Abstract:
Methods and apparatus are provided for reducing the overall radiation hardness of a semiconductor chip. A radiation detector and a failure memory are provided. A disable signal or signals is produced by the failure memory. The disable signal is a required input to a user logic function, such as an off chip driver, an off chip receiver, a clock, or a static random access memory. When the radiation detector detects radiation, that detection is stored in the failure memory. The disable signal, when active, causes some or all of the user function to be inoperative. This invention is particularly important when the semiconductor chip is produced in a silicon on insulator (SOI) Complementary Metal Oxide Semiconductor (CMOS) process, which is naturally radiation resistant.

Description:
FIELD OF THE INVENTION 
   The present invention relates to semiconductor chips. In particular, the present invention relates reducing radiation hardness on chips 
   DESCRIPTION OF THE RELATED ART 
   Very Large Scale Integration (VLSI) employing bulk substrate Complementary Metal Oxide Semiconductor (CMOS) technology has been one of the major success stories in recent years. Virtually all of the recent high performance microprocessors, as well as the Application Specific Integrated Circuit (ASIC) chips have been implemented in CMOS on a bulk substrate. 
   Bulk substrate technology provides a relatively inexpensive foundation upon which to create the N-channel Field Effect Transistors (NFETs) and the P-channel Field Effect Transistors (PFETs) needed to make modern logic circuits, such as NANDs, NORs, and memory elements, such as latches, registers, and Static Random Access Memories (SRAMs). CMOS implemented on a bulk substrate technology is called bulk CMOS. Source and drain areas on the NFETs are directly created in the bulk substrate, in the case where the bulk substrate is a P− semiconductor material. In some implementations, the bulk substrate is a P+ semiconductor material with a thin P− epitaxial layer grown on the top. A P+ substrate with a P− epitaxial layer is more costly to produce but has some advantages, such as higher resistance to latchup phenomena. PFET devices are created in N-wells that are implanted or diffused into the bulk P− or the epitaxial P− layer created on a P+ substrate as described above. 
   Microprocessors built on bulk CMOS technology have seen dramatic increases in frequency with technology advances. Microprocessor clock frequencies have risen from approximately 10 megahertz (MHz) in 1985, to approximately 100 MHz in 1991 to over a gigahertz (GHz) in 2001. 
   Several limitations of bulk CMOS are beginning to limit continued increases in frequencies. 
   A first limitation of bulk CMOS is the relatively large amount of capacitance in the reverse-biased junctions associated with NFET and PFET drains. These capacitances must be charged and discharged during switching of the logic circuits on the microprocessor chip or the ASIC chip. Delays are longer when more capacitance must be charged and discharged. Thus the capacitance of the drains of the NFETs and PFETs limit the performance of the microprocessor or the ASIC. CMOS NAND logic circuits are designed with stacked NFETs. CMOS NOR logic circuits are designed with stacked PFETs. In these circuits, the sources of most of the Field Effect Transistors (FETs) in the stacks of NFETs and PFETs also often are switched from one voltage level to another, further adding to capacitance that must be switched, and thus degrading performance. 
   Undesired capacitance not only degrades performance, but also causes more power to be dissipated. Power dissipated in CMOS circuits is usually approximated by equation (1):
 
 P= ½ *C*V   2   *f   (1)
 
where P=power in watts; C=capacitance in farads switched per cycle; V=voltage in volts; f=frequency in hertz. Some circuits, such as clocks, switch twice per cycle.
 
   Power is becoming a serious problem. Batteries power many electronic products. Lower power translates into longer battery life, or, alternatively, the ability to use a smaller, cheaper, battery. A lower power microprocessor or ASIC product therefore has a competitive edge in the battery-powered marketplace. Even when large computers or other electronic products are plugged into the wall, heat buildup calls for elaborate and expensive cooling mechanisms. Lower power microprocessors and ASIC products therefore also hold a competitive advantage in such products as well. 
     FIG. 1A  shows a cross sectional view of an NFET comprising a gate  2 , a drain  4  and a source  5  created in a bulk CMOS process, using a P− substrate  7 . Note in particular that the bottoms of source  5  and drain  4  of the NFET form junctions with P− substrate  7 . The capacitance associated with these junction areas must be charged and discharged as discussed above. 
