Abstract:
Semiconductor devices and integrated circuits that benefit from the advantages of contemporary processing technologies yet are irreparably damaged by ionizing radiation, and methods for making the same. Transistors that are particularly intolerant to ionizing radiation have a gate insulator that includes a portion of a screen layer that is used in conjunction with N- and P-well implantation. After the implantation step, the screen layer exhibits significantly degraded tolerance to ionizing radiation, so that a gate insulator incorporating a portion of such a screen layer will likewise be radiation intolerant. By selectively removing portions of the screen layer, a method is provided for co-locating radiation-tolerant and radiation-intolerant transistors on a substrate. A radiation intolerant integrated circuit is formed by adding “safeguard devices” to an integrated circuit. The safeguard devices are susceptible to relatively low doses of ionizing radiation while other “utile devices” on the integrated circuit are not. The safeguard devices are coupled to the utile devices in such a manner that when the integrated circuit is bombarded with ionizing radiation, the safeguard devices short and destroy the functionality of the utile devices, and, hence, the functionality of the integrated circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of U.S. Provisional Application No. 60/138,721, filed Jun. 11, 1999, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor processing and integrated circuits. More particularly, the present invention relates to semiconductor devices and circuits that have a low tolerance to ionizing radiation, and methods for manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     Older semiconductor processing technologies produced integrated circuits that were highly susceptible to damage from ionizing radiation. Such ionizing radiation is emitted from a multitude of galactic sources (e.g., the Sun, etc.) and exists above the ionosphere, and is also emitted when nuclear weapons are detonated. The important consequence of such radiation susceptibility is that these integrated circuits were not well suited for use in satellites or in military applications. They could, therefore, be freely sold and exported without the fear that they would be used militarily against the United States and its allies. 
     In contrast, state-of-the-art semiconductor processing technologies currently produce integrated circuits that are highly tolerant to damage from ionizing radiation. Such tolerance results, among any other reasons, from a decrease in semiconductor-feature size (e.g., interconnect line width, etc.) in integrated circuits. In particular, the “gate oxide” or “gate insulator” in field effect transistors (FETs) has thinned to the point it is inherently tolerant to ionizing radiation. 
     The relatively high radiation tolerance of state-of-the-art circuits is no benefit to most users and for most applications. This characteristic does, however, allow such circuits to be used in aerospace and military applications. In fact, such circuits may be so radiation tolerant that Department of Defense export restrictions (ITAR) are implicated. Such export restrictions are financially detrimental to a commercial CMOS fabricator since they result in added expense and a shrunken market. 
     Thus, a need exists for a method that increases the susceptibility of semiconductor devices and integrated circuits to ionizing radiation so that they can be freely exported. 
     SUMMARY OF THE INVENTION 
     Some embodiments of the present invention provide a method for manufacturing semiconductor devices and circuits that have increased susceptibility to ionizing radiation. The inventive methods use well-known processing techniques. In fact, with the exception of a few processing steps, the inventive methods follow conventional CMOS processing methodology. As such, while devices and circuits that are produced in accordance with the present teachings are more susceptible to damage/aberrant operation from ionizing radiation than devices and circuits fabricated via conventional processes, they nevertheless possess the advantages of contemporary processing technologies (e.g., small feature size, etc.). 
     In accordance with the illustrated embodiment of the present invention, a radiation susceptible device, such as a FET, is fabricated by forming a screen layer on a substrate; implanting said substrate with a dopant through said screen layer; selectively removing a portion of said screen layer; and forming an electrically-insulating layer on said screen layer. Conventional CMOS processing steps are used to complete the devices. A device exhibiting increased radiation susceptibility is formed at a location at which the screen layer is not removed. A device exhibiting a “standard” or otherwise unaffected susceptibility (relative to conventional CMOS processing procedures) is formed at a location at which the screen layer is removed. 
     In a further embodiment, a method of operating an integrated circuit in accordance with the present invention comprises: processing signals with a utile device; and interfering with the operation of the utile device with a safeguard device comprising a transistor having a gate insulator that includes a screen layer, wherein the safeguard device interferes with the operation of said utile device when and only when said integrated circuit is exposed to ionizing radiation. 
     In additional embodiments, the present invention provides an integrated circuit exhibiting increased susceptibility to ionizing radiation, and a method for fabricating such a circuit. An integrated circuit fabricated in accordance with the inventive method comprises (1) “utile” devices that provide the functionality for which the circuit was designed, and (2) safeguard devices that impart the integrated circuit&#39;s increased susceptibility to ionizing radiation. In some embodiments, a utile device comprises a first transistor that is disposed on a region of a substrate at which a screen layer is removed, and a safeguard device comprises a second transistor disposed on a region of substrate at which said screen layer is not removed. 
