverse-engineering-detection-prevention technique. As shown in FIG. 1a, for some non-operational transistors, the artifact edges 18 of a silicide layer 15 coincide with the edges 11, 13 of the gate layer 14. However, with operational transistors as shown in FIG. 2a, the artifact edges 18′ of a silicide layer 15 are offset from the edges 11, 13 of the gate layer 14 by the width of sidewall spacers 19.

[0036] FIG. 3a is a top-down view and FIG. 3b is a cross-sectional view of a false transistor in accordance with the present invention. FIG. 3a depicts artifact edges 18″ of a conductive layer 15 that do not coincide with the edges 11, 13 of gate layer 14. A conductive layer block mask 21, see FIG. 4, is preferably modified to prevent the silicide layer 15 from covering the entire active areas 12, 16. The conductive layer 15 is partially formed over a first active area 12 and a second active area 16. The result is that the conductive layer 15 has a cross-sectional width 151 that is smaller than the cross-sectional width 121, 161 of the active areas 12, 16. Thus, when a reverse engineering process, such as CMP or other etching process, is used, the artifact edges 18″ of the conductive layer 15 do not give away the fact that the transistor is a false transistor. Instead, the artifact edges 18″ are offset by a distance 17, see FIG. 3a, from the gate layer 14, with distance 17 having a width that is preferably approximately equivalent to the width of one typical sidewall spacer, as if sidewall spacers were present. Therefore, the reverse engineer can no longer rely on the placement of the artifact edges 18 of conductive layer 15 to determine if a transistor is a true transistor or a false transistor.

[0037] One skilled in the art will appreciate that the conductive layer block mask 21 will require different modifications depending on the feature size of the device. The offset distance 17 between the artifact edge 18″ of the conductive layer 15 and the edge 11, 13 of the gate layer 14 is preferably approximately equal to the width of the sidewall spacers, which varies depending on the feature size of the device. One skilled in the art will appreciate that the difference between the width of the sidewall spacer 19 and the width of the offset 17 should be within the manufacturing tolerances for the process used, and thus the offset 17 and the width of the sidewall spacer 19 are approximately equal. For 0.35 μm technology, for example, the sidewall spacer width is approximately 0.09 μm. For typical CMOS processes, the conductive layer 15 will be silicide while the gate layer 14 will be polysilicon. One skilled in the art will appreciate that regardless of the feature size of the device, the person laying out the masks should place the artifact edges 18″ of the conductive layer 15 for a false transistor in substantially the same relative locations as the artifact edges 18′ of the conductive layer 15 for a true transistor. Thus, the reverse engineer will be unable to use the artifact edges 18 of the conductive layer 15 to determine if the transistor is a true transistor or a false transistor.

[0038] Additionally, false transistors manufactured in accordance with the invention are preferably used not to completely disable a multiple transistor circuit, 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. Alternatively, what appears as an inverting input might really be non-inverting. The possibilities are endless and are almost sure to cause the reverse engineer so much grief that he or she would give up as opposed to pressing forward to discover how to reverse engineer the integrated circuit device on which this technique is utilized.

[0039] Having described the invention in connection with certain preferred embodiments thereof, modification will now certainly suggest itself to those skilled in the art. As such, the invention is not to be limited to the disclosed embodiments, except as is specifically required by the appended claims.