JPH036465A - Non-contact type voltage measuring apparatus using laser beam - Google Patents

Abstract

PURPOSE:To accurately measure a voltage waveform in a non-contact manner by calculating the voltage change of the IC electrode formed on a wafer as the intensity change of laser beam changed by Stark effect using laser beam. CONSTITUTION:The beam of a laser beam source 1 stabilized to specific frequency passes through a mirror 2, a beam splitter 3, a convex lens 4 and a microcell 5 to be condensed to the electrode of the IC 6 on a wafer. The reflected beam from said electrode again passes through the microcell 5 and the lens 4 to be reflected by the splitter 3 and this reflected beam is converted to an electric signal by a photodetector 10 and said signal is subjected to synchronous detection along with the reference signal of an oscillator 8 by a lock-in amplifier 11. Herein, when the voltage of the electrode on the IC 6 changes, the light absorbing frequency of the gas in the microcell 5 is changed by Stark effect and the intensity of the detected beam changes and the output of the amplifier 11 changes. This change of the intensity of beam is synchronously detected and can be detected with high sensitivity in a reduced noise level. Therefore, the voltage of a measuring electrode is measured on the basis of the detection of the amplifier 11.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an integrated circuit (IC,
The present invention relates to a device that non-contactly measures the voltage applied to the electrodes of an LSI.

(Prior art and its problems) Conventionally, a device called a "wafer probing device" or "prober" is used to measure the voltage applied to the electrodes of integrated circuits (hereinafter simply referred to as ICs) formed on a wafer. has been done. In principle, these devices bring a thin metal needle made of tungsten or the like into direct contact with a desired electrode of an IC formed on a wafer, thereby determining the voltage at the contact point.

In recent years, increasing the density of ICs has become an important research topic, and active research and development is underway. Accordingly, the electrode width of IC is several micrometers.
The size has been miniaturized from m to sub-μm. On the other hand, in conventional devices for evaluating such high-density ICs that measure voltage by directly contacting metal needles, the metal needles place a capacitive load on the measurement electrode. Therefore, there was a problem in that accurate voltage waveforms could not be measured in the high frequency range. Furthermore, since it is difficult to reliably contact the metal-side miniaturized electrode, there is a problem that measurement and evaluation take time and the electrode may be damaged.

(Means for Solving the Problems) An object of the present invention is to solve the above-mentioned problems, to detect changes in light absorption intensity with a good S/N ratio, and to detect changes in light absorption intensity without applying a capacitive load even in a high frequency region. Measure accurate voltage by touch,
An object of the present invention is to provide a device that can shorten IC evaluation time. In order to achieve the above object, in the present invention, when atoms and molecules having an electric dipole moment are placed in an electric field, the bodential energy changes due to the well-known Stark effect, and the light absorbed by the atoms and molecules changes. This takes advantage of the effect of shifting the resonance frequency.

In addition, the present invention places a microcell filled with a gas having a Stark effect (for example, ammonia gas) near the measurement electrode of an IC formed on a wafer, and irradiates the microcell with a laser beam. It is characterized in that a lock-in amplifier detects changes in the absorption intensity of light that change depending on the voltage, and measures the voltage of the measurement electrode.

(Example) FIG. 1 shows an embodiment of the present invention.

1 is a laser light source, 2 is a mirror, 3 is a beam splitter that divides the laser beam into transmission and reflection, 4 is a convex lens for focusing, 5
6 is an IC formed on a wafer, 6 is a DC amplifier,
8 is an oscillator, 9 is a stage for moving the wafer, 10
is a photodetector, and 11 is a lock-in amplifier. The frequency of the laser light emitted from the laser light source 1 is the reference frequency, for example, the resonance frequency of an etalon resonator, or H2O, NH3.
Negative feedback control is applied to lock to the resonance absorption frequency of molecules such as. For example, in the case of a semiconductor laser, the drive current is stabilized at a specific frequency by controlling the drive current to be locked to the resonator length of the laser or the reference frequency in the case of a cassette laser. The frequency is roughly modulated by the signal of the oscillator 8 (reference signal). Frequency modulation is achieved by modulating the drive current of a semiconductor laser, or by using an electro-optic modulator, an acousto-optic modulator, or the like. still,
The oscillation frequency of this laser is selected to be near the optical absorption frequency of the gas within the microcell 5.

The light stabilized to a specific frequency in this way has its optical path changed by a mirror 2, then passes through a beam splitter 3 and a convex lens 4.
, passes through the microcell 5 and is focused on the electrode of the IC 6. The reflected light from this electrode passes through the microcell 5 and the convex lens 4 again, becomes parallel light, is reflected by the beam splitter 3, and enters the photodetector 10. The photodetector 10 converts an optical signal into an electrical signal using a photodiode, a mercury cadmium detector, or the like. The output signal of the photodetector 10 is synchronously detected with the reference signal of the oscillator 8 by the lock-in amplifier 11, and only the component synchronized with the reference signal can be detected.

As a result, the drift in the light intensity of the laser light source 1 can be removed and a signal with a good S/N ratio can be detected. Furthermore, since the detection signal is obtained by synchronously detecting the frequency-modulated laser beam, it is a primary component of the optical absorption intensity. A transparent thin film electrode that is conductive and does not transmit light is attached to the surface of the microcell 5 facing the convex lens 4, and a voltage based on the ground potential of the IC 6 is applied to this electrode by a DC amplifier 7. The voltage of the DC amplifier 7 is determined in balance with the oscillation frequency of the laser light source 1, so that the optical absorption frequency of the gas in the microcell 5 is adjusted to the position a in FIG. The position is set so that the change in the signal is proportional. Incidentally, FIG. 2 is a diagram showing the relationship between the output signal of the lock-in amplifier 1 and the voltage of the electrode of the IC 6.