   A second limitation of bulk CMOS is that the bodies of all NFETs are coupled to ground (or another suitable lower voltage). In the view of the NFET in  FIG. 1A , the area between source  5  and drain  4  is the body, and is therefore coupled electrically to substrate  7 , which is usually coupled to ground, as stated. PFETs (not shown) are constructed in an N-well, which is usually connected to Vdd, a supply voltage that is positive with respect to ground, and thus, the bodies of all PFETs are similarly coupled to Vdd. The threshold voltage of an FET is strongly dependent on the voltage between the source of the FET and the body of the FET. Consider a CMOS NAND with two inputs. Two NFETs are stacked, with a gate of each NFET coupled to one of the two inputs. The bottom NFET has a source coupled to ground. The drain of the bottom NFET is coupled to the source of the top NFET. The drain of the top NFET is coupled to the output of the two input CMOS NAND. Assume that the top NFET has a logical “1” (i.e., Vdd) coupled to its gate, but the bottom NFET has a logical “0” (i.e., ground) coupled to its gate. The two NFETs provide the pull-down portion of the CMOS NAND circuit. A complementary pull-up portion of the CMOS NAND circuit comprises two PFETs. Each PFET has a gate coupled to one of the two inputs, a source coupled to Vdd, and a drain coupled to the output of the two input CMOS NAND. In the example above, the PFET that has a gate coupled to the same input as the gate on the lower NFET is conducting, and the output of the CMOS NAND is at Vdd. Since the top NFET is conducting but the bottom NFET is nonconducting, the source of the top NFET is also at Vdd. The source to body voltage of the top NFET is therefore Vdd, and the threshold voltage of the top NFET is relatively high. When the input coupled to the bottom NFET switches, the initially high threshold voltage of the top NFET reduces conductance of the two-NFET stack. As stated before, the charge stored on the reverse-biased junctions associated with the drains of the top and the bottom NFET, the source of the top NFET, and the drains of both PFETs must be discharged when the output of the two-input NAND is switched in the example. Reduction in the threshold voltage of the top NFET, and reduction of capacitance both serve to improve performance. 
   To reduce the limitations of bulk CMOS, use of silicon on insulator (SOI) is becoming a common way to improve performance of microprocessors, ASICs, SRAMs, and other semiconductor chips. 
   The major difference between CMOS on SOI and bulk CMOS is that SOI CMOS places an insulator directly below the FET devices, dramatically reducing the junction areas, and therefore the associated capacitances. The insulator also isolates the bodies of the FETs from the substrate in the case of NFETs, or the N-well, in the case of PFETs. 
     FIG. 1B  shows a cross section of an NFET implemented in SOI, comprising a gate  2 , a drain  4 , and a source  5 .  FIG. 1B  shows that the bottoms of the source and drain regions are in contact with a buried oxide  8 , which dramatically reduces the capacitance associated with source  5  and drain  4 . In the case of the bulk CMOS devices, junction capacitance can be relatively large. In the case of the SOI CMOS devices, the capacitance of the bottom of the source and drain regions is much less, and can be calculated using the textbook capacitance equation (2):
   C=epsilon*area/distance between plates   (2) 
where a top plate of the capacitor is the bottom of a source or drain region; a bottom plate of the capacitor is the P− substrate in the example; epsilon is the dielectric constant of the medium (the buried oxide) between capacitor plates; area is the area in which the capacitor plates overlap (i.e., the bottom of the source or drain, since the buried oxide in the example underlies the entire area); the distance between plates is the thickness of the buried oxide. The geometries involved in SOI CMOS are such that this capacitance is extremely small relative to the junction capacitance associated with sources and drains in bulk CMOS.
 
   As stated above, SOI CMOS FETs also have their bodies isolated from the ground and Vdd supplies, for NFETs and PFETs, respectively. The bodies of NFETs in SOI CMOS tend to float higher than ground; the bodies of PFETs in SOI CMOS tend to float lower than Vdd. This is particularly true for stacked devices. The resulting lower threshold voltages cause the FETs to conduct more current for a given FET size, therefore yielding logic circuits with less delay. 
