Abstract:
A semiconductor wafer includes a plurality of sensors. Each of the sensors has a field oxide transistor, and a detecting circuit electrically connected to the field oxide transistor for detecting if the field oxide transistor is switched on or off and generating corresponding detecting signals. The field oxide of a different field oxide transistor has a different thickness. Each field oxide transistor is coupled to a corresponding detecting circuit for detecting radiation impinging on the semiconductor wafer.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to a semiconductor wafer that can detect radiation, and more particularly, to a semiconductor wafer that uses a conducting state of a channel of a field oxide to measure radiance. 
     2. Description of the Prior Art 
     Due to the extensive use of semiconductor devices, semiconductor wafers are more often subjected to environments that have a large amount of radiation. This radiation can cause a semiconductor device to malfunction. Please refer to FIG.  1 . FIG. 1 is a structural diagram of a prior art semiconductor wafer  10 . The semiconductor wafer  10  has two N-type metal-oxide-semiconductor (NMOS) transistors  12 A and  12 B on a P-type substrate. Each NMOS transistor has a gate  16 A or  16 B, a source  18 A or  18 B, a drain  20 A or  20 B, and a gate oxide  22 A or  22 B. In the highly compact modern circuit layouts, distances between transistors are extremely small, and are isolated by field oxides, like field oxide  28  in the semiconductor wafer  10 , to prevent mutual electrical interference. Conductive layer  26  on the field oxide  28  provides a link to each transistor. The conductive layer  26  is typically a metallic link in the semiconductor wafer  10 . In addition, a channel stop  15  is below the field oxide  28 . 
     When a semiconductor wafer  10  is subject to radiation, the energy of the radiation will create electron-hole pairs in the oxide layer in semiconductor wafer  10 .Holes are more likely to be retained in the oxide layer because hole mobility is slower in an oxide layer. In a field oxide layer, the above phenomenon is more evident. Compared to other oxide layers (such as a gate oxide) in a semiconductor wafer, electron-hole pairs are more likely to occur in the field oxide layer, and holes are more likely to accumulate in a field oxide because the volume of a field oxide is larger. 
     When the conductive layer  26  passes over the field oxide  28 , field oxide  28 , conductive layer  26  and two electrodes  18 A and  20 B become, in effect, a field oxide transistor. Field oxide  28  is equivalent to a gate oxide capacitor. Charge carried by holes accumulated in the field oxide  28  reduces the threshold voltage of the equivalent field oxide transistor. It is well known that changing the amplitude of the threshold voltage of a metal-oxide-semiconductor is proportional to charge of the gate oxide capacitance, and inversely proportional to the capacitance of a gate oxide capacitor. In a metal-oxide-semiconductor transistor, when the gate oxide capacitance is very small, even very little net charge on the gate oxide capacitor will cause dramatic change of the threshold voltage. This change of threshold voltage in the above equivalent field oxide transistor is particularly evident. Because holes easily accumulate in the field oxide, and the field oxide is very thick, the equivalent gate capacitance of the field oxide is relatively small, and the threshold voltage caused by charge is thus affected more. If too much charge accumulates on the field oxide  28  because of radiation, the threshold voltage of the field oxide transistor is, in effect, reduced. If there is electric activity in the conductive layer  26 , a channel will form below the field oxide  28  and activate the equivalent field oxide transistor. An improper electric connection between the electrode  18 A and the electrode  20 B on two sides of the field oxide  28  is formed. Then, the functionality of the field oxide  28  to isolate transistor  12 A and transistor  12 B is damaged and causes the semiconductor wafer  10  to malfunction. 
     In the prior art, the semiconductor wafer  10  has no advance warning that the semiconductor wafer  10  is being influenced by radiation. When negative affects induced by radiation accumulate to cause a prior art semiconductor wafer  10  to malfunction, normal operations of a microprocessor system based on the semiconductor wafer  10  is severely and adversely influenced. 
     SUMMARY OF INVENTION 
     It is therefore a primary objective of the present invention to provide a semiconductor wafer that can detect radiation and provide a warning signal when the semiconductor wafer is subjected to radiation-induced damage in the early stages of exposure. 
     Briefly, in a preferred embodiment, the present invention provides a semiconductor wafer having at least one sensor comprising. The sensor includes a field oxide transistor, and a detecting circuit electrically connected to the field oxide transistor for detecting if the field oxide transistor is switched on or off and generating corresponding detecting signals. 
     It is an advantage of the present invention that the semiconductor wafer according to the present invention can detect radiation and provide a corresponding warning. Malfunctions of a semiconductor wafer can thus be prevented. 