Since the present invention has such a configuration, the light from the laser light source 1 stabilized at a specific frequency passes through the mirror 2, the beam splitter 3, the convex lens 4, and the microcell 5, and is focused onto the electrode of the IC 6. The light reflected from the microcell 5 passes through the convex lens 4 again, is reflected by the beam splitter 3, is converted into an electric signal by the photodetector 10, and is synchronously detected with the reference signal of the oscillator 8 by the lock-in amplifier 11. Here, if the voltage of the electrode on IC6 is constant, lock-in amplifier 1
The output of the lock-in amplifier 1 does not change, but when the voltage changes, the optical absorption frequency of the dregs in the microcell changes due to the Stark effect, the detected light intensity changes, and the output of the lock-in amplifier 1 changes. Changes in light intensity are detected synchronously, so signals can be detected with little noise, that is, with high sensitivity. When the gas pressure is low, the light absorption spectrum shape is represented by a Gaussian distribution curve, and near point a in FIG. 2, it changes in proportion to the voltage of the electrode of IC6. Therefore, it is possible to accurately detect the electrode voltage from the measured signal intensity by using the substantially straight line portion centered on point a. Furthermore, a more accurate voltage waveform can be measured by measuring the light intensity change in response to the voltage change in advance, creating a light intensity/voltage conversion table, importing the measured intensity change into a computer, and correcting it using the computer.

Although the shape of the microcell 5 is shown as a square in FIG. 1, it may have any shape.

In this way, changes in the voltage of the electrodes on the IC are detected using laser light, and changes in light intensity due to the Stark effect are determined by synchronous detection. Voltage waveforms can be measured with sensitivity.

FIG. 3 is a block diagram showing another embodiment of the present invention. In the example of the present invention, the voltage applied to the microcell 5 is modulated instead of modulating the laser light source 1 in the previous example. 12 is an adder, and the other configurations are the same as in the previous embodiment.

The light from the laser light source is stabilized at a specific frequency as in the embodiments described above. The stabilized light has its optical path changed by the mirror 2, passes through the beam splitter 3, the convex lens 4, and the microcell 5, and is focused onto the electrode of the IC 6. The reflected light from this electrode is turned into parallel light by the convex lens 4 again, reflected by the beam splitter 3, converted into an electric signal by the photodetector 10, and detected synchronously with the reference signal of the oscillator 8 by the lock-in amplifier 11. . Convex lens 4 of microcell 5
The voltage of the DC amplifier 7 and the voltage of the oscillator 8 are added by an adder 12 and applied to the twisted electrodes.

Therefore, the signal detected by the photodetector 10 is subjected to a Stark shift (Stark modulation) proportional to the reference signal. The voltage of the DC amplifier 7 is set to be at the position a in FIG. 2, as in the previous embodiment.

With this structure, the light from the laser light source 1 stabilized at a specific frequency passes through the mirror 2, beam splitter 3, convex lens 4, and microcell 5, and is focused on the electrode of the IC 6, and then The light reflected from the micro cell 5 and the convex lens 4 become parallel light again, and the beam splitter 3
The light is reflected by the photodetector 10 and converted into an electrical signal. The electrical signal is detected in lock-in amplifier 11 in synchronization with the reference signal of oscillator 8 . Here, if the voltage of the electrode on the ICB is constant, the output of the lock-in amplifier 11 will not change, or if the voltage of the electrode changes, the Stark effect will cause the microcell 5
The optical absorption frequency within the signal changes, and the output of the lock-in amplifier 11 changes. From this change, the voltage of the electrode on IC6 and the voltage waveform can be determined.

Roughly speaking, since changes in light intensity are synchronously detected, detection can be performed with high sensitivity as in the previous embodiment. In both embodiments, the laser beam passes through the space, but optical fibers are used between the laser light source 1 and the beam splitter 3, between the beam splitter 3 and the convex lens 4, and between the beam splitter 3 and the photodetector 10, and roughly the beam splitter 3 is replaced by optical fibers. A directional coupler using optical fibers may also be used.

Further, if the voltage waveform of the electrode of the IC 6 is a periodic signal, by averaging the output signal of the lock-in amplifier 11, a signal with less noise can be detected and an accurate voltage waveform can be measured.

(Effects of the Invention) As explained above, since the change in the voltage of the IC electrode formed on the wafer using laser light is determined from the change in the intensity of the laser light that changes due to the Stark effect, the voltage waveform is accurate without contact. has measurable benefits. Roughly speaking, changes in light absorption intensity can be detected with a high S/N ratio, and since no capacitive load is applied even in a high frequency region, accurate voltage measurement is possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG.
The figure shows the relationship between changes in electrode voltage and photodetection intensity.
The figure is a block diagram showing another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Laser light source, 2... Mirror, 3... Beam splitter, 4... Convex lens, 5... Microcell, 6... IC formed on wafer, ? . . . DC amplifier, 8 . . Oscillator, 9 . . . Moving stage, 10
...Photodetector, 11...Lock-in amplifier.

Claims (2)

[Claims] (1) In a device for measuring the voltage applied to the electrodes of an integrated circuit, a cell is equipped with a means for generating frequency-stabilized laser light, a substance having a Stark effect, and a transparent electrode on one side. a lock-in increaser for detecting a light absorption property of the material that varies in proportion to the voltage at the integrated circuit electrode; A non-contact voltage measuring device using a laser beam, characterized in that: (2) A non-contact voltage measuring device using laser light, wherein a modulated voltage is applied to the transparent electrode in the device according to claim 1.