   A textbook providing much more detail on the advantages of SOI CMOS over bulk CMOS is “SOI Circuit Design Concepts”, by Kerry Bernstein and Norman J. Rohrer, Published by Kluwer Academic Publishers, ISBN 0-7923-7762-1, (hereinafter Bernstein). Bernstein also describes several methods of providing the insulating layer required in an SOI semiconductor process. 
   Bernstein suggests a rule of thumb that reduction in junction capacitance is responsible for approximately half the advantage of SOI CMOS; reduction of average FET threshold is responsible for the other half. 
   The advantages of SOI CMOS have been exploited in some applications to provide a faster microprocessor, ASIC, or other semiconductor product at the same power. Other applications choose to operate the microprocessor, ASIC, or other semiconductor product at the same frequency that a bulk CMOS implementation would run, but at significantly lower power. 
   Creation of semiconductor chips involves relatively high fixed costs. The products have to be designed, requiring expensive design tools and engineering labor. In addition, the fabrication of the products requires extremely expensive semiconductor tools and factories. Those making such products therefore need to have the largest possible world market, so that the fixed costs can be spread out over as much volume of product as possible. 
   A characteristic of SOI CMOS is that it is much less susceptible to failures induced by radiation than is bulk CMOS. These failures are often called soft errors. Citing from Bernstein, page 141,
          Soft errors refer to false data states induced in the microprocessor logic or memory by the instantaneous introduction of high amounts of unexpected charge, created by a radiation event. Charge can be created as a result of incident alpha particles or high energy protons or neutrons (cosmic rays)” . . . “Radiation effects cause less charge generation and collection in SOI, as the sensitive region through which the alpha particle passes is now limited to the active silicon layer above the SIMOX (or BOX).”
 
SIMOX (Separation by the Implantation of Oxygen) is one process for creating the insulating layer beneath the FET devices. BOX stands for “Buried Oxide”.
       

   SOI CMOS is far less susceptible to radiation than is bulk CMOS. Furthermore, as the supply voltage, Vdd, decreases, the SOI CMOS advantage becomes far more pronounced. This is significant, because as technology improves, and semiconductor device sizes shrink, supply voltages are decreasing to prevent breakdown and/or wearout acceleration on the product. Supply voltages have dropped from 5 volts in 1985, to 3.3 volts in the early 1990s, to 2.5 volts in the mid 1990s. 1.8 volt was common in the late 1990s. Current CMOS products are being designed to operate at a supply voltage of a volt or less. Bernstein shows on page 143 that for identical memory array hardware built in SOI and bulk CMOS, the bulk CMOS version has over an order of magnitude higher rate of soft errors than the SOI version. As supply voltage decreases further, that margin will increase. 
   Although reducing the likelihood of soft errors is, in general, an advantage, a product with a high resistance to radiation can and does limit the size of the marketplace to which the product can be sold. Exports of items on the U.S. Munitions List (22 C.F.R. 121, et seq.), under State Department cognizance, generally require a license or other authority. The United States government restricts foreign sale or export of such products to some countries. 
   Code of Federal Regulations, Title 22, Chapter 1, part 121, “The United States Munitions List”, under “Category XV—Spacecraft Systems and Associated Equipment” includes as restricted items:
         d. Radiation-hardened microelectronic circuits that meet or exceed all five of the following characteristics:
           1. A total dose of 5×10 5  Rads (SI);   2. A dose rate upset of 5×10 8  Rads (SI)/Sec;   3. A neutron dose of 1×10 14  N/cm 2 ;   4. A single event upset of 1×10 −7  or less error/bit/day;   5. Single event latch-up free and having a dose rate latch-up of 5×10 8  Rads (SI)/sec or greater   
               

   SOI CMOS microprocessors, Static Random Access Memories (SRAMs), Dynamic Random Access Memories (DRAMs), and ASIC products are resistant enough to radiation to fall into the radiation-hardened microelectronic circuit category, and are therefore subject to export restrictions and market limitations. 
   There exist other semiconductor processes that also provide relatively high resistance to radiation. Semiconductor products produced on such processes also have their markets limited by the United States Government munitions restrictions. Although SOI CMOS is the primary focus of this invention, the spirit and scope of the invention includes any semiconductor process that results in a radiation resistant product. 
   Therefore, a need exists for a method and apparatus that allow semiconductor products that are produced in processes that are radiation resistant, such as SOI CMOS products, to be marketed free from export and trade restrictions. 
