Abstract:
A method for characterizing the quality of the interface between a silicon and a gate insulator in a MOS device includes the steps of: applying at least one current to the MOS device through the gate; detecting at least one electroluminescent signal corresponding to the silicon bandgap energy after the current flows through the MOS device; and outputting the electroluminescent waveform in the time domain. The quality of the interface between a silicon and a gate insulator in the MOS device is determined by analyzing the minority carrier lifetime in silicon. The invention also discloses a characterization system for implementing the method.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a system and method for characterizing the quality of a semiconductor device and, more particularly, to a system and method for characterizing the quality of the interface between a silicon and a gate insulator in a MOS device.  
           [0003]    2. Description of the Related Art  
           [0004]    Metal-oxide-semiconductor transistors (hereinafter referred to as MOS devices) are the most widely used and most important transistors in the Ultra-Large-Scale-Integrated (ULSI) circuits using the silicon semiconductor crystal. Various kinds of integrated circuits (ICs) such as microprocessors used in personal computers are usually constituted by up to several millions of MOS devices. Therefore, the properties of the MOS devices have to be well controlled so as to obtain an integrated circuit with good performance. For example, it is well known that the lattice defects, impurities, interface state densities (D it ) and minority carrier lifetime near the gate insulator/silicon interface, are closely related to the quality of the gate insulator and the performance of a MOS device. As a result, it is very important to exactly measure the defects, impurities, interface state densities (D it ) and minority carrier lifetime near the gate insulator/silicon interface in MOS devices.  
           [0005]    Conventional methods such as the high/low frequency capacitance-voltage analysis, conductance method, charge-pumping method, and transient capacitance-time Zerbst analysis, etc., have been successfully used to measure the interface state densities and minority carrier lifetime of MOS devices with relatively thick gate insulator. However, when the line width has been reduced to 0.13 to 0.09 microns in the advanced ULSI technology, the thickness of the gate insulator in a MOS device is in the range of 15 to 30 angstroms, which is about the thickness of 3 to 6 layers of atoms. The ultra-thin gate insulator with such small thickness may result in significant leakage current tunneling through the gate insulator, thereby increasing series resistance of MOS capacitors. The significant series resistance in MOS devices due to the ultra-thin gate insulator complicates the analysis and causes the modeling based on the above measurement technologies to be difficult. As a result, it is necessary to develop a novel and simpler method for measuring the defects, impurities, interface state densities, and minority carrier lifetime near the gate insulator/silicon interface in the MOS devices with ultra-thin gate insulator  
         SUMMARY OF THE INVENTION  
         [0006]    To solve the above-mentioned problems, the invention provides a characterization system and method utilizing the characteristics of great gate tunneling currents in the MOS device with ultra-thin gate insulator. The minority carrier lifetime near the gate insulator/silicon interface in the MOS device is obtained by measuring the temporal electroluminescenct signal corresponding to the silicon bandgap energy. The characterization system and method can be used to investigate the parameters related to the quality of the gate insulator/silicon interface, such as lattice defects, impurities, interface state densities.  
           [0007]    To achieve the above-mentioned object, a method to characterize the gate insulator/silicon interface quality in the MOS device with ultra-thin gate insulator includes the steps of applying a current to the MOS device through the gate; detecting at least one electroluminescent signal corresponding to the silicon bandgap energy after the current flows through the MOS device; and outputting the electroluminescent waveform in the time domain. The minority carrier lifetime can be extracted from the temporal electroluminescent waveform. The quality of the gate insulator/silicon interface in the MOS device is determined by analyzing the minority carrier lifetime in silicon.  
           [0008]    To implement the above-mentioned method, a characterization system may include a current source, a photodetector and an output sub-system. The output sub-system may include a gate integrator and boxcar averager, a gate scanner and an X-Y recorder. The current source applies at least one current to the MOS device. The photodetector detects at least one electroluminescent signal corresponding to the silicon bandgap energy after the current flows through the MOS device. The gate integrator and boxcar averager receives the electroluminescent signals, integrates the signals during the gated period, and averages the received electroluminescent signals over many shots of the injection current. The gate scanner controls the position of the gate in the gate integrator and boxcar averager in the time domain, so that the gate can scan in the time domain to allow the retrieval of entire temporal electroluminescent waveform. The X-Y recorder records the voltage signals output from the gate scanner as the time axis, and records the voltage signals output from the gate integrator and boxcar averager as the intensity of the electroluminescent signals, to obtain the electroluminescent waveform in the time domain. According to the measured temporal electroluminescent waveform, the minority carrier lifetime near the gate insulator/silicon interface can be obtained.  
           [0009]    The current supplied from the current source may be a square wave or a pulse. The electroluminescent signal from the photodetector may be amplified by an amplifier. The photodetector may be an InGaAs or a germanium photodetector. The bandwidths of the current source, photodetector, amplifier and gate integrator and boxcar averager have to be greater than 10 MHz.  
