1. Field of the Invention
The present invention relates to a technique of testing an electric circuit in its operating state. More particularly, it relates to a constitution of an apparatus or system which uses a novel probe suitable for effecting diagnosis and analysis of the operation of an integrated circuit such as an LSI including minute wirings and effects a voltage measurement to thereby be adapted for testing the integrated circuit.
In development and manufacture of semiconductor integrated circuits (LSIs), it is essential to test a semiconductor element and, where a malfunction of the element is found, check the cause (i.e., effect a fault analysis). With a recent high degree of integration of LSIs and a multiplication of input/output (I/O) pins, however, it has been difficult to effect an exact design verification and a fault analysis only by measuring signals at the I/O pins by means of an LSI tester or the like. Accordingly, it becomes necessary to effect a voltage measurement of a minute wiring in the LSI, an internal diagnosis and an analysis such as a measurement of operational waveforms. For example, there is known a method of contacting a probe having a pointed end portion directly to a measurement point, amplifying a signal detected by the probe and measuring the signal by means of an oscilloscope or the like. The method is simple and a basic approach. However, it is very difficult to measure a voltage at a much minuter internal wiring or internal electrode than the size of the probe. Also, the precision of measurement is not sufficient and there is a possibility of a secondary trouble such as a short circuit between wirings. Thus, the approach possesses a limitation in itself. In view of this, it is demanded to develop a novel measurement method or approach.
2. Description of the Related Art
As an apparatus suitable for a wiring voltage measurement of a minute pattern, there is known an apparatus using an electron beam or a light beam.
In an apparatus using an electron beam, the wiring voltage measurement is carried out by applying the electron beam to a minute wiring (measurement point) in a semiconductor integrated circuit and detecting the amount of secondary electrons emitted from the measurement point. Namely, the measurement using an electron beam utilizes the fact that the amount of secondary electrons to be detected correlates with the voltage at the measurement point.
On the other hand, as an apparatus using a light beam, for example, an apparatus utilizing an electro-optic effect is known. In the apparatus, the wiring voltage measurement is carried out by applying the light beam to a certain crystal arranged near the measurement point and detecting the amount of polarization of a light beam transmitted through or reflected from the crystal. Namely, the measurement using a light beam utilizes the fact that, when an external electric field (i.e., wiring voltage) is applied to a crystal, the refractive index of the crystal is changed. The principle of the electro-optic effect is disclosed, for example, in the publication: Valdmanis J. A., Electron. Lett. 23, 1308-1310, 1987. In this connection, the measurement of a change of a short time (e.g., below 1 [ns]) in voltage is carried out by means of a so-called sampling method which effects a voltage measurement by a pulsed beam, not a continuous beam.
Also, as another apparatus using a light beam, an apparatus utilizing a photoconductive gate is known. A photoconductive gate has characteristics in which the conductive index is very small (gate OFF state) when light is not applied thereto, and in which the conductive index is increased for a short time (gate ON state) when light is applied thereto. Therefore, it is possible to measure a voltage applied to the gate, i.e., wiring voltage by detecting a current in the photoconductive gate by means of the above characteristics.
According to the above apparatus using an electron beam, it becomes necessary to make the cross section of the electron beam small in accordance with the size of a minute measurement point. In this case, since the number of electrons contained in the beam is decreased and accordingly the number of secondary electrons is also decreased, a problem occurs in that the signal to noise (S/N) ratio is deteriorated. To cope with this, it is effective to increase the number of shots of a pulsed electron beam on a measurement point. In this case, however, another problem occurs in that time required for the measurement is prolonged. Also, the like problem is posed where a time width of an electron beam pulse is shortened for the purpose of an improvement in "time resolution" of the measurement. In this connection, considering the transit time effect of second electrons (fluctuation of the measurement timing occurring due to a slow transit speed of second electrons), an upper limit of the time resolution is approximately 5 [ps] at present and thus the realization of a higher time resolution than that value is very difficult in principle.
On the other hand, according to the apparatus using a light beam utilizing an electro-optic effect, a very high time resolution exceeding 0.5 [ps] is realized and a voltage resolution corresponding to the S/N ratio is also excellent compared with the above apparatus using an electron beam. However, since the space. resolution is determined depending on the wavelength of light, a problem occurs in that it is very difficult to measure a wiring voltage of a very minute pattern. In this connection, although it is at present possible to effect a voltage measurement with respect to a minute pattern of approximately 1 .mu.m or more, it is very difficult to effect a voltage measurement with respect to a minuter pattern than that value. Namely, the apparatus using an electron beam is excellent in respect of "space resolution" although it is insufficient in respect of "measurement time" and "time resolution", while the apparatus using a light beam is excellent in respect of "measurement time" and "time resolution" although it is insufficient in respect of "space resolution". Namely, both types have advantages and defects contrary to each other.
Also, according to the apparatus using a light beam utilizing a photoconductive gate, it is possible to obtain a voltage measurement sensitivity ten times or more compared with the apparatus using an electro-optic effect. However, a few problems are posed. For example, where a photoconductive gate is formed by applying a radiation damage to an SOS (Silicon on Sapphier), there is a disadvantage in that the photoconductive gate must be formed in advance on a LSI chip under test. Although there is a case that a photoconductive gate need not be formed in advance on a LSI chip under test, in this case, a problem occurs in that it is impossible to effect a probing to a minute wiring because the size of an electrode pad to be electrically contacted is relatively large. Also, even if the size of the contact electrode can be made small, it is very difficult to effect a precise probing to the contact electrode with respect to a minute wiring of submicron width.