   SUMMARY OF THE INVENTION 
   The present invention is a method and apparatus that preserves the power and performance competitive advantages of SOI CMOS or similar radiation-hard semiconductor products, but which is less radiation-hard and therefore can be more easily and more broadly marketed. 
   In an embodiment, a radiation detector on a semiconductor chip detects presence of radiation and stores information about the detection in a failure memory. An output of the failure memory is used to disable portions of the user&#39;s desired function on the chip. 
   In another embodiment, the failure memory is volatile memory so that the chip is not permanently disabled upon encountering a detection of radiation. In an alternative embodiment, the failure memory is nonvolatile memory, so that the chip is permanently disabled upon encountering a detection of radiation. 
   In another embodiment of an SOI CMOS product built using the Separation by the Implantation of Oxygen (SIMOX) process, the radiation detector comprises an area constructed without the Buried Oxide (BOX). A radiation detector logic function that is especially vulnerable to radiation, such as an SRAM, latches, or dynamic logic is placed on the area. This logic function has radiation hardness of a bulk CMOS product, since no BOX exists in that area. 
   In an embodiment of the invention, the radiation detector stores information about detection of radiation in a failure memory. If radiation is detected, one or more disable signals is sent from the failure memory to one or more functions of the SOI CMOS portion of the chip, which are disabled respondent to the disable signal or signals; thereby causing the chip to be nonfunctional for its intended purpose. 
   In another embodiment of the invention, the disable signal(s) disables a clock function on the semiconductor chip. In another embodiment of the invention, the disable signal(s) disables off-chip drivers and/or receivers of the chip. In another embodiment of the invention, one or more SRAMs are disabled by the disable signal(s). Use of the disable signal to disable any function in the radiation-hard area of the SOI CMOS or other radiation-hard type of chip is within the spirit and scope of the present invention. 
   In an embodiment, semiconductor products produced in a method that results in radiation resistant products, are made susceptible to radiation failures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a cross section of a bulk CMOS chip, showing an NFET. 
       FIG. 1B  shows a cross section of an SOI chip, showing an NFET. 
       FIG. 2  shows a high level sequence of process steps to create a buried oxide layer on a semiconductor chip. 
       FIG. 3  shows a high level sequence of process steps, including a mask, to create a buried oxide layer on a semiconductor chip, while leaving an area of the semiconductor without a buried oxide layer. 
       FIG. 4  shows a high-level block diagram of the invention, comprising a radiation detector, and a failure memory. 
       FIG. 5  shows a semiconductor chip with a radiation detector and a failure memory. The failure memory is coupled to a clock on the chip and to input and output (I/O) circuits on the chip. 
       FIG. 6  shows a dynamic circuit suitable to detect radiation via soft error rate. 
       FIG. 7  shows a number of the dynamic circuit shown in  FIG. 6  implemented and logically OR&#39;ed to increase probability of radiation detection. 
       FIG. 8  shows an exemplary circuit that can be used to set the dynamic circuit of  FIG. 6  during power-up of the chip. 
       FIG. 9  shows exemplary voltage waveforms of the exemplary circuit of FIG.  8 . 
       FIG. 10  shows a circuit capable of storing nonvolatile information in the failure memory. 
       FIGS. 11A-11D  show several exemplary user functions that can be disabled, using a value or values stored in the failure memory. 
       FIG. 12  shows block diagram of a first block coupled to two other blocks, with attention drawn to the physical connections to the blocks. 
       FIG. 13  shows a flow diagram of an exemplary process embodying the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Having reference now to the figures, the present invention will be described in detail. 
     FIG. 1A  shows a cross section of a conventional N-channel Field Effect Transistor (NFET), comprising a gate  2 , a gate oxide  3 , a drain  4 , a source  5 , and shallow trench isolation  6 , constructed in a bulk P− silicon substrate. Such a structure is sensitive to radiation. A high-energy particle (not shown) will ionize a region in the bulk P− substrate  7 . Positive charges will be drawn off to a low voltage supply (usually ground) to which the substrate is coupled. Negative charges will be attracted to a positively charged drain  4 . In precharged circuits, such as Dynamic Random Access Memory (DRAM) precharged logic, or in high-impedance circuits such Static Random Access Memory (SRAM), the accumulation of the negative charges on a positively-charged N+ drain, can cause faulty operation. Similarly, accumulation of positive charges on a negatively precharged P+ drain can cause faulty operation. Such faults are called soft errors, and the rate of occurrence of the faults is called the soft error rate. 
