JPS6339099B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention scans a laser beam, focuses this scanning beam to a fine spot diameter using an optical lens system, and irradiates it onto a sample semiconductor device, thereby emitting a signal associated with a light absorption current from this sample semiconductor device. In a laser scanning microscope device that detects and observes light absorption current images, while operating a sample semiconductor device,
The present invention relates to a laser scanning microscope device capable of measuring dynamic characteristics of semiconductor devices that can measure characteristics at each phase.

In recent years, semiconductor integrated circuits have been moving toward higher integration density and larger capacity due to rapid advances in microfabrication technology. As a result, it has become difficult to test large-scale ICs, analyze failures, etc. using only the input/output pins of integrated circuits (hereinafter referred to as ICs) due to the huge amount of test patterns. Even direct current testing is fraught with such difficulties;
Dynamic tests that take into consideration the current situation and timing are nearly impossible due to time constraints. On the other hand, I.C.
The method of mechanically applying a probe to the pattern of the circuit of interest is becoming more difficult as the patterns become finer due to higher integration densities.

Conventionally, SEMs (scanning electron microscopes) and laser scanning microscopes have been used as non-contact inspection and analysis devices for ICs. Since it is necessary to pull the method, there are major disadvantages in that the device becomes large-scale and the operation time becomes long. Also,
Since the sample is in a vacuum, the state is different from the actual semiconductor surface, and the charge-up phenomenon caused by electron beam irradiation changes the surface state, making it impossible to observe the phenomenon under actual usage conditions. There is also a drawback.

Next, semiconductors in use can be easily exposed to the atmosphere.
The conventional measurement method for laser scanning microscopes that can observe IC surface phenomena is to change the input conditions of the IC to focus on the operating state of the circuit, and then measure the light absorption current image at that time. Although it is possible to measure the IC's internal circuit in a static state without contact, the power supply current generally changes when the IC is operated. The disadvantage of this method was that it was not possible to measure the dynamic characteristics of the IC. In other words, it is impossible to measure dynamically operating ICs such as dynamic MOS memories that require refresh operations using conventional laser scanning microscope equipment.

FIG. 1 shows a block diagram of the configuration of a conventional laser scanning microscope device. A laser beam emitted from a laser light source 1 is transmitted through an X-axis scanning drive device 2 and a Y-axis scanning drive device 3 to After being raster scanned in 3D, a few μ
Semiconductor devices (IC,
The surface of the sample 5 (such as a transistor) is irradiated while being scanned two-dimensionally. The power supply current to sample 5 is
Since the light is supplied through the series resistor 7, the light absorption current of the semiconductor generated by light irradiation is
Detected as a voltage change on the IC's power pin. This detection signal is amplified by an amplifier 9 provided in a control section 8 of the laser scanning microscope device, further modulated by a brightness modulation circuit 10, and applied to a monitoring oscilloscope 14 as a Z signal. The X-axis scanning of the laser beam is detected by the photodetector 16, and in synchronization with it, the monitoring oscilloscope 1
The X signal for electron beam scanning No. 4 is generated by an X signal generation circuit 11, and the Y signal is generated by a Y signal generation circuit 12. The Y signal is also applied to the laser beam Y-axis scanning drive device 3. Therefore, a two-dimensional brightness modulation image of the light absorption current from the sample in the laser scanning area appears on the monitoring oscilloscope 14. In addition, by mixing the light absorption current signal obtained from the sample 5 and the Y signal of the monitoring oscilloscope 14 in the mixing circuit 13,
A dimensional contour image can be displayed. The display image is changed over by the display image changeover switch 15, and when the switch is set to A, a brightness modulated image appears on the motor oscilloscope 14, and when it is set to B, a contour line display image appears on the motor oscilloscope 14. Input signal generation circuit 17, sample (IC) 5
By observing the optical absorption current image when changing the input conditions and changing the operating state, you can measure the internal characteristics of the IC under each static circuit state by detecting the difference in modulation degree. Ru. As described above, in the conventional laser scanning microscope apparatus, although it is possible to measure the internal characteristics of an IC in a static circuit state, it is not possible to measure the dynamic characteristics of the IC.