     In one embodiment of such an integrated circuit, safeguard devices are coupled into the logic of the integrated circuit in such a manner that when the integrated circuit is exposed to ionizing radiation, the safeguard devices irreparably destroy the functionality of the integrated circuit. 
     In general, the safeguard devices can destroy the functionality of the integrated circuit in two ways. First, one or more safeguard devices can interfere with the logical operation of an integrated circuit by, for example, shorting a signal lead to ground. Second, one or more safeguard devices can interfere with the electrical operation of an integrated circuit by, for example, shorting V DD  to ground. This technique works by depriving the utile devices on the integrated circuit of electrical power. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a portion of a conventional semiconductor circuit showing two field-effect transistors. 
     FIG. 2 depicts a screen-oxide layer deposited on a wafer in accordance with conventional CMOS manufacturing processes. 
     FIG. 3 depicts a block-flow diagram of a method for forming semiconductor devices and circuits in accordance with the illustrative embodiment of the present invention. 
     FIG. 4 depicts a method by which the screen insulator is selectively removed in accordance with the present invention. 
     FIGS. 5,  6 ,  7 A and  7 B depict a dose-soft device and a dose-hard device in various stages of fabrication in accordance with the illustrative method. 
     FIG. 8 depicts a block diagram of an integrated circuit in the prior art. 
     FIG. 9 depicts a block diagram of an integrated circuit in accordance with the illustrative embodiment of the present invention. 
     FIG. 10 depicts a schematic diagram of an illustrative safeguard device in accordance with an illustrative embodiment of the present invention. 
     FIG. 11 depicts a graph of the effect of total ionizing dose on the threshold voltage of the safeguard device depicted in FIG.  10 . 
     FIG. 12 depicts a block flow diagram of a method of fabricating the integrated circuit of FIG.  9 . 
    
    
     DETAILED DESCRIPTION 
     For clarity of explanation, the illustrative embodiments of the present invention are presented as comprising individual “operational” blocks. The functions or operations indicated by these blocks are provided via various well known semiconductor-processing techniques, including, for example, photolithography, and doping or implantation. Since those skilled in the art are quite familiar with such processing techniques, they will be referenced without description as appropriate. 
     For the purposes of this Specification, the terms “circuit” and “device” are used interchangeably, unless other noted. Circuits exhibiting increased susceptibility to radiation are often termed “dose soft” or “radiation soft,” and such terminology is adopted for use herein. For the purposes of this Specification, a circuit that is characterized as “dose soft,” “radiation soft,” “radiation susceptible,” or “radiation intolerant” means that it is more susceptible to damage or aberrant operation due to ionizing radiation than circuits formed via conventional CMOS processing. The aberrant operation typically manifests as a substantial current leakage from, for, example, a transistor. 
     By way of illustration, not limitation, a dose-soft circuit is typically intolerant of (e.g., damaged by, deleteriously affected by, etc.) a total ionizing (radiation) dose (“TID”) above about 50 Krad (Si), and even as low as 1 to 3 Krad (Si). “Total” dose signifies that the radiation exposure may occur over a period of time; in other words, TID is a cumulative dose. As the amount of radiation absorbed by different materials for a given exposure varies, radiation tolerance must be referenced to a particular material, such as silicon. The designation “dose-hard” is adopted for use herein to refer to circuits exhibiting a tolerance to TID consistent with circuits fabricated via conventional CMOS processing. Again, by way of illustration, not limitation, a dose-hard circuit should be expected to exhibit a TID tolerance in the range of about 50 to about 200 Krad (Si). 
     FIG. 1 depicts two conventional FETs  102  and  122  on wafer  100 . Transistor  102  is depicted as an “N-channel” device, and transistor  122  is depicted as a “P-channel” device. 
     N-channel transistor  102  includes gate  106 , source/drain  108  and P-well  104 . Gate oxide or gate insulator  110  is disposed beneath gate  106 . Dielectric  120 , typically silicon dioxide, provides intra- and inter-transistor electrical insulation. Contacts  112 , typically an electrical conductor such as aluminum, provide electrical contact to the gate, source and drain of the transistor. P-channel transistor  122  has substantially the same structure as transistor  102 , and includes gate  126 , gate oxide  130 , source/drain  128  and electrical contacts  132 . Since transistor  124  is a P-channel device, it has an N-well (i.e., N-well  124 ) rather than a P-well. 