     FIG. 1B  shows a conventional Silicon On Insulator (SOI) NFET in cross-section. The structure is similar to the conventional NFET shown in  FIG. 1 , with the addition of an insulating layer, the buried oxide (BOX) that is physically between the bottoms of the source and drain areas of NFET devices and the bulk P− substrate in the example. Advantages of SOI were described earlier. SOI CMOS circuits are far less susceptible to radiation than are bulk CMOS circuits. A high-energy particle (not shown) cannot ionize nearly as much semiconductor volume, because of the insulating layer. Furthermore, negative charges (except in the small volume of the FET body itself) cannot reach the drain regions. 
     FIG. 2  shows a set of semiconductor processing steps used by the SIMOX (Separation by the Implantation of Oxygen) process to create the buried oxide layer. A bulk silicon wafer substrate  11 , usually doped P−, is the starting point. High-energy oxygen implant  12  is implanted into wafer, causing oxygen atoms to penetrate some distance into substrate  11 . At the completion of oxygen implant  12 , a damaged thin layer  13  of P− is at the top of the substrate. Oxygen atoms are in layer  14 . P− substrate  15  is essentially unchanged, since few or none of the oxygen atoms penetrate deeply enough to reach that area. Finally an anneal step repairs the damage done to thin layer  13 , creating high-quality P− layer  16 . The anneal step also causes the oxygen atoms to form a silicon dioxide layer  17 , which is the insulating, buried oxide layer, discussed above. 
     FIG. 3  shows a process similar to the process of  FIG. 2 , with the addition of mask  19 , which blocks oxygen implant  12  from reaching selected areas of substrate  11 . CMOS devices created in such selected regions are conventional bulk CMOS because no buried oxide layer  17  exists in the selected regions. 
     FIG. 4  is a high-level block diagram of a semiconductor chip  30  comprising a radiation detector  31 , a failure memory  32 , and a user function  33 . Radiation detector  31  on the chip is sensitive to radiation and activates one or more signals responsive to the detection of radiation. Failure memory  32  can be a volatile memory or a nonvolatile memory. A volatile memory will retain information about the detection of radiation as long as the chip is powered on. The volatile memory is implemented as a conventional static or dynamic latch, or in an SRAM or DRAM on the chip. Preferably, the volatile memory is built in a radiation-hard region of the chip so that further exposure to radiation does not erase the information. A nonvolatile memory is built using a fuse, a ferroelectric memory, a flash memory, or other similar memory device that retains information even when the chip is not powered on. Embodiments using ferroelectric memory or flash memory, or other memory that can be written to are preferably designed such that the state of the memory can not be reset by a customer, following shipment of the chip to the customer. 
   User function  33  is the logical function desired by the user, or customer. User function  33  can comprise microprocessor function, SRAM, DRAM, latches, Programmable Logic Arrays, clock functions, Input/Output (I/O) circuitry, or any other logic function the customer desires on the chip. Some or all of user function  33  is disabled by a signal from failure memory  32  when radiation detector  31  detects radiation and stores information about the detection in failure memory  32 . 
     FIG. 5  shows chip  30  in more detail from a chip schematic view. Radiation detector  31  and failure memory  32  are shown as taking up space on chip  30 . User function  33  from  FIG. 4  takes up some or all of the remainder of the physical area of chip  30 . For example,  FIG. 5  shows I/O areas  35  on the edges of chip  30 . Input and output circuits (I/O), such as off chip drivers (OCDs) and off chip receivers (OCRs) are placed in areas  35 . In many modern chips, I/O circuitry is not confined to special areas  35 , but is intermingled with other circuitry on the chip, however I/O circuits are specialized in many respects, in general, having slew rate control, higher current drive capability, electrostatic discharge protection, and special disabling functions for test purposes. Areas  35  are shown separate only for exemplary purposes. Clock  36  represents one or more clock circuits that are common on chips, and these circuits control when latches can drive and receive data on the chip. Clock  36  is shown coupled to clock receiver  37 . Special clock receiver  37  comprises circuitry used to receive an external clock. Clock receiver  37  can be a differential receiver or a single-ended receiver. Clock  36  can simply repower signals received by clock receiver  37 . Clock  36  can be circuitry that uses signals received by clock  37  as a reference, such as when clock  36  comprises one or more phase locked loops. Any clock  36  is to be considered within the spirit and scope of this invention. In chip designs featuring I/O regions  35  at the periphery of the chip, often regions  34  are unused or are used for special purpose circuitry such as test circuitry. DISABLE  38  is a signal or signals produced by failure memory  32  that is coupled to one or more circuits in user function  33  and prevents such user function  33  from operating when DISABLE  38  is active. In  FIG. 5 , clock  36  and I/O function  35  are exemplary user functions  33 . 