     These and other objects and the advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic diagram of a structure of a prior art semiconductor wafer. 
     FIG. 2 is a schematic diagram of a structure of a semiconductor wafer of the present invention. 
     FIG. 3 is a schematic diagram of a sensor embodiment of a semiconductor wafer of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In addition to common transistors and circuits for performing various functions, a semiconductor wafer of the present invention has a detecting transistor for detecting radiation in order to provide a warning of the radiation. Please refer to FIG.  2 . FIG. 2 is a schematic diagram of a structure of a detecting transistor  30  in a semiconductor wafer  25  of the present invention. The semiconductor wafer  25  of the present invention has a p-type base  40  and a field oxide  32  for isolating common transistors. The function and set-up of the field oxide  32  is the same as the field oxide  28  in the prior art semiconductor wafer  10 . The detecting transistor  30  of the present invention also uses a part of the field oxide  32  of the semiconductor wafer  25  and forms two conductive contacts, drain  36  and source  38  respectively, on both sides of the field oxide  32 . In addition, a conductive gate  34  is set on the field oxide  32 . In the detecting transistor  30 , arrangement of the gate  34 , the drain  36  and the source  38  with the field oxide  32  in between is equivalent to a field transistor. The biggest difference between the detecting transistor  30  and other ordinary metal-oxide-semiconductor transistors is a bulky and thick field oxide  32  isolating the gate  30  and the channel in the detecting transistor  30 . Oxide layers in other ordinary metal-oxide-semiconductor transistors are a thinner gate oxide. The two conductive contacts in the detecting transistor  30 , the drain  36  and the source  38 , are respectively formed by two n-typed doped regions on the p-type substrate  40  and of a conductive material. A channel stop  41  is below the field oxide  32 . 
     As described above, a thicker field oxide is easily affected by radiation and the corresponding accumulation of charge. For the same reason, when the detecting transistor  30  of the present invention is subjected to radiation, electron-hole pairs are also produced, and holes accumulate in the field oxide  32  of the detecting transistor  30  because of lower mobility. Because the field oxide  32  in the detecting transistor  30  is equivalent to a gate capacitor, charge produced by hole accumulation will lower the threshold voltage of the detecting transistor  30 . As a well-known physical phenomenon, change of the threshold voltage of the detecting transistor  30  will change a conducting condition between the drain  36  and the source  38 . So, measurement of the conducting condition between the drain  36  and the source  38  can reveal the degree of exposure of the detecting transistor  30  to radiation, and thus provide a measurement of radiation exposure of the semiconductor wafer  25  of the present invention. 
     To warn of radiation-induced damage, the field oxide  32  of the detecting transistor  30  of the present invention can be thickened to a thickness that is thicker than any other isolating field oxide in semiconductor wafer  25 . The thicker the field oxide is, the less the capacitance of the equivalent gate capacitor is. And change of the threshold voltage influenced by hole charge is greater. In other words, the conducting condition between two conductive contacts (i.e., the drain  36  and the source  38 ) adjacent to a thicker field oxide (i.e., the field oxide  32 ) is more susceptible to charge accumulation and is thus more sensitive to exposure to radiation. The field oxide  32  in the detecting transistor  30  should thus be thickened to a thickness that is thicker than any other isolating field oxide in semiconductor wafer  25  so that current conduction between the two conductive contacts (drain  36  and source  38 ) in the detecting transistor  30  is changed by radiation before other field oxides isolating ordinary transistors are influenced. In this arrangement, current conduction change between the two conductive contacts (drain  36  and source  38 ) in the detecting transistor  30  means that the semiconductor wafer  25  has received a certain dosage of radiation. If the semiconductor wafer  25  continues to be subjected to radiation exposure, the normal functioning of the semiconductor wafer  25  will be adversely influenced. So, the influence of radiation on the operations of the semiconductor wafer  25  can be forewarned by the detecting transistor  30 . 
