MetalPolyimide Interfaces Characterized by Secondary Ion Mass

Chapter 22

Metal—Polyimide Interfaces Characterized by Secondary Ion Mass Spectroscopy

Downloaded by GRIFFITH UNIV on September 4, 2017 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch022

Β. K. Furman, S. Purushothaman, E. Castellani, S. Renick, and D. Neugroshl T. J. Watson Research Center, IBM Corporation, Box 218, Yorktown Heights, NY 10598

The effects of deposition process conditions and post-deposition annealing on the adhe sion of electron beam evaporated Ti, Zr, Cu, and Cr to fully cured polyimide have been characterized by Secondary Ion Mass Spectrometry (SIMS) , Auger Spectroscopy and me chanical peel testing. Mechanical peel testing was used to evaluate the relative strength of metal-polyimide adhesion and it's subsequent degradation, as a function of deposition condi tions, and post processing. Auger and SIMS analysis provided complementary characteriza tion of peel failure loci and interfaces associated with observed failure. SIMS analysis of as-deposited metal films was also used to characterize impurity levels, both metallic and gaseous, incorporated throughout the metal during deposition. SIMS was also used to char acterize the absorption and redistribution of water during processing, using isotopically en riched water exposure. Together these techniques provide a us with a comprehensive understanding of factors influencing metal/polyimide adhesion degradation. Two modes of degradation are reported in this study. One mode results in the reduction of peel strengths by 25-35% to 45-55 g/mm. This is characterized, through Auger analysis, as a cohesive failure in the polyimide. This region has been identified by SIMS as reactive to isotopically tagged water,suggesting, that the reaction of water with the damaged polyimide decreases the polyimide strength in this area. The ob served reaction of water in the region directly below the metal interface is consistent with me chanical weakening of this region during thermal or T&H processing. This is further supported by the fact that similar samples ,identically annealed without exposure to water vapor in air show little or no degradation in peel strengths. A more dramatic failure results in peel strengths of 0-10 g/mm and is characterized as an ad hesive failure at the polyimide/metal oxide interface.This was the only failure mode observed in Ti and Zr films. Isotopically tagged water used with SIMS analysis shows that on annealing water reacts with the Ti with oxygen segregating to the metal/polyimide interface and hydro gen penetrating into the bulk of the Ti, in these samples. Similar results were observed for samples exposed to light RF sputter cleaning and a Cr ad hesion layer. SIMS identified a Cr-oxideregionto be enriched in O18 if samples are exposed to H 0 water prior to annealing. Extending the time used to ramp the sample to higher tem peratures has been demonstrated to delay this interface failure. Again no degradation is ob served if samples are not exposed to air prior to thermal processing. In this case we propose that the water absorbed in the polyimide segregates at the metal/polyimide interface and reacts with the metal during the elevated temperature annealing resulting in the formation of an ox ygen rich region at the interface and loss of adhesion. 2

0097-6156/90/O44O-0297$06.00/0 © 1990 American Chemical Society Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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METALLIZATION OF POLYMERS


Polymeric materials are increasingly being used as dielectrics in microelectronic applica tions [1,2]. Among the various polymeric materials being explored and used most is polyimide primarily because of its low dielectric constant , ability to withstand elevated temperatures and resistance to solvents when fully cured. One of the major challenges in building a reliable metal/polymer structure is the adhesion between the metal and the polymer. Numerous studies are reported in the literature where adhesion between metals and polyimide has been examined [3,4,5]. Many of these studies focus on understanding the chemistry of the bonding process between sub-monolayer additions of the metal atoms onto a clean as-cured polyimide surface inside a UHV system. Although such studies provide valuable insight into the fundamental aspects of metal/polyimide bonding under very idealized conditions, they are not necessarily applicable in total to the situation encountered in a practical metal/polyimide structure. Commonly UHV systems are not used for metal deposition and gaseous impurities can be incorporated into the metal/polyimide interface. In addition, polyimide surfaces are sub jected to in situ precleaning using energetic inert ions from a plasma or an ion gun to remove contaminant layers induced from prior processing and/or storage. It has been recently shown that this can lead to significant alterations in the polymer surface [6,7] and incorporation of metallic impurities. The details of the bonding process and hence the adhesion will no doubt be affected by such treatments. Further, metal/polyimide structures of practical interest expe rience thermal and ambient exposures after deposition as a result of subsequent process steps. Answers to such process related issues are not readily deducible from the fundamental studies. Accordingly, one needs to study metal/polyimide adhesion on samples fabricated under real istic processing environments and subjected to thermal and ambient exposures to understand the relevant issues. In this paper, we address metal/polyimide adhesionfromthis point of view. Using thin film peel stripes of evaporated onto in situ sputter cleaned polyimide samples under high vacuum process conditions (10 Torr to 10 Torr), we have performed peel strength measurements to characterize adhesion and surface analysis of the peel failure surfaces to un derstand the failure mechanisms. From the data on the as-deposited samples as well as those annealed at 350°C in N or forming gas, we propose a mechanism for the observed adhesion degradation after annealing.The model is based on the premise that water absorbed by the polyimide during processing or from the ambient can react with the damaged region of the polyimide or the metal adhesion layer resulting in adhesion degradation. The model was crit ically tested by using isotopically tagged water exposure of the polyimide and SIMS, to track the source of the oxygen as well as by an in situ annealing experiment that precluded water absorption and hence the suspected source of oxygen in the polyimide. The results from these experiments support the proposed adhesion degradation mechanism. -7