     FIG. 6  shows a exemplary circuit that provides a combined, or merged, radiation detector  31  and failure memory  32 . In this circuit, failure memory  32  is volatile. Cell  40  comprises the merged radiation detector  31  and failure memory  32 . The circuit is a dynamic, precharged noninverting buffer, with an input coupled to ground. When input SET is low, PFET P 1  conducts, charging NODEA high (to Vdd). PFET P 3  and NFET N 2  comprise a static inverter. When NODEA is high, node OUT is low. PFET P 2  is then active, holding NODEA high even when input SET returns to high. P 2  is a very weak PFET, advantageously designed with a very long and very narrow channel. NFET N 1  has a drain coupled to NODEA and a gate and a source coupled to ground. N 1  is needed to provide an NFET drain that will collect negative charges when radiation particles pass through the chip near the NFET drain. In an embodiment, only the N+ drain region is implemented, with no FET gate or source. 
   When a high-energy radiation particle passes near the drain of N 1 , NODEA is discharged. The inverter comprising P 3  and N 2  causes node OUT to rise to a high level (Vdd), and P 2  is turned off, as the gate of P 2  is coupled to node OUT. A high (logic “1”) level on node OUT signals that radiation has been detected. The circuit shown in  FIG. 6  comprises a half-latch. NODEA, if pulled down, may float high enough, in time, to cause the static inverter comprising P 3  and N 2  to again cause node OUT to be pulled low. To prevent this possibility, the gate of N 1  can be coupled to node OUT, making the circuit a full latch. P 1  would then have to be designed strong enough relative to N 1  to guarantee that P 1  can set the full latch. 
   Discharge of NODEA by radiation is probabilistic, that is, NODEA has some probability of being affected by a particular particle of radiation. Many particles of radiation pass through the chip without passing through or near the drain of N 1 . Some particles may not have enough energy to discharge NODEA enough to cause node OUT to rise. 
   To raise the likelihood of radiation detection, circuit  40  is replicated many times, as shown in FIG.  7 . Circuit  40  is replicated as instances  40 A through  40 N, where N can indicate, for example, a thousand instances of circuit  40 . A technologist must determine the soft error rate probabilities for his or her technology and design the radiation detector with enough instances of circuit  40  so as to raise the soft error failure rate sufficiently high to avoid the product being categorized as a munition, per the earlier discussion. A logical combination of the outputs of circuit  40  instances  40 A- 40 N is shown performed by OR circuit  41 , which then produces a node OUT which is driven high when any of the outputs OUTA-OUTN of instances  40 A- 40 N is driven high. Although OR  41  is shown as a single element, in practice a 1000-input OR is impractical, and multiple circuits with a logical culmination of OR would be needed. Embodiments of circuit  40  that output a logical “0” when radiation has been detected is also contemplated. In such case, a logical AND would be used instead of the logical OR  41 . 
   In the example of  FIG. 6 , the radiation detector is a precharged dynamic circuit. Any circuit known to be susceptible to radiation-caused soft error fails can be used. For example, an SRAM could be used. For a second example, a DRAM could be used. 
   In a preferred embodiment, on an SOI semiconductor chip, the area of the chip in which radiation detector  31  is placed should be defined with a mask  19 . This will cause the FETs comprising radiation detector  31  to not have the insulating layer under them, and therefore, those FETs will not be SOI FETs, but rather, will be bulk FETs and will have a soft error rate associated with bulk CMOS, instead of SOI CMOS. Soft error rate in bulk CMOS is at least an order of magnitude larger for CMOS technologies operating at one volt or less, as described earlier. 
