Extrapolation of Highly Accelerated Electromigration Tests on Copper to Operation Conditions

Jochen von Hagen, Robert Bauer, Sabine Penka, Andreas Pietsch, Wolfgang Walter, Anke Zitzelsberger lnfineon Technologies AG, Otto-Hahn-Ring 6, D-8 1739 Munich, Germany, Phone: +49 89 234 50452, Fax: +49 89 234 746759

E-mail: jochen.hagen@infineon.com

Abstract Compared to aluminum the electromigration resistance of copper is higher. This leads to longer test times. Higher acceleration factors are needed to reach reasonable test times. Because of the oven hardware there is a limitation in the stress temperature for standard iso-current tests on package level at 35OOC. Self-heated test methods, such as SWEAT and accelerated iso-current test on wafer level, use Joule-heating to to reach stress temperatures up to 600°C and therefor to much lower test times. For aluminum a qualitative analysis with these tests has already been conducted successfully. It would be a major advantage, if quantitative analysis, e.g. extrapolation to operation conditions, would he feasible. During this investigations, we have used three different electromigration stress methods, iso-current test on package level, iso-current test on wafer level and SWEAT, with different stress conditions on the same structure and material. We show an extrapolation of all of the results of the different test methods to operation conditions and a way to calculate the necessary Black's parameters from the results of the highly accelerated self-heated tests

Introduction: One of the reasons to replace aluminum with copper

metallization is the higher electromigration resistance of copper leading necessarily to longer test times. Higher acceleration factors compared to aluminum are needed for reasonable test times. The stress temperature T i s limited to 350 'C for standard iso-current tests on package level (PL) due to degradation of sockets, packages etc. resulting in test times in the range of hundreds of hours depending on the test structure and current densities used. Highly accelerated electromigration tests on wafer level (WL), such as SWEAT (standard wafer level electromigration accelerated test) [ l][2], iso-thermal or iso-current test, use Joule-heating to reach stress temperatures up to 600 "C in order to reduce the stress time to several seconds. A qualitative analysis of the results, such as process comparisons, have already successfUlly been conducted for aluminum. However, it would he a major advantage, if quantitative analysis, i.e. extrapolation to operation conditions, would also he feasible. Fast test methods would be a very good addendum to the standard long-term tests by reducing the number of long-term tests and test times, besides improving the statistics. A successful procedure for aluminum has already been presented [3], and its validity is also expected for copper.

We show a comparison between highly accelerated tests (SWEAT and iso-current) at wafer level and standard iso- current tests at package level for narrow lines. The results are extrapolated to operation conditions. Different stress conditions (current density and temperature) were performed to determine acceleration limits and kinetics,

Experimental: Via-terminated metal test lines in copper were used for self-

heated electromigration tests (Fig.1). The electron flow goes from metal layer 2 to layer I . In this investigation wafers with the same ground rules and the same process flow are used.

< . . Figure I: 0.4pm wide copper line, length 800pm

Iso-currenl Wafer Level and Package Level Test The current stress at elevated temperature is kept constant

for tests on package level (PL) and on wafer level (WL). The temperature level is preset by oven and chuck temperature, respectively. The line temperature due to Joule-heating is determined by the temperature coefficient of resistance (TCR) during the initial phase of each test.Both, oven and chuck temperatures are 300 "C for various current densities (Table I ) . The stress current density on PL is set to a lower value to prevent an increase of the stress temperature due to Joule heating.

Because the chuck temperature is limited to 3OOOC for iso- current WL tests, high current densities lead to the wanted stress temperatures of up to 400°C. There is no regulation of the stress temperature driven by the Joule-heating during the complete test flow.

S WEAT-Test 11.21 The idea of the SWEAT test is to control the target time to

failure (Q as estimated in Black's Equation (eq.1). This /f serves as a measure of the severity of the stress. In the control loop, target failure times are maintained constant by regulating current density and temperature. The line temperature due to Joule-heating is calculated by means of the dissipated power and the thermal resistance during the initial phase ofeach test.

Although, the algorithm controls the target failure time, the current density J during the test and accordingly the temperature T(J), varies less than I % except for the last few seconds of the test. For the later extrapolation, the medians of the line temperature and current density are used.

2002 IRW FINAL REPORT 0~7803-7558-01021$17.00 "2002 IEEE 41

von Hagen et al. Extrapolation of highly accelerated electromigration tests on copper to operation conditions

Table 1: Stress conditions and results for conducted tests

E, k . T ( J ) ttr = A . J-" . e

This method considers the influence of both values to control and keep stress conditions constant until the failure occurs.