The present invention has been made in view of the above-mentioned circumstances, and is achieved by simply adding a simple circuit to the light absorption current measurement circuit of a conventional laser scanning microscope device, specifically, the operation of a drive circuit that controls the operation of a sample semiconductor device. 1/N of frequency (N is an integer, N≧1)
By providing a sample and hold circuit synchronized at the frequency of It is an object of the present invention to provide a laser scanning microscope device that can observe and measure the dynamic characteristics of a semiconductor device in the atmosphere and without contact.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows an embodiment of this invention.
In the figure, the same parts as those in FIG. The structure shown in FIG. 2 is particularly different from the structure shown in FIG. a timing signal generation circuit 18 that obtains various timing signals; a pattern generation circuit 19 that obtains signals for operating a sample semiconductor device (hereinafter simply referred to as a sample) based on timing pulses obtained from the timing signal generation circuit 18; The above timing signal generation circuit 1
A variable sampling pulse generation circuit 20 obtains a desired sampling pulse based on the timing pulse obtained from the sample 5, and an alternating current (AC ) component, an impedance conversion circuit 21 that converts the impedance of the signal extracted by the coupling capacitor 24, a level shift circuit 22 that corrects the level of the signal obtained from the impedance conversion circuit 21, and A sample hold circuit 23 is provided which samples the signal obtained from the level shift circuit 22 in synchronization with the sampling pulse obtained from the variable sampling pulse generation circuit 20.
The point is that the configuration is such that the signal is supplied to the amplifier 9.

The effect will be explained here. The operations of each of the parts 1 to 16 shown in FIG. 2 are the same as in the case of FIG. 1 described above, so the explanation thereof will be omitted here.
The basic clock pulses generated by the clock signal generation circuit 25 are applied to the timing signal generation circuit 18 to produce timing pulses for the pattern generation circuit 19 and timing pulses for the variable sampling pulse generation circuit 20.

In general, the power supply current of a semiconductor IC varies depending on the circuit operating state according to the input conditions, so
During dynamic operation of the IC, the power supply current value changes with the clock frequency. However, since the measurement of the light absorption current by laser beam irradiation on the sample (IC) 5 is performed by detecting changes in the power supply current, during dynamic operation of the IC, the power supply current due to changes in the circuit state Since the change in the light absorption current is superimposed on the change, the light absorption current image cannot be observed simply by continuously detecting and amplifying the change in the power supply current. Therefore, changes in the power supply current including the light absorption current are converted into voltage changes by the series resistor 7, and the coupling capacitor 24
Only the alternating current component is taken out, passed through an impedance conversion circuit 21, inputted into a level shift circuit 22, the level of which is adjusted, and added to a sample hold circuit 23. The sampling pulse at this time is generated by the variable sampling pulse generation circuit 20, and its frequency is 1/N of the clock pulse frequency (N is an integer,
N≧1) and synchronized. The output from the sample and hold circuit 23 is input to the amplifier 9 and amplified, then subjected to brightness modulation or contour modulation and displayed as a light absorption current image on the monitoring oscilloscope 14. Here, the sampling pulse is the period of the sampling pulse, i.e.
Since it is variable within the range of NX (clock period) (N is an integer, N≧1), the circuit state can be changed at various phases within the sampling period during IC operation, as if the circuit state were stationary. Absorption current images can be observed.

Next, the relationship between the X-axis and Y-axis scanning frequencies on the monitoring oscilloscope 14 and the operating frequency of the sample 5 will be explained. FIG. 3 is a part of a connection diagram of a simple sample (IC) for explaining the sample and hold operation of the light absorption current, and the power supply current to the sample 5 is carried out from the power supply 6 through the series resistor 7. It is. Therefore, changes in the power supply current including the light absorption current are converted into voltage changes, and only the alternating current (AC) component is transmitted to the sample and hold circuit by the coupling capacitor 24. I ₁ to I _N are the signals applied to the input terminals, C _P is the clock pulse,
QA and QB are output signals.

A timing diagram corresponding to FIG. 3 is shown in FIG. The power supply current i _cc shown in Figure 4 d shows different current values due to differences in the circuit operating state according to the input conditions including the clock, so the sampling pulse S _P shown in Figure 4 e shows different current values when the circuit operating state is the same. It must be output with a period T _S such that Therefore,
In FIG. 4, if the period T _C of the clock pulse C _P shown in FIG _. 4A is, the period _{T S} of the sampling pulse S _P shown in FIG. In other words,
When the clock period T _C is set to 1, the repetition coefficient N for achieving the same circuit operation state is 4. Furthermore, the third
Input signals I ₁ to I _N of sample 5 in the figure are all assumed to remain unchanged for the sake of simplicity, but when they change as in normal operation, the number of combinations of input signals increases, so Generally, the repetition factor N for reaching the circuit state is greater than 4. Here, T _C is the clock period of sample 5, _C is the clock frequency, T _S is the sampling period, _S is the sampling frequency, N is the repetition coefficient to achieve the same circuit state for T _C , and is the X-axis scanning frequency, that is, the line frequency.
_L , the Y-axis scanning frequency, that is, the frame frequency, is _F , and the display image according to the present invention is a type of sampling image, but the sample point per X line is m, and the number of X lines per frame is number
Assuming N _L , the following relational expression holds true.