     There are many possible embodiments to measure current conduction between the two conductive contacts in the detecting transistor  30 . Please refer to FIG.  3 . FIG. 3 is a circuit schematic diagram of an embodiment of a sensor  50  according to the present invention. The sensor  50  uses a detecting circuit  42  to measure current conduction between two conductive contacts in the detecting transistor  30 , and transmits a detecting signal of radiation-induced effects of the detecting transistor  30 . The sensor  50  is disposed on the semiconductor wafer  25  of the present invention, and has a detecting circuit  42  for detecting current conduction between two conductive contacts (drain  36  and source  38 ) in the detecting transistor  30 . Please note that, for convenience of the present disclosure of the function and structure of the sensor  50 , the detecting transistor  30  in FIG.2 is shown as a circuit symbol. As described above, the detecting transistor  30  can be viewed as an n-type metal-oxide-semiconductor transistor (i.e. a field transistor). In the sensor  50 , two conductive contacts in the detecting transistor  30 , drain  36  and source  38 , are respectively connected to node N 2  and ground of the detecting circuit  42 . Gate  34  of the detecting transistor  30  is connected to node N 1  of the detecting circuit  42 . In the detecting circuit  42 , there is a p-type metal-oxide-semiconductor transistor M 1 . A source, a drain and a gate of the p-type metal-oxide-semiconductor transistor M 1  are respectively connected to a direct current Vdd, a node N 2  and a node N 1 . Connected to N 1  are a resistor R and a capacitor C. Two inverters I 1  and I 2  of a latch  46  are connected to the node N 2 . Finally, detecting signal  52  of the sensor  50  is output from a node N 4 . Please note that all metal-oxide-semiconductor transistors (including transistor Ml) in the detecting circuit  42  are common metal-oxide-semiconductor transistors. The field oxide in gates, drains and sources of these metal-oxide-semiconductor transistors is a gate oxide. Only in the detecting transistor  30  is the thickened field oxide used between gate  34 , drain  36  and source  38 . 
     Operation of the sensor  50  is described below. The direct current Vdd charges the capacitor C through the resistor R and makes a steady-state voltage of the node N 1  that is close to a voltage of the direct current Vdd. When the sensor  50  is not subject to radiation (or the radiation dosage is low), the field oxide in the detecting transistor  30  normally isolates drain  36  and source  38 . That is, the threshold voltage of the detecting transistor  30  is very large. Even connecting the gate  34  and the node N 1  will not turn on the detecting transistor  30 . At this time, the detecting signal  52  is at a low level. Once the detecting transistor  30  is subject to radiation and accumulates holes in the oxide, the threshold voltage of the detecting transistor  30  declines. The threshold voltage of the detecting transistor  30  declines down to a certain degree, and then the drain  36  and the source  38  will be connected, and the voltage of N 2  will decline, and the detecting signal  52  output from the latch circuit  46  will become high. In other words, if the detecting signal  52  of the sensor  50  goes high from a low state,then the detecting signal  52  should be inferred as a warning that radiation-induced effects are possible. 
     In fact, a detecting circuit for detecting current conduction between the two conductive contacts of the detecting transistor  30  may have many other embodiments, such as a comparator comparing a reference current and a current between the two conductive contacts of the detecting transistor  30 , and a comparison result being used as a detecting signal for warning. In addition, several sensors of differing sensitivity to radiation can be disposed on the semiconductor wafer of the present invention to quantize radiation warning effects. If the radiation dosage causes one sensor S 1  to produce a warning but not the other less sensitive sensor S 2 , then it can be inferred that the radiation dosage is beyond a warning level of S 1  but below a warning level of S 2 . Changing the warning levels of sensors can be performed by setting the respective field oxides to different thicknesses. As previously discussed, the thickness of the field oxide  32  of the detecting transistor  30  influences the capacitance of the equivalent gate capacitor. When charge accumulates on the field oxide, threshold voltage lowering is different because of different equivalent capacitances, and influence upon the threshold voltage of the detecting transistor is different. In other words, changing of the thickness of the field oxide  32  of the detecting transistor  30  changes the corresponding sensitivity (i.e., the warning level) of the sensor  50 . In addition, connecting gate  34  of the detecting transistor  30  to different voltages also influences the warning level of the detecting transistor  30 . Doping different concentrations of the channel stop below the field oxide  32  of the detecting transistor  30  can also change warning levels of the detecting transistor  30 . 
     Compared with the prior art semiconductor wafer, which is unable to detect and warn of radiation dosages, a semiconductor chip of the present invention has the detecting transistor  30 . The field oxide  32  of the detecting transistor  30  will change the conduction of the detecting transistor (i.e. the field transistor) under exposure to radiation. And through detecting conductance of the detecting transistor  30  by the detecting circuit  42  of the sensor  50 , a detecting signal is transmitted to warn of excessive radiation levels that may lead to adverse circuit performance. The present invention is particularly suitable for integration into flash memory, or other electronic circuits that are vulnerable to radiation, and thus enables prior warning of radiation dosages that are harmful to normal operation. If properly designed, the semiconductor wafer of the present invention can also quantize radiance dosages. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the radiation sensor may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.