     In the fabrication of a FET via conventional CMOS fabrication, a layer  234  is grown on the surface of a substrate  100  (e.g., a wafer, etc.) prior to N-well and P-well implantation. Layer  234 , which is typically an oxide such as silicon dioxide, has a thickness of about 100 to 200 angstroms. Layer  234  aids in the distribution of N-type dopant atoms (e.g., antimony, arsenic, phosphorous) and P-type dopant atoms (e.g., boron, indium) into the wafer and also reduces out-gassing during annealing. In conventional CMOS processing operations, layer  234  is removed after N-well and P-well implantation and then the “gate oxide” or “gate insulator” is grown. 
     In recognition of this “screening” function, layer  234  is usually referred to as a “screen” layer. And, since layer  234  is typically an oxide, it is often referred to as a “screen oxide.” To make it clear that layer  234  can comprise a material other than an oxide, it will be referred to herein as a “screen layer.” 
     It is known that implanting through a screen layer comprising, for example, silicon dioxide or oxynitride, significantly degrades its radiation tolerance. In application of this fact, the present inventors recognized that a dose-soft device (e.g., transistor) can be fabricated via an easily-implemented modification to conventional CMOS technology. Specifically, a dose-soft device can be made by not removing the screen insulator at any location on the substrate where such a dose-soft device is desired. 
     Thus, in a method  300  for fabricating a dose-soft FET in accordance with the illustrated embodiment of the present invention, conventional CMOS processing is used up to and including the well-implant step, as indicated in operation block  302 . Subsequently, in operation block  304 , method  300  deviates from conventional CMOS processing in that the screen layer is not removed from the substrate (e.g., wafer, etc.). 
     Operation  304  advantageously facilitates removal of the screen layer at selected locations. The screen layer remains intact at other locations. This partial removal, (hereinafter “selectively removed,” “selective removal,” “selectively removing,” etc.) provides a way to co-locate dose-soft and dose-hard devices on a substrate. Dose hard circuits are suitably formed at the locations at which the screen layer is removed, and dose soft circuits are suitably formed at the locations at which the screen layer remains. 
     Operation  304 , selective removal of the screen layer, is advantageously performed using standard photolithographic processing steps. Further detail of operation  304  is provided in FIG.  4 . In particular, as indicated at operation block  402 , the screen insulator is covered with photoresist at locations on the substrate at which the screen insulator is to remain intact (i.e., locations at which dose-soft devices will be formed). 
     FIG. 5 depicts a stage in the fabrication (i.e., after operation  402 ) of two transistors on a portion of a wafer  500 . A dose-soft transistor is to be formed at region  502 , and a dose-hard transistor is to be formed at region  522 . At the stage of fabrication shown, P-well  504  and P-well  524  have already been implanted and screen layer  234  is partially covered by resist  536  in preparation for selective removal. 
     With continuing reference to FIG. 4, in operation block  404 , any portion of the screen layer that is not covered by resist is stripped off. Dose hard devices can be formed at locations at which the screen layer is removed. Selective removal of the screen layer is performed using well known methods, such as, for example, placing the wafer in a standard tool containing HF acid (e.g., spray, vapor, tank or the like). After selectively removing the screen layer, the photoresist that covers portions of the screen layer is removed using standard resist-strip methods in operation block  406 . FIG. 6 depicts the substrate after the uncovered portion of screen layer  234  has been removed, and after resist  536  has been removed. A portion  634  of the original screen layer thus remains at region  502 . 
     Referring again to FIG. 3, after resist removal, the gate insulator is grown as per operation block  306 . FIG. 7A depicts gate insulator  710  underneath screen layer  634  and at the exposed surface of wafer  500 . FIG. 7B depicts, after patterning, gate insulator  712  of the nascent transistor at region  502 , which is an unconventionally-thick gate insulator that comprises screen layer portion  634  and newly-grown layer of insulator (e.g., oxide)  710 . Gate  702  overlies gate insulator  712 . As already described, a screen layer that has been implanted through, such as the screen layer portion  634 , exhibits significantly increased radiation susceptibility. Moreover, growing additional oxide under the screen layer, which, in some embodiments, is itself an oxide layer, results in an overall increase in thickness of the oxide layer under gate  706 , which further reduces dose hardness. A transistor formed in this manner will therefore be dose-soft. 
     FIG. 7B also depicts the gate and gate insulator regions of a nascent transistor at region  522 . The gate insulator of the transistor being fabricated at region  522  comprises only the layer  710  of insulator grown in operation  306 , since the screen layer over region  522  was removed. Gate  726  overlies the gate insulator. Since the TID-sensitive screen layer is not present at region  522 , the transistor formed there is dose hard. 
