1. Field of the Invention
The present invention relates to a probe system for testing the electrical characteristics of a substrate, e.g., a semiconductor wafer or an LCD substrate, while electrically connecting probes to the electrode pads of the substrate.
2. Description of the Related Art
In a probe system, a technique which has a first image pickup means for picking up the image of the distal ends of probes from the wafer side and a second image pickup means for picking up the wafer image from the probe side and which aligns the wafer and the probes by using the first and second image pickup means is described in, e.g., Jpn. Pat. Appln. KOKAI Publication Nos. 1-94631 and 1-119036.
In the conventional probe system, the mounting position of a probe card and the position of an XYZ stage in alignment of a wafer must have absolutely high precision. For example, a position for picking up the image of the wafer and a position for actually testing the substrate are rather separated from each other, and the positional relationship between them is guaranteed only by the XYZ stage.
However, when probes are to be aligned with the electrode pads of a wafer, even when the linearity, the flatness, the orthogonality, and the like of the rails for guiding the XYZ stage are set at high precision, when the temperature changes, a thermal deformation occurs to cause an error. When the temperature changes by, e.g., 10.degree. C., an error of 20 to 30 microns per 100 mm occurs by the thermal expansion and contraction of a ball screw that drives the XYZ stage. An error on the order of microns is also caused by a temperature change in the positional relationship between the probe card connected to the head plate and the image pickup means for picking up the image of the wafer.
Ordinarily, the probe card is exchanged in accordance with the type of the substrate. Every time the probe card is exchanged, its position is displaced due to the gap between the inner diameter of the hollow portion of the head plate and the outer diameter of the probe card. This error also makes it difficult to perform accurate alignment of the probes and the electrode pads of the wafer.
In a conventional typical probe system, an alignment region for picking up the image of the wafer and a probing region under a probe card used for testing the wafer are separated from each other. In this case, the data on the wafer obtained in the alignment region is transferred to the probing region and used. Accordingly, if the state of the three-dimensional coordinate system of the moving mechanism of the table in the alignment region and that in the probing region are the same, the wafer moves in the probing region in the same manner as in the alignment region. In fact, however, the two states differ due to the following reason, thus causing an error in movement of the wafer (movement of IC chips).
More specifically, in the probe system, a probe card having probes aligned to correspond to the electrode pad array of an IC chip on a wafer W is arranged above a wafer table which is movable in the X, Y, Z, and 8 directions. The electrode pads of the IC chip are brought into contact with the probes, and the electrical characteristics of the chip are tested by a test head through a contact ring.
In order to perform an accurate electrical test, the probes must be surely brought into contact with the electrode pads. For this purpose, the table must be controlled at high precision, and the electrode pads must be correctly aligned with the probes before measurements. As a large number of circuit components and wires are incorporated in the test head, a wafer image pickup means is arranged at a position remote from the test head, and a portion under this image pickup means is defined as an alignment region for wafer alignment.
The relative positions of the probing and alignment regions with respect to each other, the driving amount of the ball screw, and the like are set such that alignment of the probes and the electrode pads of the chip in the probing region is automatically performed. When the relative positions of the electrode pads and probes are aligned with respect to each other, as the moving distance is already obtained in advance in the alignment region, all the electrode pads are correctly brought into contact with the corresponding probes by moving the table based on the moving distance data.
The equality of the postures and positions of the wafer in the three-dimensional axial directions in the probing and alignment regions is determined by the machining precision of the ball screw between the probing and alignment regions and the influences of yawing (right-to-left shift on the X-Y plane), pitching (to-and-from inclination), and rolling (inclination about the moving axis) of the guides. The equality is also influenced by the expansion and contraction of the ball screw caused by an environmental temperature during the measurement. In addition, it is also influenced by the thermal expansion caused by the frictional heat of the ball screw which is generated when the wafer moves from the alignment region to the probing region. As a result, when the wafer moves from the alignment region to the probing region, the wafer has a posture different from the original posture, e.g., the wafer slightly shifts to the right or left or is inclined forward or backward.
Therefore, the moving pattern of the wafer in the probing region is not completely the same as that anticipated in the alignment region. The operation in the alignment region is based on the premise that the relative positions of the wafer image pickup means and probe card are prefixed with respect to each other. However, the positional relationship between the probe card mounted on the head plate and the image pickup means slightly changes in accordance with a temperature change. Also, a positional error of the probe card occurs, although slight, due to the gap between the probe card mounting portion and the probe card in accordance with the type of the wafer.
Although the errors described above are not very large, they pose a problem as the integration degree of the device increases, like in a transition of the DRAM to 32M and further to 64M, as the size of the electrode pad decreases, and as the number of electrode pads increases. When such an error in position of the wafer (an error in position of an IC chip) occurs, it becomes difficult to correctly bring the probes into contact with all the chips on the wafer, so that high-precision electrical testing cannot be performed.