The failure criterion for all tests is defined by a 10 % resistance increase. The quantitative analysis of the test results requires an accurate determination of the stress temperature. The necessary accuracy can he obtained by considering the change of the TCR for copper and the th<xmal resistance of the silicon-oxide with the stress temperature higher than 3OOOC [4].

Stress conditions To compare the highly accelerated tests on WL with long-

term PL-tests, a wide variation of stress conditions (current density and temperature) has been used. An overlap of the stress conditions between both WL-tests may show similarities between the methods. I moved the table to be @ page bottom.

The test conditions for the different test types are shown in Table 1. Due to the Joule-heating characteristics of the high current densities for the WL-is0 current test, only the resulting temperature after ramp-up to stress conditions is shown. For the SWEAT, the estimated time to failure results in the displayed temperatures and current densities.

Results and Discussion

Failure Dktributwn The failure distribution of all the tests are shown in a

lognormal chart (FIG.2). The same shape factor of all failure distributions indicate the same quality of all used wafers.

I 2.1 , I

Figure 2: Electromigration tests results

Activation Energy E. and Current Den& Exponent n Extrapolation to operation conditions by using Blacks'

equation is the standard quantitative analysis for electromigration tests. Required is the knowledge of activation energy E, and the current density exponent n.

E, and n are obtained by a combination of the test results of all tests. Since highly accelerated tests use Joule-heating, the temperature T,,, and the current density J,,,, cannot be vaned independently. Therefore, for Ea determination the test results have to he nomalized to a current density J,, and for n determination to a temperature T,, , respectively, using Black's Equation (eq. 2).

Activation energy E, Using (Jnom = 25mA/pm') one can determine Ea with the

standard method plotting t50.nom vs. (likT,,m) (Figure 3). The slope of the regression line is the activation energy E,. For the investigated structure and technology, its value is determined to 0.92 eV. The wide range of temperatures improves the accuracy of the E,determination.

6

5

I 1 I k T

Figure 3: E.-determination

Current Density Exponent n Accordingly, for the determination of U the median failure

times t5O have to be normalized to a norm temperature T,,, as shown in ea. 3:

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von Hagen et at Extrapolation of highly accelerated electromigration tests on copper to operation conditions

Figure 4 shows the t50,nom vs. J,,,, diagram.. Here, the value of n is determined to 1.0. As a consequence of the different current density ranges for the different tests the data points are clustered.

Figure 4: n-determination

Since the current densities of only the SWEAT test do not differ enough it is necessary to use data from highly accelerated tests and from tests with a smoother acceleration to get a sufficient accurate n.

Both, Ea and n found in this study are typical values for this copper metallization and have heen published [SI.

Scalability ofSWEAT results The SWEAT-test can he executed in a wide range of stress

conditions. During this investigation stress temperatures in a range of 340 up to 645 "C were reached with a corresponding current density of 500 - 810 m4ipm2, by setting the target failure time of 40s up to 16000s. If materials are of the same quality, a linear correlation between the target failure time and the achieved failure time is expected. Otherwise, there would ; a change in failure mechanism or quality.

mow

10 , , , , , . . , ~ , , -, , , . . ,

10 1w ,ma 1" lOWm Target Ill(*)

Figure 5: SWEAT Failure times vs. target failure time

In figure 5, the achieved failure time were plotted vs. the target failure times for all executed tests mentioned in table 1. The near perfect line proves the scalability of the stress conditions and also the consistency of the method over a wide range of stress conditions.

Extrapolation of Tests ResulIs to Operation Condition It is the main target of a quantitative analysis of

electromigration test results to get a lifetime prediction of the device for operation conditions using Black's equation assuming a constant shape factor.

Here, typical operation conditions of T = 125 "C and J = 5 mA / pm' are used for extrapolation (Ea = 0.92eV and n = 1 .0 as determined beforehand; Fig.3 and 4).

For all tests of table I the extrapolation has been performed and plotted in the lognormal plot of Figure 6. The utilization of Black's parameter from Figure 3 and 4 leads to

stributions at operation conditions in a quite narrow range.

Figure 6: Extrapolated test results

Failure Analysis Failure analysis from a chip tested with SWEAT and iso-

current WL test is shown in Figure 7. Both chips were exposed to similar stress conditions. For both methods the voiding is found in the line close to the via or underneath the via. The failure locations, shown in the images of Figure 7, are well known from standard electromigration tests, confirming the same failure mechanism for the wafer level test methods despite the very high current densities.