           [0010]    The output sub-system also may be a combination of an amplifier and an oscilloscope. Under the condition that the amplification of the amplifier is great enough and the bandwidths of the amplifier and oscilloscope are large enough (&gt;10 MHz), the oscilloscope may directly display the temporal electroluminescent waveform. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic illustration showing the structure of a MOS device.  
         [0012]    [0012]FIG. 2 is a schematic illustration showing a band diagram of the MOS device when the electroluminescence takes place.  
         [0013]    [0013]FIG. 3 is a graph showing the electroluminescent spectra from the MOS device.  
         [0014]    [0014]FIG. 4 is a block diagram showing a system for measuring the temporal electroluminescent signals from a MOS device in accordance with the preferred embodiment of the invention.  
         [0015]    [0015]FIG. 5A is a graph showing a waveform of a square current with duration of 150 microseconds.  
         [0016]    [0016]FIG. 5B is a graph showing the electroluminescent waveforms in the time domain from the MOS device at various injection currents.  
         [0017]    [0017]FIG. 6 is a graph showing that the carrier recombination rates 1/τ versus the injected current level at the rising and falling edges of the electroluminescent signals in FIG. 5B; in which the dashed lines are the theoretical fitting.  
         [0018]    [0018]FIG. 7 is a graph showing the electroluminescent waveforms in the time domain from another MOS device at various injection currents. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The system and method to characterize the gate insulator/silicon interface in a MOS device in accordance with a preferred embodiment of the invention will be described with reference to the accompanying drawings, wherein the same reference numbers denote the same elements.  
         [0020]    [0020]FIG. 1 shows the structure of a MOS device  10 . An ultra-thin gate insulator  12  is grown on the surface of a silicon substrate  11 , and a gate  13  of the MOS device is deposited on the gate insulator  12 . The gate  13  may be metal or polysilicon. Finally, a metal layer  14  is deposited on the backside of the silicon substrate  11  as another electrode of the MOS device.  
         [0021]    [0021]FIG. 2 shows the band diagram of the MOS device when electroluminescence takes places. When the MOS device is operated in the accumulation region, a large number of majority carriers are accumulated near the gate insulator/silicon interface. The gate insulator is thin enough to allow significant tunneling current taking place. The carriers tunneling from the gate radiatively recombine with the majority carriers near the gate insulator/silicon interface. Thus, infrared radiation corresponding to silicon bandgap energy is generated.  
         [0022]    [0022]FIG. 3 shows the electroluminescent spectra of the MOS device at room temperature. The photon energy at the peaks of the spectra is around 1.09 eV, which is slightly below the bandgap energy of silicon crystal at room temperature. This is due to the participation of optical phonon for the requirement of momentum conservation in the radiative recombination.  
         [0023]    [0023]FIG. 4 shows a system for measuring the electroluminescent signal in the time domain from the MOS device in accordance with the preferred embodiment of the invention. The electrodes of a current source  41  are connected to the gate  13  of the MOS device  10  and the electrode  14  on the back of the silicon substrate  11 , respectively. A square current is injected into the MOS device  10  via the gate  13  and the backside electrode  14  so as to generate the electroluminescence. Then, an InGaAs photodetector  42  is closely placed to the gate  13  of the MOS device to collect the infrared radiation.  
         [0024]    The signal from the InGaAs photodetector  42  is sent to an output sub-system  40  to show the electroluminescent waveform in the time domain. In this embodiment, since the electroluminescent signal from the InGaAs photodetector  42  is weak, an amplifier  43  is used to amplify the electroluminescent signal. The amplified signal from the amplifier  43  is further fed to a scanning-gate integrator and boxcar averager system.  
         [0025]    After electroluminescent signals are sent to a gate integrator and boxcar averager  44 , the intensities of the electroluminescent signals within the sampling interval are integrated and averaged. The gate integrator and boxcar averager  44  is triggered by the current source  41  and averages the electroluminescent signals over many shots of the injection current so as to reduce the noise. The sampling interval of the electroluminescent signals is determined by the width of the gate, while position of the gate in the time domain is controlled by a gate scanner  45 . The gate scanner  45  outputs a voltage signal to the gate integrator and boxcar averager  44  so as to control the position of the gate in the time domain. Accordingly, the gate scans in the time domain to measure the intensities of the electroluminescent signals in the time domain continuously. Finally, the gate scanner  45  outputs voltage signals to an X-Y recorder  46  as horizontal axis (X-axis), which represents the time axis for the electroluminescent signals. On the other hand, the gate integrator and boxcar averager  44  outputs voltage signals to the X-Y recorder  46  as vertical axis (Y-axis), which represents intensities of the electroluminescence signals. As a result, the electroluminescent signals in the time domain of the MOS device  10  are acquired.  