     As indicated at operation block  308 , the transistors are completed using conventional CMOS processing methodology. 
     In a further embodiment of the present invention, the present invention provides a method for fabricating an integrated circuit having increased susceptibility to ionizing radiation. 
     FIG. 8 depicts a block diagram of integrated circuit  800  in the prior art, which typically comprises a plurality of utile devices, utile devices  801 - 1  through  801 -N, electrical conductors  811  for connecting utile devices  801 - 1  through  801 -N to power, and electrical conductor  812  for connecting utile devices  801 - 1  through  801 -N to ground. It will be clear to those skilled in the art that integrated circuits typically comprise many other elements (e.g., pads for receiving power and ground, etc.) than are depicted in FIG. 8, but these are not shown so that attention can be focused on those elements that are germane to an understanding of the present invention. 
     For the purposes of this specification, a “utile device” is a device that processes an information-bearing signal. The word “device,” when used in the term “utile device,” is defined as a transistor (i.e., both the operating transistor and a parasitic transistor, if one exists) and the surrounding materials (e.g., the field oxide, the gate dielectric, etc.) that affect the operating parameters (e.g., the effective threshold voltage, V T , etc.) of the transistor. Utile devices can be operate in either analog mode or digital mode or both. Typically, the utile devices on an integrated circuit provide the functionality for which the circuit was designed, fabricated and utilized. For example, the utile devices on an integrated circuit might function as a microprocessor with a control sequencer and an arithmetic logic unit, a plurality of memory cells, an amplifier, etc. 
     As shown in FIG. 8, utile devices typically have: (i) input signals, which might be received from off of integrated circuit  800  or that might be generated by other utile devices, and (ii) output signals, which might be sent off of integrated circuit  800  or might be fed into other utile devices. It will be clear to those skilled in the art how to make and use one or more utile devices in accordance with the illustrative embodiment of the present invention. 
     Electrical conductor  811  and electrical conductor  812  can be metal bus lines or specific transistor diffusion nodes, as are well-known in the art, and it will be clear to those skilled in the art how to make and use electrical conductor  811  and electrical conductor  812  in conjunction with utile devices  801 - 1  through  801 -N. It is well-known to those skilled in the art how to make and use integrated circuit  800 . 
     FIG. 9 depicts a block diagram of integrated circuit  900  in accordance with the present teachings. Integrated circuit  900  advantageously comprises a plurality of utile devices, utile devices  901 - 1  through  901 -N, electrical conductors  911  for connecting utile devices  901 - 1  through  901 -N to power, and electrical conductor  912  for connecting utile devices  901 - 1  through  901 -N to ground. 
     In addition to these elements, integrated circuit  900  also advantageously comprises one or more “safeguard devices,” such as safeguard devices  902 - 1  through  902 -M. It will be clear to those skilled in the art that integrated circuit  900  can comprise elements (e.g., pads for receiving power and ground, etc.) in addition to those depicted in FIG.  9 . These elements are not shown so that attention can be focused on those elements that are germane to an understanding of the present invention. 
     For the purposes of this specification, a “safeguard device” is defined as a device that is designed to interrupt the functioning of all or part of an integrated circuit when the integrated circuit is exposed to ionizing radiation. In general, the safeguard devices on the integrated circuit do not provide any functionality to the end-user and only exist so that they can completely or partially destroy the functionality of the integrated circuit when the integrated circuit is exposed to ionizing radiation. As a practical matter, most end users would probably prefer that the integrated circuit not contain a safeguard device because the presence of the safeguard device increases the susceptibility of the integrated circuit to failure. 
     The details of a safeguard device are discussed with respect to FIG. 10 below. Although one lead of a safeguard device is always tied to ground, the other lead can be either connected to power (i.e., V DD ) or to a signal lead (e.g., signal lead  913 -i, etc.). The theory of operation of the illustrative embodiment is as follows: when integrated circuit  900  is exposed to ionizing radiation, a safeguard device shorts its two terminals together. When the safeguard device is connected between power and ground, the safeguard device shorts power to ground. When the safeguard device is connected between the signal lead and ground, the safeguard device shorts the signal lead to ground. In either case, the integrated circuit is affected. And this effect advantageously occurs at radiation dose levels that would not otherwise affect the operation of utile devices  901 - 1  through  901 -N and integrated circuit  900 . 
     The advantage of placing the safeguard devices between power and ground is that they can completely disable integrated circuit  900  from operating after exposure to a sufficient dose of radiation. The disadvantage of placing the safeguard devices between power and ground is that they might also short out all of the integrated circuits in the vicinity of integrated circuit  900 . 