SWEAT - MO 'C l496mAfpm' Iso-I PL - 367 "C I16OmN~m' Figure 7: Failure Analysis

Open Issues The results of PL and WL agree very well despite the high

current densities used in WL tests. Although, high current densities in the via occur voiding in the line directly underneath the via was found. However, in this paper only one type of test stmcture (Figure 1) was stressed in a down stream mode showing a lognormal distribution even at the moderate PL test conditions. From other investigations [SI, bimodal distributions are well known for Cu metallizations, indicating two different types of voiding mechanisms in the metal line and via, respectively.

It has to he shown in future investigations that the bimodality can also he displayed in highly accelerated tests. Also the agreement of fast WL and PL tests for the upstream test mode of this test structures as well as other test structures have to he confirmed.

In principle the fast WL tests can also he performed on fat wire metallizations. The necessary power dissipation for sufficient heat and stress times were reached. Lognormal failure distributions were obtained. The correlation to package level tests however has to be shown.

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von Hagen et al

For monitoring purposes it is necessary to resolve even small process deviations in highly accelerated tests to ensure a stable process quality.

Summary and Conclusions For a wide range of stress conditions, with stress

temperatures of up to 645 "C and the corresponding stress current density of 810 mAipm2, an excellent agreement of tso,em values are found using the activation energy and current density exponent determined with the results of all perfoimed tests. We have shown a possibility to perform highly accelerated, Joule-heated electromigration tests on narrow line copper metallizations to get reasonable lifetime predictions under operation conditions. A comparison of the results from the SWEAT-test, the iso-current WL- test and the standard iso-current test on PL is very promi!;ing. Fast test methods are a very good addendum to the standard long-term tests and reduce the number of long- term tests and test times besides improving the statistics.

One requirement for any comparison IS a good knowledge of the temperature in the metal line throughout the stress. Careful investigation of Ea and n is fundamental. Confinniog results with failure analysis is required to ensure that samples were not overstressed. Summarized, we have made a first step to make electromigration tests faster and cheaper but thert: are still open issues.

Extrapolation of highly accelerated electromigiation tests on copper t o Opeiation conditions

Questions and Answers Q: For aluminum a comparison between standard package

level test and iso-thermal test do not show the same good correlation like shown for copper in this paper. Is there an explanation ? For aluminum there is a change in the kinetics (diffusions paths) of the electromigration due to the high temperatures and current densities of the highly accelerated EM-tests such as SWEAT or iso-thermal test. The kinetics in copper metallizations are different. For a wide range of temperatures and current densities the same value of n = 1 .O was obtained. This indicates same kinetics for all investigated tests.

The temperature of the ambience is 30 "C and the temperature has a level of up to 600 "C. Does this temperature gradient between the connecting line, the via and the stress line have an influence on the electromigration result ? For highly accelerated EM-tests, there is not only Joule- heating in the stressed line, but also in the via and in the connecting line. The via has a similar size like the stressed line, the connecting line is not critical. So the temperature in the via is expected to he a bit lower than the stressed line and the connecting line may reach half of the stress temperature. For a stress temperature of 600 "C, the temperature gradient is assumed to eo from

A:

Q:

A:

300°C to 6OOOC. There is no possibility to irevent a temperature gradient and its influence is still a topic for further investigations. Temperature simulations will be carried out for venfication.

Literature: [ I ] A Procedure for Executing SWEAT; JEP 119,

Sept. 1994; JEDEC [2] J.v.Hagen, et.al.; New SWEAT Method for Fast,

Accurate and Stable Electromigration Testing on Wafer Level; IRW 2000, pp.85-89

[3] A.Zitzelsberger et al.; Electromigration Testing on Via Line Structures with Fast Wafer Level Tests in Comparison to Standard Package Level Tests; IITC

[4] J.v.Hagen, H. Schafft, Temperature Determination Methods on Copper Material for Highly Accelerated Electromigration Tests (e.g. SWEAT); IRW 2002

[ 5 ] Standard Method for Measuring and Using the Temperature Coefficient of Resistance.; EIA/JESD33-A, Oct. 1995, Electronic Industries Association

[6] T.C.Lee et al.; Electromigration Study of AI and Cu Metallization Using WLR Isothermal Method, IRPS 2002

[7] A.Marathe et al.; Isothermal Test as a WLR Monitor for Cu Interconnects; SPIE 2000

[8] A.v. Glasow, A. H. Fischer; EM and SM Investigations on Dual Damascene Copper Interconnects, AMC 2001

2001, pp. 180-182

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