         [0026]    In this embodiment, it should be noted that the bandwidth of each unit (the current source  41 , the InGaAs photodetector  42 , the amplifier  43  and the gate integrator and boxcar averager  44 ) of the measuring system has to be greater than 10 MHz. Thus, the measuring system can possess enough resolution in the time domain to measure the minority carrier lifetime in silicon. In addition, one of ordinary skill in the art may also record or observe the temporal electroluminescent signal by another method. For example, an oscilloscope with bandwidth greater than 10 MHz can be used to directly show the electroluminescent waveform in the time domain.  
         [0027]    The waveforms of the electroluminescent signals generated from the MOS device under the injection of a square current are shown in FIGS. 5A and 5B. FIG. 5A shows a square current waveform with duration of 150 microseconds, while FIG. 5B shows the temporal waveforms of the electroluminescence from the MOS device at various injection currents. Compared to the waveform of the square current in FIG. 5A, the rising and falling edges of the electroluminescent waveforms shown in FIG. 5B vary relatively slowly. Thus, the measured electroluminescent waveform can tracks the variation of the excess minority carrier density with time. Therefore, the rising or falling time of the electroluminescent waveform represents the minority carrier lifetime. By measuring the rising or falling time of the electroluminescence waveform, we can determine the minority carrier lifetime near the gate insulator/silicon interface.  
         [0028]    The variation of excess minority carrier density with time can be expressed using the following equation:  
                    P          t       =     G   -     (       A   ·   P     +     B   ·   NP     +       C   ·     N   2          P       )               (   1   )                               
 
         [0029]    where “P” represents the excess minority carrier density, “N” represents the majority carrier density, “G” represents the generation rate of the excess minority carrier density due to the injection current, and “t” represents time. “A” is the coefficient of the Shockley-Read-Hall (SRH) recombination through the nonradiative recombination states near the gate insulator/silicon interface., while “B” and “C” represent the radiative recombination coefficient and the Auger recombination coefficient, respectively. The dotted lines in FIG. 5B are obtained by solving Equation (1) to fit the experimental data, which show a good match between the theoretical calculation and the experimental data. The rising and falling edges of temporal electroluminescent waveforms are fitted using the exponential function with the time constant c related to the coefficients A, B and C:  
               1   τ     =     A   +     B   ·   N     +     C   ·     N   2                 (   2   )                               
 
         [0030]    where τ is the minority carrier lifetime, and 1/τ represents the recombination rate of carriers.  
         [0031]    The carrier recombination rates 1/τ corresponding to the rising and falling edges of the electroluminescent waveforms under various injection current densities in FIG. 5B are shown in FIG. 6. At the rising edge of electroluminescent waveform, the recombination rate is independent of the injection current density. Because the majority carrier density N near the gate insulator/silicon interface increases with the injection current, it can be understood from Equation (2) that the SRH nonradiative recombination represented by the coefficient A is the dominant carrier recombination mechanism at the rising edge of the electroluminescent signal. On the other hand, the carrier recombination rate increases with the injection current density at the falling edge of the electroluminescent waveform. It also can be understood from Equation (2) that the carrier radiative recombination and Auger recombination represented by the coefficients B and C cannot be neglected at the falling edge.  
         [0032]    As can be known from the above description, the minority carrier lifetime extracted from the rising edge of the temporal electroluminescent waveform is mainly determined by the SRH nonradiative recombination close to the gate insulator/silicon interface in the MOS device. Thus, the effect of radiative recombination and Auger recombination can be ignored. The rising edge of the electroluminescent waveform in the time domain can be used to characterize the SRH nonradiative recombination near the gate insulator/silicon interface. The minority carrier lifetime will be small if the density of the SRH nonradiative recombination states is great. The lattice defects, impurities and interface states densities near the gate insulator/silicon interface, are the sources of the SRH nonradiative recombination states. By measuring the minority carrier lifetimes corresponding to the rising edges of the temporal electroluminescent waveforms, one can compare the quality of the gate insulator/silicon interface for the ultra-thin gate insulators grown under different conditions.  
         [0033]    [0033]FIG. 7 shows the temporal electroluminescent waveform of another MOS device under the injection of a square current of 150 microseconds. The ultra-thin gate insulator of the MOS devices in FIGS. 5B and 7 are grown under different conditions. By solving Equation (1) to fit the rising edges of the temporal electroluminescent waveforms, we can obtain that the minority carrier lifetimes are 18 and 27 microseconds corresponding to FIGS. 5B and 7, respectively. Consequently, in comparison with the MOS device of FIG. 5B, the crystal lattice defects, impurities and interface state densities near the gate insulator/silicon interface in the MOS device of FIG. 7 are smaller. Therefore, the characterization of temporal electroluminescent signals from MOS devices can be an effective method to investigate the minority carrier lifetime near the gate insulator/silicon interface of MOS devices with ultra-thin gate insulator.  
         [0034]    While the invention has been described by way of an example and in terms of a preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. To the contrary, it is intended to cover various modifications. For instance, the filter may be a high-pass filter or a low-pass filter. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.