     FIG. 8  shows a circuit that provides a SET signal suitable for use by the circuit of FIG.  6 . The circuit in  FIG. 8  maintains a logic “0” output during Vdd power-up of the chip until Vdd reaches approximately three threshold voltages above ground. When Vdd becomes high enough to activate PFET P 4 , PFET P 5 , and NFET N 3 , N 3  will conduct. Resistors R 2  and R 1  are high-resistance resistors. R 2  ensures that the gate of N 3  is at ground potential until P 4  and P 5  begin to conduct. When N 3  begins to conduct current, NODEB falls from a high voltage to a low voltage. An inverter comprising PFET P 6  and NFET N 4  is coupled to NODEB and responds to NODEB falling by raising SET. A variation on this circuit is to replace N 4  by a high resistance resistor (not shown) coupled between SET and ground. 
     FIG. 9  shows the waveforms of the circuit of FIG.  8 . In the top graph, Vdd is shown ramping linearly. A linear increase in Vdd with time as shown is exemplary only and is not a requirement. Most Vdd power supplies take microseconds or milliseconds to reach their operating Vdd voltage. In the middle graph, NODEB follows (is the same voltage as) Vdd, since N 3  is nonconducting and no current flows through R 1 . However, when Vdd exceeds the sum of the threshold voltages of P 4 , P 5 , and N 3 , N 3  begins to conduct; NODEB then falls to a low level. In the bottom graph, the voltage of SET is shown, remaining at ground until NODEB falls; then, responsive to NODEB falling, SET rises, and quickly becomes the same voltage as Vdd. 
     FIG. 10  shows a nonvolatile embodiment of failure memory  32 . Node OUT of the circuits of  FIG. 6  or  FIG. 7  is coupled to the radiation-detected input of FIG.  10 . When no radiation has been detected, the radiation-detected input of  FIG. 10  is low and NFET N 5  is nonconducting. However if the radiation-detected input is driven high by radiation detector  31 , N 5  conducts. N 5  is coupled to FUSEA, and N 5  is designed to conduct sufficient current to cause FUSEA to change from a conductor to a nonconductor. Familiarly, this is often called “blowing the fuse”. Resistor R 3  is a high-resistance resistor. If FUSEA is a conductor, output DISABLE will be essentially at Vdd. If FUSEA is a nonconductor, R 3  will pull output DISABLE to ground. 
   Output DISABLE of  FIG. 10  is suitable to drive DISABLE  38 , shown in FIG.  5 . 
     FIGS. 11A-11D  are examples of possible user functions  33  that can be disabled by DISABLE  38 . 
     FIG. 11A  shows an exemplary SRAM that has a DISABLE input that can be coupled to DISABLE  38 . If the signal driving the DISABLE input is active, the SRAM will not operate. Many methods of disabling the SRAM will be apparent to those skilled in the art, including forcing some or all outputs of the SRAM to predetermined voltage levels, degating the SRAM CLK input, or other such means. 
     FIG. 11B  shows an exemplary Off Chip Driver (OCD). The OCD is shown having a DRIVER DATA input, a FUNC ENABLE input, and a DISABLE input. DRIVER DATA is coupled to a signal the user wants to drive off the chip. FUNC ENABLE is an input often present on an OCD that can be used to force the output of the OCD to a high impedance state, driving neither a logic “1” nor a logic “0”. Such high impedance state is often used when multiple OCDs, usually from separate chips, are coupled onto the same signal interconnect. Disabling an OCD with a DISABLE input can include forcing the output, DRIVER OUT, of the OCD to a predetermined voltage, advantageously Vdd or ground, or causing the OCD to be in a high impedance state. 
     FIG. 1C  shows an exemplary Off Chip Receiver (OCR), having a RECEIVER DATA input and a DISABLE input. The OCR normally receives signals driven from other chips and drives those signals onto the current chip. Often, an OCR performs signal shaping, hysteresis function, or level shifting. The OCR in  FIG. 11C  is designed such that when DISABLE is active, RECEIVER OUT is driven to a predetermined voltage, advantageously Vdd or ground. 