     The advantage of placing the safeguard devices between a signal lead and ground is that they can be used to selectively disable portions integrated circuit  900  from operating. The disadvantage of placing the safeguard devices between a signal lead and ground is that they might not sufficiently degrade the operation of integrated circuit  900 . 
     From reading this specification, it will be clear to those skilled in the art how and where to place the safeguard devices on an integrated circuit to achieve a desire effect. 
     Electrical conductor  911  and electrical conductor  912  can be metal bus lines or specific transistor diffusion nodes, as are well-known in the art, and it will be clear to those skilled in the art how to make and use electrical conductor  911  and electrical conductor  912  in conjunction with utile devices  901 - 1  through  901 -N, and safeguard devices  902 - 1  through  902 -M. 
     FIG. 10 depicts a schematic diagram of illustrative safeguard device  902 -i, in accordance with the illustrative embodiment of the present invention. Safeguard device  902 -i advantageously comprises one or more n-type metal-oxide semiconductor field-effect transistors  1002 -i (i.e., MOSFET) that are ganged in parallel, and so that their gates and sources are electrically connected to ground and so that their drains are electrically connected to either V DD  or a signal lead. 
     This signal lead could be, for example, a chip enable that must have a high voltage in order for integrated circuit  900  to function. In this case, when integrated circuit  900  is exposed to ionizing radiation, safeguard device  902 -i shorts. As a consequence, the chip enable, and, hence, integrated circuit  900 , are disabled. 
     The salient characteristic of safeguard devices  902 - 1  through  902 -M is that they are fabricated so as to be more susceptible to ionizing radiation than the utile devices  901 - 1  through  901 -N, and, in fact, susceptible enough to ionizing radiation to pass the ITAR restrictions. Safeguard devices  902 - 1  through  902 -M are advantageously fabricated in accordance with the teachings presented herein wherein the screen layer is not removed prior to growing the gate oxide. 
     Advantageously, each basic cell or group of utile devices is associated with one or more safeguard devices that are capable of preventing at least the associated utile devices from functioning. To accomplish this: 
     1. the safeguard devices are advantageously placed near their associated utile devices; 
     2. the amount of electrical resistance between the safeguard devices and their associated utile devices should be kept to a minimum; and 
     3. the safeguard devices should be designed and fabricated to have enough current capacity to affect the voltage on whichever of V DD  or the signal lead they are attached to. 
     FIG. 11 depicts a graph of the threshold voltage, V T , of safeguard device  902 -i as a function of the amount of total ionizing dose radiation to which integrated circuit  900  has been exposed. In normal operation (i.e., when the total ionizing dose radiation is zero), the threshold voltage is high, the safeguard device is “off” (i.e., an open circuit) and the operation of the utile devices is unaffected. In contrast, when integrated circuit  900  has been exposed to ionizing radiation and the total ionizing dose increases, the threshold voltage eventually drops to the point where the safeguard device turns “on” (i.e., a closed circuit) and the operation of the utile devices is affected. It will be understood to those skilled in the art that the total ionizing dose might occur over an extended period (e.g., days, months, years, etc.). 
     FIG. 12 depicts a block flow diagram of a method  1100  for fabricating a dose-soft integrated circuit, such as integrated circuit  900 . Operation  1102  of method  1100  comprises the operations of method  300 . Method  1100  further includes operation  1104  wherein the dose-soft safeguard devices are electrically connected to elements of the integrated circuit such that on exposure to a sufficient dose of TID, utile devices are prevented from functioning. Electrical connection is effected as previously described. That is, one lead of a safeguard device is always tied to ground and the other lead can be either connected to power (i.e., V DD ) or to a signal lead (e.g., signal lead  913 -i, etc.). 
     Additional detail concerning the fabrication and structure of radiation-susceptible integrated circuits is provided in the following commonly-owned, co-pending applications, filed on the same date as this application which was filed. These applications are incorporated by reference: 
     1. “Apparatus and Method for Manufacturing a Semiconductor Circuit,” Ser. No., 09/590,809, (Attorney Docket 280-1/FE-00444); 
     2. “Increasing the Susceptibility of Integrated Circuits to Ionizing Radiation,” Ser. No., 09/590,805, (Attorney Docket 280-3/FE-00439); and 
     3. “Semiconductor Circuit Having Increased Susceptibility to Ionizing Radiation,” Ser. No., 09/592,473, (Attorney Docket 280-4/FE-00442). 
     It is to be understood that the above-described embodiments are merely illustrative of the invention and that many variations may be devised by those skilled in the art without departing from the scope of the invention and from the principles disclosed herein. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.