Abstract:
Methods and circuits to measure the speed of silicon test structures using direct current test equipment. Each test structure comprises an oscillator and a detector. Oscillations started by a direct current input signal are rectified by the detector into a direct current output signal. Start of oscillations cause a jump in the output signal and that point is correlated with the input signal strength which in turn is correlated to the speed of the test circuits. By knowing the speed of the test circuits the quality of the manufacturing process can be checked. Direct current greatly simplifies measurement so that 100% testing can be performed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the testing of high frequency test structures as incorporated in semiconductor device fabrication. 
     2. Description of the Related Art 
     U.S. Pat. No. 4,523,312 (Takeuchi) deals with testing of an integrated circuit (IC) mounted in an IC socket, where the IC tester sends test patterns to the IC and receives responses from the IC to be checked for their logic levels. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to show a method and describe a test structure on a semiconductor wafer that allows a fast determination of the speed of semiconductor devices. 
     It is another object of this invention to increase the speed of testing through direct current measurements so that 100% testing can be performed. 
     These objects have been accomplished by the use of a plurality of test structures on the semiconductor wafer, manufactured by the same process as the semiconductor devices. Each test structure, consisting of an oscillator and a detector, is connected via probes to test equipment. The oscillator is induced to oscillate by increasing a direct current signal to its inputs. The detector rectifies these oscillations resulting in a jump of the direct current output signal. The magnitude of the input signal is noted for that point and correlates to the frequency or speed of the test circuit. Having earlier correlated the magnitude of the input signal with the frequency of oscillation for a given fabrication process, it is possible to know the speed and, therefore, the quality of the semiconductor manufactured by the same process. A high frequency measurement is, therefore, replaced by simply measuring a direct current input signal and output signal of the test structure. Because the test equipment has to deal only with direct currents, a switching matrix can now be used to quickly measure 100% of the test structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a is a flow diagram showing the method of the invention for measuring a test structure. 
     FIG. 1b is a block diagram showing the method of the invention for measuring a test structure. 
     FIG. 2a is a graph showing the correlation between cut-off frequency or device speed, and the input signal. 
     FIG. 2b is a schematic of one of the test structures of the invention, the test gear and switching matrix. 
     FIG. 3 is a graph showing the correlation between input current and output voltage. 
     FIG. 4 is a circuit diagram of one of the test structures of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In semiconductor fabrication test structures are deposited on the semiconductor wafer to allow verification of the quality of the process. These test structures operate at the same high frequencies as the semiconductor devices themselves. Direct measurements at these very high frequencies are difficult and time-consuming because high frequency test gear cannot be routed through a switching matrix, since the latter has an inherently low cut-off frequency. 
     Referring now to FIG. 1a, we show a flow diagram depicting the methods of this present invention. In Block 2 a test structure is incorporated into a semiconductor wafer during the regular fabrication process having, therefore, the same device characteristics as the other semiconductor devices. A direct current signal is supplied in Block 3, and increased in strength until it causes electrical oscillations. These electrical oscillations are then converted to a direct current signal and monitored at an output, as shown in Block 4. In Block 5 the input signal is measured when the output signal jumps. Block 6 correlates the input signal measurement to the speed of the test structure device or the quality of the fabrication process. This correlation is possible because the relationship between input signal, output signal, and frequency of oscillation had first been investigated on other semiconductor test structures. 
     Referring now to FIG. 1b, we show another block diagram depicting in more detail the methods of this present invention. Only one test structure 20 is shown, however, there normally will be a plurality of these. Input 10 supplies a direct current signal of rising magnitude to the Oscillator 30 part of the test structure. At a certain strength of the input signal oscillations will start. These oscillations are fed to the Detector 40, where they are rectified, resulting in a direct current (D.C.) output signal, measured by Output 50. The point where this sudden rise in voltage occurs is noted and the input signal at that point is labeled, e.g. I CO  ; see Correlation 60. By having first correlated the actual speed, or for example the cut-off frequency f T , to I CO  for a given semiconductor process, subsequent I CO  measurements of the test structure can be used to characterize the f T , and, therefore, the quality of semiconductor devices manufactured by the same process. 
     FIG. 2a is a graph with CURVE 2, showing the relationship of the speed of a test structure, or as an example the f T , and the input signal, i.e. I CO . CURVE 2 is seen to decline monotonically, providing good correlation between the input signal on the y-axis and the device speed on the x-axis. 
     Referring now to test structure 20 of FIG. 2b, we describe the principle of the present invention. A test oscillator 30 having an input 31, equal to Pad 1, is placed on a semiconductor wafer during semiconductor wafer fabrication. Connected to that test oscillator is a test detector 40 having an output 41, equal to Pad 3. That test detector is also placed on the semiconductor wafer during semiconductor wafer fabrication. A test apparatus or some other means attached to input 31, supplies a direct current signal to input 31 and also measures it. The test oscillator will start oscillating at a certain input signal strength applied to input 31. Test detector 40 will rectify those oscillations and produce a direct current signal at output 41. That test apparatus or some other means attached to output 41 detects the presence of a direct current signal at that output. I.e. the detection or measurement of very high frequency oscillations has been turned into the detection of a direct current signal, which is a very simple task as compared to detection, or measurement, of a very high frequency. 
     The important prior correlation of input signal magnitude with the high frequency oscillations, and therefore, the speed and quality of the devices on the semiconductor wafer has been described previously and need not be repeated here. Test structure 20 also shows the presence of a Pad 2 and 4. These pads may provide e.g. ground or some other reference but are not important to the principle of the invention or its understanding. Similar considerations apply to the pictorial representation of rectification of an alternating current signal by a capacitor and a diode as part of the test detector. Other embodiments for rectification may be chosen as is understood by those skilled in the art. 