     FIG. 11D  shows an exemplary CLOCK  36  circuit. CLOCK  36  is shown in  FIG. 11D  having inputs CLOCK IN and DISABLE. When DISABLE is active, CLOCK OUT is forced to a predetermined voltage, advantageously Vdd or ground. 
   The user functions  33  of  FIGS. 11A-11D  are exemplary only. Any user function  33  that can be disabled responsive to detection of radiation is contemplated by this invention. A preferred embodiment implements all, or substantially all, of the OCDs of the chip as shown in FIG.  11 B. 
     FIG. 12  shows three logic blocks coupled by on-chip interconnect wiring. BLOCKA is coupled to the interconnection at node  50 ; BLOCKB is coupled to the interconnection at node  51 ; BLOCKC is coupled to the interconnection at node  52 . Such connection nodes are called ports or pins of the blocks. A common feature of design systems used to manage, assemble, and check the use and interconnection, of logic blocks on chips is the ability to assign pintypes to pins of blocks, and to use this pintype information to check that blocks are properly connected. 
   For example, in an embodiment, BLOCKA is failure memory  32 , and pin  50  is the driving pin of a DISABLE  38  signal. BLOCKB in the example is an OCD, and pin  51  is the DISABLE input of the OCD. BLOCKC is a CLOCK and pin  51  is the DISABLE input of CLOCK  36 . A technologist codes pintypes associated with pin  51  and pin  52  to require that they be connected to a pin  50  (i.e., a DISABLE  38  signal source). The design system, when doing its checking to verify that signals on the chip are properly connected, will ensure that if a pin  51  or a pin  52  is found on an interconnection, that a pin  50  is also found on the interconnection. Checking rules can also ensure that a radiation detector  31 , a failure memory  32 , and a user function  33  that can be disabled are selected by a designer for inclusion in the design. 
     FIG. 13  shows an exemplary flowchart that shows a set of steps that can be used to implement the invention. 
   In step  60 , the technologist provides a circuit library that a user can choose logic functions from to implement his or her design. The user interfaces with a design system in choosing logic functions and interconnecting the logic functions. Such logic functions can include, but are not limited to, NANDs, NORs, Exclusive Ors, And-Or-Inverters, latches, registers, SRAMs, DRAMs, Arithmetic Logic Units (ALUs), OCD&#39;s, OCR&#39;s, Clocks, and PLAs. Some or all of these functions are designed with a DISABLE input. When the DISABLE input is driven to an active state, the logic function is disabled. The technologist includes in the library one or more radiation detectors and one or more failure memories. 
   In step  61 , the technologist creates checking rules and checking information that the design system utilizes to ensure correctness in the use and interconnection of the logic functions. Among those rules are pintype rules, described earlier, which are used by the design system to ensure that particular pins of each logic block are coupled to pins required by the pintype rule for those particular pins. In particular, DISABLE input pins must have pintypes that require coupling to a suitable pin driving a DISABLE signal, as described earlier. Furthermore, the checking rules verify that at least one radiation detector  31  and at least one failure memory  32  have been selected for inclusion in the design. (Note that in some embodiments, as described earlier, radiation detector  31  and failure memory  32  may be merged). 
   In step  62 , the user&#39;s design is read in by the design system. 
   In step  63 , the design system checks the user&#39;s design for completeness and correctness, according to the checking rules. Typically, a great deal of checking is done, for examples, verifying timing correctness, checking for pins that have been left unconnected, checking test methodology correctness, and the like. In accordance with the current invention, pintype checking is done to ensure compliance with the pintype rules. If, for example, a pintype on a DISABLE input requires coupling to a pin of a block that drives a DISABLE signal  38 , but the coupling to a pin of a block that drives a DISABLE signal  38  is not found on the interconnection, that check will result in an error. 
   In step  64 , the results of the design system&#39;s checking for completeness and correctness is examined. If one or more errors have been discovered, the chip will not be built, as shown in step  65 . Normally, in such a case, the user is notified and informed of the error(s) in his design. The user can then choose to correct the error(s) or abandon the effort. If no errors were discovered by the design system the design is sent to the semiconductor fabrication facility, where the chip will be built. 
   The above process ensures that the chip, as sent to the user, will not be radiation hard enough to be categorized as munitions. The technologist can more widely and freely market the chip. 
   While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawings, these details are not intended to limit the scope of the invention as claimed in the appended claims.