T _S = N × T _C Therefore, _C = N × _S ………(1) _S = m × _L ………(2) _L = N _L × _F ………(3) (1), (2) , (3), _C = N × m × _L ………(4) N × m × N _L × _F ………(5) In general, if the number of sample points m per X line is at least 256, the resolution is sufficient. From this point of view, I am satisfied.
Assuming a case where the number N _L of X lines per frame is 256, the lower limit of the frame frequency _F for visually observing a light absorption current image when using an oscilloscope with some afterglow is It is around 2Hz. Therefore, the relationship between the repetition coefficient N for achieving the same circuit state for T _C and the lowest value of the operating frequency of the sample IC, that is, the clock frequency _C , is expressed by the above equation (5).
Therefore, _C min = N x m x N _L x _F = N x 256 x 256 x 2 = N x 1.31 x 10 ⁵ (Hz). Therefore, when N becomes large, that is,
As the IC becomes larger and the number of combinations of input signals increases, the minimum clock frequency _Cmin must be raised, but there are cases where it exceeds the upper limit of the IC operating frequency. In such a case, it is necessary to devise an input signal pattern that makes N small, but on the other hand, if a memory oscilloscope is used as the monitor oscilloscope 14, it is possible to adjust the input signal pattern according to the upper limit of the operating frequency of the sample IC. Therefore, the frame frequency _F can be sufficiently lowered. Also, instead of a memory oscilloscope, a memory device is used to temporarily store the optical absorption current at a sampling frequency _S that is 1/N of the clock frequency _C of the sample IC (N is an integer N≧1). There is also a method of reading out the image at a frame frequency _F necessary for visual observation and displaying it on the monitor oscilloscope 14. A block diagram of the configuration of this embodiment is shown in FIG.
8A in the figure is a laser scanning microscope device equipped with a sample hold system circuit according to the present invention,
X, Y, Z analog signals are output. The X signal and Y signal corresponding to the address are sent to the A/D converter 5.
1 and an A/D converter 52, and is temporarily held in a data register 54 through a switching gate 53 operated by a sampling pulse S _P of a sample and hold circuit. Z
The signal is also converted into a digital signal by an A/D converter 55 and applied to a memory device 56 via a switching gate 53 and a data register 54. The memory device 56 operates at the sampling pulse frequency _S , and the data register 54 X,
The Z output of the data register 54 is written to the address determined by Y. Writing (WRITE) and reading (READ) to and from the memory device 56 are controlled by an R/W controller 57. When the R/W controller 57 is set to read mode, the contents of the address determined by XY of the data register 54 are read from the memory device 56, and the Z signal converted to an analog signal by the D/A converter 58 is sent to the monitoring oscilloscope. In addition to the Z terminal of 14, the X signal from the laser scanning microscope device 8A,
With the Y signal, a light absorption current image is displayed on the monitoring oscilloscope 14. Since the readout operation is proportional to the frame frequency _F of the monitoring oscilloscope 14, it is preferable that the readout operation be as fast as possible.
Therefore, the clock frequency controller 59 switches between the high speed clock pulse H _P during reading and the sampling pulse S _P of the sample and hold circuit during writing, and the control signal is sent from the R/W controller 57.
By using the memory device 56, when the phase of the sampling pulse S _P is changed within the sampling period T _S , it is possible to store all the light absorption current images corresponding to each phase, and also to store other sample IC ambient conditions. When various changes are made, optical absorption current images corresponding to each change can be stored. This operation is performed by a condition selection circuit 60, and in response to a signal from this circuit 60, the light absorption current images under each condition are distinguished and all stored in the memory device 56. By using this function, you can easily read out two light absorption current images under different conditions at the time of readout, automatically compare them using hardware, and display the resulting images on the monitor oscilloscope 14. Its uses are very wide.

In the embodiment shown in FIG. 2, the case where the relationship between the scanning of the laser beam and the operation of the sample IC is asynchronous is explained.
If a digital device such as a pulse motor is used for the axis scanning drive device 3, laser beam scanning and sample
Synchronization with IC operation can be achieved. In this way, the corresponding sampling points on the surface of the sample 5 will be fixed, and more detailed and accurate data can be obtained than by randomly sampling the surface of the sample 5 asynchronously.

