Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1998-02-25
2001-01-30
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S537000
Reexamination Certificate
active
06181144
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor manufacture and specifically to a probe card for testing semiconductor wafers. This invention also relates to a method for fabricating the probe card, to a method for testing using the probe card, and to a test system employing the probe card.
BACKGROUND OF THE INVENTION
Semiconductor wafers are tested prior to singulation into individual dice, to assess the electrical characteristics of the integrated circuits contained on the dice. A typical wafer-level test system includes a wafer handler for handling and positioning the wafers, a test controller for generating test signals, and a probe card for making temporary electrical connections with the wafer. In addition, a performance board associated with the probe card contains driver circuitry for transmitting the test signals to the probe card.
The test signals can include specific combinations of voltages and currents transmitted through the performance board and probe card to the wafer. During the test procedure response signals such as voltage, current and frequency can be analyzed and compared by the test controller to required values. The integrated circuits that do not meet specification can be marked or mapped in software. Following testing, defective circuits can be repaired by actuating fuses (or anti-fuses) to inactivate the defective circuitry and substitute redundant circuitry.
Different types of probe cards have been developed for probe testing semiconductor wafers. The most common type of probe card includes elongated needle probes adapted to electrically engage corresponding contacts on the wafer. An exemplary probe card having needle probes is described in U.S. Pat. No. 4,563,640 to Hasegawa et al. Another type of probe card includes buckle beam probes adapted to flex upon contact with the wafer. This type of probe card is described in U.S. Pat. No. 4,027,935 to Byrnes et al. Yet another type of probe card, referred to as a “membrane probe card”, includes a membrane, such as polyimide, having contacts in the form of contact bumps thereon. An exemplary membrane probe card is described in U.S. Pat. No. 4,918,383 to Huff et al. Still another type of conventional probe card includes a silicon substrate and probe tips that have been micro machined and covered with a conductive layer. Such a probe card is described in U.S. Pat. No. 5,177,439 to Liu et al.
With any of the above types of probe cards, contacts on the probe card (e.g., probe needles, contact bumps, probe tips) must electrically engage contacts on the wafer (e.g., test pads, bond pads). A problem with making these electrical connections is that the electrical resistivity of the probe contacts can increase with continuous use of the probe card. Probe cards are designed to be used over extended periods of time with periodic cleaning and adjustments. However, a single probe card may test thousands of wafers prior to being cleaned and adjusted.
With extended use, the probe contacts can become covered with contaminants, such as particles, and residual photoresist from wafer fabrication processes. These contaminants tend to increase the electrical resistivity of the electrical connections between the probe contacts and the wafer contacts. The increased resistivity can increase the time required for test signals to be transmitted and received from the wafer. Also voltage and current values of the test signals can be adversely affected by the increased resistivity. The wafer contacts (e.g., aluminum bond pads) can also include layers that may affect the resistivity of the temporary electrical connections with the probe card, and the test signals transmitted through these connections.
In addition to contaminants which may affect resistivity, the metallic surfaces of the probe contacts and wafer contacts will typically be covered with a film of some type. Base metals such as copper, aluminum and nickel include a surface film comprising a metal oxide. This surface film can be up to several hundred Angstroms thick. Noble metals such as gold, can also include adsorbed gases, water vapor and organic molecules. The films are electrically insulative and can interfere with the free flow of electrons between the mating contacts.
It would be advantageous to be able to evaluate the electrical resistivity of the probe contacts and of the wafer contacts. In addition, it would be advantageous to be able to evaluate the contact resistance between the probe contacts and wafer contacts during a test procedure. These resistivity measurements could then be used during transmission and evaluation of test signals. This information could also be used to indicate cleaning of a probe card is required.
In view of the foregoing, the present invention is directed to an improved probe card that includes resistivity measuring circuitry, and to a method for fabricating the probe card.
SUMMARY OF THE INVENTION
In accordance with the present invention, an improved probe card for testing semiconductor wafers, a test system employing the probe card, a method for testing using the probe card, and a method for fabricating the probe card are provided. The probe card includes a substrate formed of a rigid material, such as silicon or ceramic, and a pattern of probe contacts formed on the substrate. The probe contacts are configured to electrically engage wafer contacts contained on a wafer under test.
Some of the probe contacts comprise resistivity contacts, which are configured to electrically engage selected wafer contacts. Preferably, the selected wafer contacts comprise interconnected power (Vcc), or ground (Vss) pads, of semiconductor dice contained on the wafer. With the resistivity contacts in electrical engagement with the selected wafer contacts an electrical path between the resistivity contacts is provided.
The resistivity contacts are adapted for use with a resistivity measuring circuit. The resistivity measuring circuit evaluates a total resistance R
x
of the electrical path between the resistivity contacts. A high value for R
x
can indicate a high contact resistance between the probe contacts and the wafer contacts, such as would occur with misaligned or damaged contacts. A high value for R
x
can also indicate high resistivity in the probe contacts or wafer contacts, such as would occur with thick metal oxides, or contaminants.
The resistivity contacts include two impedance sense leads, and two impedance source leads configured as a four point Kelvin structure. With this arrangement, a test current can be applied through a known resistance RL to the resistivity contacts. In addition, a sense current can be applied through known resistances to the resistivity contacts. The sense current is very low (e.g., pico-amps) such that the I-R drop is low, and the voltage seen by the sense terminals is the same as the voltage developed across the resistivity contacts. This enables a total resistance R
x
of the electrical path between the resistivity contacts to be quantified. The resistance R
x
can be used to provide feed back for adjusting test signal voltages and currents. The resistance R
x
can also be used to indicate that the probe card (or the wafer) requires cleaning.
Several different embodiments of the probe card are provided. In a first embodiment, the probe contacts comprise etched projections covered with a conductive layer. In a second embodiment, the probe contacts comprise microbumps formed on an electrically insulating polymer layer. In a third embodiment, the probe contacts comprise indentations covered with conductive layers, and configured to electrically engage bumped wafer contacts (e.g., solder balls). In a fourth embodiment, the probe contacts comprise microbumps deposited in openings in an elastomeric mask layer.
A system constructed in accordance with the invention includes the probe card, a wafer handler and a tester. The tester includes the resistivity measuring circuitry and test circuitry. The wafer handler includes a wafer chuck for moving the wafer in x and y directions for aligning the waf
Akram Salman
Hembree David R.
Gratton Stephen A.
Hollington Jermele M.
Metjahic Safet
Micro)n Technology, Inc.
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