     FIG. 2b is a schematic diagram of the invention incorporating the methods illustrated in FIG. 1b and, adding as an example, a matrix switch and test apparatus. The matrix switch and the test apparatus are for the purpose of illustration only and are not meant to limit the scope of the invention. Similarly this particular embodiment shows a direct current as the input signal and a voltage signal as the output signal but these choices are not meant to limit the scope of the invention since other embodiments regarding the input and output sources are equally possible. 
     Still referring to FIG. 2b, the schematic shows one of a plurality of test structures 20, consisting of oscillator 30 and detector 40 (depicting a simplified rectifying circuit). Oscillator inputs 31, 32 and detector outputs 41, 42 are connected via metalized lines 11 to metalized pads 51. These pads are located in the scribe line area 50 of the semiconductor wafer where they do not take up valuable silicon real estate. Movable probes 52 are lowered onto contact pads 51 and connect one-to-one to a unique input 65 of the switching matrix 60. Inputs 65 are connected through suitable switching contacts 66 to multiplexed outputs 61, 62, 63, and 64. These outputs are in turn wired to terminals 71, 72, 73, and 74, respectively, of test apparatus 70. 
     Because there is a set of four (modulo 4) probes for every test structure 20 (two inputs, two outputs), the number of switching inputs 65 is also modulo 4. The switching matrix multiplexes sets of four inputs to four outputs 61, 62, 63, and 64. Contacts 66 are controlled by suitable devices such as reed relays (not shown). Switching matrix 60 is located in reasonable proximity to the semiconductor wafer station. After testing of the first test structure, the switching matrix will connect another set of probes to outputs 61 to 64, and the test apparatus 70 will test a second test structure until all test structures are tested. 
     Direct current is supplied from terminals 71 to oscillator input 31, while oscillator input 32 is the direct current return to terminal 72. At a certain value of the input current, oscillator 30 will start to oscillate. Detector 40 rectifies these oscillations and produces a D.C. voltage between outputs 41 and 42. This voltage is measured by test apparatus 70 at terminals 73 and 74. The oscillations typically range from about 1000 to 4000 Megahertz. The D.C. output voltage ranges from microvolts to hundreds of millivolts. 
     FIG. 3 is a graph with CURVE 1, showing the relationship of the detector D.C. output voltage V 34  on the ordinate to the direct current input I 12  on the abscissa. As can be seen, the jump in D.C. voltage V 34  is quite distinct when oscillations start and is marked on the abscissa by I CO . While V 34  continues to rise as I 12  rises, the increase is not nearly as dramatic. It is apparent from the foregoing that determining point I CO  is non-ambiguous. I CO  is, therefore, a reliable measure of the oscillator&#39;s cut-off frequency f T . The lower I CO  is, the higher f T  is as shown in FIG. 2a. f T , and thus I CO , is a direct measure of the fabrication process. 
     FIG. 4 is a circuit diagram of one implementation of the invention to characterize the cut-off frequency f T  of semiconductor devices. A plurality of test structures are deposited during semiconductor fabrication for the purpose of verifying the quality of the fabrication process. One of these test structures, consisting of oscillator circuit 10 and detector circuit 20, is described. 
     The oscillator circuit 10 is made up of two transistors 11 and 12, where the base of each transistor is connected to the collector of the other transistor. Both emitters are tied to GND (pad 2) which is the direct current return path. An inductor 15 is connected between the collector of transistor 11 (Terminal A) and the collector of transistor 12 (Terminal B). Capacitors 13 and 14 connect from Terminal A and B, respectively, to GND. Resistor 16, between input 19 (pad 1) and Terminal A, is the collector resistor for transistor 11. Resistor 17, between V CC  and Terminal B, is the collector resistor for transistor 12. Pads 1 and 2 are the inputs to the oscillator circuit, while Terminal A is the output. When no or little input current I 12  flows, Terminal B is more positive than Terminal A and the base of transistor 11 is forward biased. The transistor conducts and its collector is near GND, thereby cutting off transistor 12. As I 12  rises, the voltage at Terminal A rises as well and transistor 12 starts to conduct, cutting off transistor 11. Capacitor 13 charges up, while capacitor 14 discharges through transistor 12; this causes current to flow through inductor 15. When current through inductor 15 stops flowing, the emf of the collapsing magnetic field makes Terminal B more positive and Terminal A more negative, thus repeating the cycle and sustaining oscillations. 
     Terminal A, the output of oscillator circuit 10, is also the input to detector circuit 20 and connects via capacitor 26 to Terminal E. Transistor 24 is wired as a diode by connecting collector and base to Terminal E, while the emitter connects to Terminal C. Capacitor 21, in parallel to resistor 22, connects between Terminal C and GND. Resistor 27, connecting V CC  to Terminal E, is the collector resistor for transistor 24. Connected between V CC  and GND and acting as a voltage divider are resistor 28, transistor 25 (also in diode configuration) and resistor 23. 
     High frequency oscillations from the oscillator circuit are rectified by transistor 24 and a voltage is developed across resistor 22, while capacitor 21 reduces the high frequency component of this D.C. voltage. Transistor 25, at Terminal D, produces the reference voltage needed to compensate for V CC  variations. Terminals C and D connect to pads 3 and 4, respectively. The voltage seen at the output of the detector circuit is the potential at Terminal C with respect to Terminal D. 
     Advantages of the present invention are simplification of measurements for high frequency circuits, requiring only the use of D.C. input and output signals to determine the circuit speed. In a production environment significant savings can be realized by using the measurement of, for example, I CO  as a process control variable, and do correlation to f T  to determine proper control limits. By replacing high frequency measurements with D.C. measurements it is now possible to quickly measure many test sites and to better characterize the fabrication process. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.