Regarding the embodiment shown in FIG. 2, the laser light source 1
Due to the structure of semiconductor devices (ICs, transistors, etc.), He-Ne lasers with a wavelength of 6328 Å are generally used, and most of the light beam is absorbed at a depth of 1 to 2 μm from the surface of typical silicon semiconductor materials. I end up. However, the laser light source for the laser scanning microscope device of the present invention is not limited to He--Ne lasers, but may be lasers with any wavelength from short wavelengths in the ultraviolet region to long wavelengths in the infrared region. The wavelength of the laser may be selected depending on the state of the measurement location of the sample IC and the type of measurement to be performed. A short wavelength laser is used to measure characteristics in a shallow area, and a long wavelength laser is used to measure characteristics in a deep area. Another feature of this method is that by using a long-wavelength laser, it is possible to perform laser scanning from the back side of a silicon wafer and measure the optical absorption current image.

Furthermore, although the laser light source 1 is described as a continuous wave (CW) type in the embodiment shown in FIG. 2, it is not limited to this, and may be a pulse modulated laser light source. If the operation of the sample IC is synchronized with laser scanning and laser pulse modulation, measurement of diffusion distance, diffusion rate, etc.
It has the feature that it can be done while operating. In addition, in the embodiment shown in Fig. 2, the light absorption current is measured by detecting the power supply current of the sample IC using the power supply pin, but it is not limited to the power supply pin; Measurements may be made on any of all pins, or multiple pins may be measured simultaneously.

As is clear from the above explanation, the sample IC is operated by sampling and holding the optical absorption current at a frequency that is 1/N (N is an integer, N≧1) of the operating frequency of the sample semiconductor device. The light absorption current can be measured with , and the phase of the sampling pulse can be varied within the sampling period T _S to observe the light absorption current image at each phase. Characteristics of IC internal circuits can be measured from these optical absorption current images.
In other words, the circuit state of an operating semiconductor device can be measured non-contact in the atmosphere with the phase of the sampling pulse stopped, so non-contact measurement of the logic level at each phase while the semiconductor device is operating is It is also possible to analyze and measure transient phenomena in the internal circuits of semiconductor devices. For example, phenomena that operate normally at low speeds but abnormally operate only at high speeds can be analyzed in the atmosphere without contact.

In addition, semiconductor devices such as ICs may have combinations of input patterns that are prone to errors, that is, pattern sensitivity, but this analysis cannot be done from the conventional measurement of static optical absorption current images. Although analysis is not possible, if the measurement method of the present invention is used, the circuit state inside the semiconductor device can be observed for each phase while performing dynamic operation in a pattern that is prone to errors, making it possible to analyze pattern sensitivity. Become. In addition, as highly integrated and high-density semiconductor devices become available, the amount of heat generated increases and hot spots may occur.In such cases, changing the duty ratio of semiconductor device operation can improve semiconductor device operation. It is possible to measure changes in characteristics due to heat generation over time, and furthermore, it is possible to investigate the influence of hot spots on semiconductor device operating characteristics. Furthermore, for dynamic operation ICs such as dynamic MOS memories that require refresh operation as sample semiconductor devices, it is necessary to measure the characteristics while operating, but the laser scanning microscope device of the present invention allows measurement of dynamic characteristics. This type of property can be measured because of the In the future, the ability to measure dynamic characteristics in the atmosphere and without contact will be very effective in the inspection and failure analysis of ICs with high integration density and large capacity.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the configuration of a conventional laser scanning microscope, Fig. 2 is a block diagram showing an embodiment of the present invention, and Fig. 3 is a simple sample for explaining the sample hold operation in the above embodiment. FIG. 4 is a timing diagram corresponding to the operation of the sample IC shown in FIG. 3, and FIG. 5 is a block diagram showing another embodiment to which the present invention is applied. DESCRIPTION OF SYMBOLS 1... Laser light source, 2... X-axis scanning drive device, 3... Y-axis scanning drive device, 4... Optical microscope, 5... Sample semiconductor device, 6... Power supply, 7
...Series resistance, 8...Control section, 14...
Monitoring oscilloscope, 18... Timing signal generation circuit, 19... Pattern generation circuit, 2
0...Variable sampling pulse generation circuit, 21...
... Impedance conversion circuit, 22 ... Level shift circuit, 23 ... Sample hold circuit, 24 ...
...Coupling capacitor.

Claims (1)

[Claims] 1 Scan a laser beam, focus this scanning beam to a fine spot diameter using an optical lens system, and irradiate the sample semiconductor device, detect the signal accompanying the light absorption current from this sample semiconductor device, and observe the light absorption current image. In the laser scanning microscope apparatus, a detection circuit extracts the light absorption current as a voltage change signal, and a 1/N detection circuit synchronizes the signal obtained from the detection circuit with the operation of the sample semiconductor device.
(N is an integer, N≧1) A laser scanning microscope apparatus comprising: a sample hold circuit that performs sampling using a sampling pulse having a frequency of (N is an integer, N≧1), and a light absorption current image is obtained from an output signal of the sample hold circuit.