Method and apparatus for resistance measurements on a semiconductor element

Abstract: A method for measuring the resistance or conductivity between two or more conductors which are placed against a semiconductor element, wherein in order to bring the contact resistance between the conductors and the element to and hold it at a predetermined value during measuring, the conductors are held at a constant distance and/or under constant pressure relative to the semiconductor element.

Description: A known technique for measurement on a semiconductor element is called Spreading Resistance Probe. A semiconductor element is herein cut obliquely and the resistance between in each case two probe points placed on the surface is measured. These probe points or conductors are placed at an interval of approximately 15 to 50 µm. The angle of inclination is in the region of several minutes to several degrees. The conductors are stepped over the obliquely cut portions with a step size in each case of 2.5 to 5 µm. In the SRP technique the resistance measurement can take place with both direct voltage and alternating voltage. A known technique is described in Solid State Technology, vol. 27, no. 12, December 1984, US, pages 125-132, R. Brennan et al "Determination of Diffusion Characteristics Using Two- and Four -point Probe Measurements", wherein a method is disclosed for measuring the resistance or conductivity between two or more conductors which are placed against a semiconductor element, wherein the conductors are held under constant pressure relative to the semiconductor element in order to bring the contact resistance between the conductors and the semiconductor element to a predetermined value and hold said contact resistance at that value during measuring. A problem of prior art arrangements is the deviation in the contact resistance between conductor and semiconductor element. The point of the conductor is accurately polished, which can sometimes take days, until the contact resistance on a calibration sample reaches a predetermined value. Even after the time-consuming calibration of the points of the conductors the deviation in the contact resistance results in inaccuracies in the measurements. The present invention has for its object to provide a novel method wherein the accuracy of resistance measurements on a semiconductor element is considerably improved. The present invention provides an improvement to the above method, in that the position of the conductors relative to the semiconductor element is controlled with piezo-crystals of an Atomic Force Microscope (AFM) or Scanning Tunneling Microscope (STM) arrangement, wherein a first of said piezo-crystals controls the pressure or distance of said conductors relative to the semiconductor element, and two other piezo-crystals control the position of said conductors along the surface of the semiconductor element. Further the present invention provides a STM arrangement for performing such method. Furthermore the present invention provides an AFM arrangement for performing the above mentioned method. Further details, features and advantages of the present invention will become apparent in the light of the following description of a preferred embodiment according to the present invention with reference to the annexed drawing, in which: Fig. 1 shows a graph of a current-voltage characteristic measured on a first semiconductor element and according to a first preferred method according to the present invention; and Fig. 2 shows a graph of a current-voltage characteristic measured on a second semiconductor element and according to a second preferred method according to the present invention. Fig. 1 and 2 are measured using an AFM arrangement (Atomic Force Microscopy). Herein a semiconductor element is movable three-dimensionally with piezo-crystals, a first of which adjusts the pressure or distance of a conducting needle relative to the semiconductor element, while two others enable a movement over the surface of the semiconductor element. A laser beam is directed onto the contact point of the conductor and the semiconductor element. The reflection of the laser beam is picked up in a photodiode, wherein the output of the photodiode is fed back to the first mentioned piezo-crystal. In the measured graphs use is made of a tungsten wire of 20 µm diameter which is cut to a point. The curve of fig. 1 is measured on a semiconductor element with a relatively high resistivity, ρ ≈ 4.7 Ω.cm. Applying a pressure force of constant value between the conductor and the semiconductor element results in an approximately linear current-voltage characteristic l 1 at negative voltage and l 2 at positive voltage. Fig. 2 shows measurements on a substrate with a comparatively smaller resistivity ρ ≈ 0.0084 Ω.cm. Different measurements are indicated with different curves in fig. 2 and show an approximately linear relation (l 3 and l 4 ), wherein the angles of slope of the lines (l 3 and l 4 ) are dependent on the applied pressure between conductor and semiconductor element. By applying the pressure force between the semiconductor element and the conductor, this conductor penetrates through a thin oxide layer which in practice is almost always present on the semiconductor element. The pressure force can be held very constant because of feedback to the first mentioned piezo-crystal. Using the two above mentioned piezo-crystals the conductors are moved over the surface for measuring the resistance or conductivity along the sloping surface, wherewith the resistance at different depths in the semiconductor element becomes known. Another technique which makes use in accurate manner of the control of a conductor over a for instance semiconducting element is so-called Scanning Tunneling Microscopy, wherein use is made of the tunnel flow between conductor and that element for holding the conductor at constant distance relative to the surface of that element. The oxide layer over the semiconductor element can form a problem here. An important novel feature of the method according to the present invention relates to the possibility of measuring either with DC current and/or voltage, or AC voltage or current. Good results have been obtained by measuring with AC voltage and using a DC bias voltage.

Claims: A method for measuring the resistance or conductivity between two or more conductors which are placed against a semiconductor element, wherein the conductors are held at a constant distance and/or under constant pressure relative to the semiconductor element in order to bring the contact resistance between the conductors and the semiconductor element to a predetermined value and hold said contact resistance at that value during measuring, characterized in that the position of the conductors relative to the semiconductor element is controlled with piezo-crystals of an Atomic Force Microscope (AFM) or a Scanning Tunneling Microscope (STM) arrangement, wherein a first of said piezo-crystals controls the pressure or distance of said conductors relative to the semiconductor element, and two other piezo-crystals control the position of said conductors along the surface of the semiconductor element. A method according to claim 1, wherein the conductor is manufactured from tungsten. A method according to claim 2, wherein use is made of a tungsten thread of 20 µm diameter on which a point has been cut. A method according to claim 1, 2 or 3, wherein a force is applied between the semiconductor element and the conductor such that the latter penetrates through the layer of natural oxide on the semiconductor element. A method according to any one of claims 1-4, using DC voltage or current and/or AC voltage or current. Scanning Tunneling Microscope (STM) arrangement for performing a method according to any one of claims 1-5. Atomic Force Microscope (AFM) arrangement for performing a method according to any one of claims 1-5.

Inventors: MEURIS MARC, VANDERVORST WILFRIED, MEURIS, MARC, VANDERVORST, WILFRIED
Applicants: IMEC INTER UNI MICRO ELECTR, INTERUNIVERSITAIR MICRO-ELEKTRONICA CENTRUM VZW
Application year: 1991
Main classification: G01R27
IPC codes: G01R31, G01R27