Method and apparatus for dynamic testing of thin-film conductor

Claim 2

Method for dynamic testing of thin-film conductors to determine kinetic parameters of activation energy and pre-­exponential factor which quantize the electromigration failure property of a thin-film conductor that is formed as a part of an integrated circuit on a semiconductor wafer which is subject to high current densities at elevated temperatures, and which method is carried out by electrically stressing the thin-film conductor by applying a uniform current thereto, heating the stressed conductor to increase the conductor temperature at a rate that is linear with respect to time, measuring changes in resistance in the current-stressed conductor as it is being heated at the linear rate, and calculating the activation energy and the pre-­exponential factor by relating the measured changes in resistance over time produced by the electromigration process; characterized in that in order to test the thin-film conductor while in an unfinished state or in uncut wafer form, the method comprises placing said semiconductor wafer (12) on a test platform (44) in which a plurality of probes (24, 28) can be moved into electrical contact with said thin film conductor (16), and in which the platform (14) contains a resistive heating element (30) which is in thermal communication with said semiconductor wafer (12), and supplying a ramp current from a controlled current source (34, 36), to said resistive heating element (30) for heating the stressed conductor (16) to increase its temperature at said linear rate.

Claim 3

The method of claim 1 further characterized in that the activation energy and the pre-exponential factor are calculated using a zeroth order rate expression wherein 1/R₀ · dR/dt = A exp(-Q/kT) Where: R₀ is the initial resistance of the thin-film conductor dR/dt is the variation in the conductor resistance produced by the electromigration process, Q is the activation energy for the process, A is the pre-exponential for the electromigration process, k is the Boltzmann's constant, and T is absolute temperature.

Claim 4

The method of claim 1, further characterized in that the measured changes in resistance are automatically compensated for changes in resistance due to temperature dependent components to determine the changes produced as a result of the electromigration process.

Claim 5

The method of claim 1, further characterized in that for testing for a plurality of like integrated circuits (18) each of which contains a plurality of said thin-film conductors (16), said method is carried out sequentially on a number of said thin-film conductors (16) while said wafer (12) remains in place in said test platform (44).

Claim 6

The method of claim 1, further characterized in that said controlled current source includes a temperature controller device (36) coupled to a temperature sensor (32) that is in thermal communication with said wafer (12).

Claim 7

The method of claim 1, further characterized in that the activation energy for this electromigration process is calculated by plotting 1n[T⁻² (ΔR em /R₀ )] vs 1/T, Where: ΔR em is the change in resistance of the conductor produced by the electromigration process.

Claim 8

The method of claim 6, further characterized in that the activation energy of the process is determined from the plot according to the relationship Q = -ks where s is the slope of the plotted line.

Claim 9

The method of claim 7, further characterized in that the pre-exponential factor is calculated from the relationship I = 1n (Ak/βQ) so A = -S exp I where β is the heating rate.

Claim 10

Apparatus for dynamic testing of a thin-film conductor that is contained in an integrated circuit on a semiconductor wafer to determine the kinetic parameters of activation energy and pre-exponential factor leading to electromigration failure in the conductor, in which an electrical current source supplies a constant current flow through the thin-­film conductor to electrically stress the same, and a heating device heats the stressed thin-film conductor as to increase its temperature linearly over time, and an electrical sensor measures changes in resistance in said thin-film conductor as the temperature thereof increases at said linear rate over a predetermined period of time, so that the change in conductor resistance due to the electromigration failure process can be calculated by relating such changes to said activation energy and said pre-exponential factor; characterized in that a semiconductor wafer unit (42) includes a test platform (44) on which said wafer (12) is mounted and with a plurality of probes (24, 28) which can be moved to contact with one said thin-film conductor (16) on said semiconductor wafer (12); said heater device includes a resistive heating element (30) mounted in said platform (44) to be in thermal contact with said semiconductor wafer (12); a controlled current supply (34, 36) supplies a ramp current to said resistive heating element (30) to effect the linear increase in temperature of said thin-film conductor (16); one set of said probes (24) is connected to a constant current source (22) for supplying a stressing current to said thin-film conductor (30) while it is being heated; the other set of probes (28) is connected to a device (26, 38) to measure changes in resistance of said conductor (30) as the temperature thereof increases at said linear rate over a predetermined period of time; and a sensor (32) in said test platform (44) measures the change in temperature of said wafer (12) so that the change in conductor resistance due to the electromigration failure process can be calculated by relating such changes to said activation energy (Q) and pre-exponential factor (A).

Claim 11

Apparatus for dynamic testing of a thin-film conductor that is contained in an integrated circuit on a semiconductor wafer to determine the kinetic parameters of activation energy and pre-exponential factor leading to electromigration failure in the conductor, in which an electrical current source supplies a constant current flow through the thin-­film conductor to electrically stress the same, and a heating device heats the stressed thin-film conductor as to increase its temperature linearly over time, and an electrical sensor measures changes in resistance in said thin-film conductor as the temperature thereof increases at said linear rate over a predetermined period of time, so that the change in conductor resistance due to the electromigration failure process can be calculated by relating such changes to said activation energy and said pre-exponential factor; characterized in that a semiconductor wafer unit (42) includes a test platform (44) on which said wafer (12) is mounted and with a plurality of probes (24, 28) which can be moved to contact with one said thin-film conductor (16) on said semiconductor wafer (12); said heater device includes a resistive heating element (30) mounted in said platform (44) to be in thermal contact with said semiconductor wafer (12); a controlled current supply (34, 36) supplies a ramp current to said resistive heating element (30) to effect the linear increase in temperature of said thin-film conductor (16); one set of said probes (24) is connected to a constant current source (22) for supplying a stressing current to said thin-film conductor (30) while it is being heated; the other set of probes (28) is connected to a device (26, 38) to measure changes in resistance of said conductor (30) as the temperature thereof increases at said linear rate over a predetermined period of time; and a sensor (32) in said test platform (44) measures the change in temperature of said wafer (12) so that the change in conductor resistance due to the electromigration failure process can be calculated by relating such changes to said activation energy (Q) and pre-exponential factor (A).

Claim 12

The apparatus of claim 9, further characterized in that a recording device (38) produces a record of changes in resistance and changes in temperature over said predetermined period of time.

Claim 13

The apparatus of claim 10, further characterized in that a plotter (40) coupled to said recording device (38) automatically draws a plot of the changes in resistance versus the changes in temperature.

Claim 14

The apparatus of claim 9, further characterized in that said controlled heating current source (34, 36) includes a programmable temperature controller (36) coupled to a control input of a current supply (34) so that the latter automatically increases the current to said heating element (30) to produce the desired increases in temperature for said wafer (12).

Claim 15

The apparatus of claim 9, further characterized in that said test station device (42) includes probe adjusting devices (64) for adjusting the positions of said probes (24, 28) while said semiconductor wafer (12) remains positioned on said test platform (44) so that similar thin-film conductors (30) on a number of integrated circuits on said wafer (12) can be tested sequentially to determine the kinetic parameters of activation energy and pre-exponential factor for each said conductor.