-6

2

EXPERIMENTAL DETAILS Substrates used for this study were 57 mm silicon wafers which were spin coated with a 5 μιη layer of commercial rx>lyimide (PMDA-ODA based) which was fully cured in a flowing nitrogen ambient to a maximum temperature of 400°C. A 200 nm edge coating of Copper was applied on top of the rxjlyimide to act as a peel initiating release layer. Prior to metal deposition, the polyimide surface was sputter cleaned in situ in an RF Ar+ plasma to remove surface contaminants and to normalize the surface conditions on all the samples studied. Two general conditions were used: 50 W,10 min. and 200W. for 30 min. . These conditions repre sent the forward power applied to the plasma for a given time. Since these conditions are system dependent they can be better compared using by there resulting etch rates of Silicon dioxide.The above conditions represent etch rates of 15 and 200 nm/min respectively. e

After RF sputter etching the substrates were heated to 90°C or 150 C and peel strips of 10 or 50 nm Τι,ΖΓ,0·,Οι/8μιη Cu thickness were then electron beam evaporated without

Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

22.

Secondary Ion Mass Spectroscopy

FURMAN ET AL.

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breaking vacuum on these wafers, through Cu coated stainless steel masks with 1.6mm wide slits in them. A chamber pressure of either low 10 Torr or low 10 Torr was achieved just prior to the onset of metal deposition, to investigate the effect of background pressure on ad hesion. The wafers were stored in dry nitrogen after deposition until they were removed for peel testing or annealing. 90° peel testing was done in a mini-peel tester at a peel rate of 4.5 mm/s and peel forces were measured using a Sensotec compact load cell that was calibrated to a sensitivity of 50 g/volt. Peel strengths in g/mm were obtained from the peel force traces over peel lengths of at least 30 mm and by averaging the resultsfromat least two to three peels per condition. Some of the wafers were annealed at 350°C in a tube furnace in a flowing ambient of N or forming gas for 30 minutes and peel tested to evaluate the effect of such an exposure. -7

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2

Surface analysis was carried out on the peeled metal strips as well as the substrate side of the peel failure to characterize the failure loci. Auger analyses were performed using a Physical Electronics (PHI) Model 595 Scanning Auger Spectrometer. A 3 keV electron beam with a 50 nA beam current was used for the analysis of fracture surfaces and elemental depth profiles. A 2 keV Ar+ ion beam was used for depth profiling. For SIMS analysis, a CAMECA IMS 3F Ion Microscope was used. Depth profiles were obtained by monitoring negative sec ondary ions of H, D, O , O while sputtering with 10 keV CJ ions. Mass surveys and Cu distributions were obtained using 12.5 keV Oi primary ions while detecting positive secondary ions. Typical beam currents were 0.5 μΑ rastered over an area of 250 χ 250 μπι. Average sputtering rates were 2.5 nm/s., determined by measuring the crater depth after sputtering . 18

16

+

RESULTS AND DISCUSSION Table 1 shows the effect of sputter cleaning, metal, and post deposition annealing on peel strength. In all cases .except Cu .as-deposited samples were observed tofracturecohesively within the rxriyimide 10-20 nm below the metal adhesion layer. The strength and depth of the fracture showed little difference regardless of metal deposited. It was noted that poor adhesion was observed if no in situ cleaning preceded metal deposition and that differences in cohesive peel strengths were observed when RF sputter parameters were changed. SIMS surface surveys of as-received and RF sputtered polyimide are shown infigure1. These results identified that our in-situ "clean" was indeed more complex than the simple removal of surface contaminants and rx)lyimide . In fact metal ions coated onto the walls of the evaporator were being im planted into the near surface region of the rx>lyirnide. The level and species of metal are ob served to influence the initial peel strength and subsequent thermal degradation rate. To establish consistency within our experiments the evaporation chamber was pre-coated with 200nm of Cu prior to RF sputter cleaning polyimide samples. Figure 2 shows a SIMS in-depth profile of the Cu incorporated during in-situ RF sputter cleaning. Auger analysis has also quantified the level of Cu as 5 % Atomic approximately 5-10 nm deep in the polyimide. This Cu appears innocuous to the as deposited cohesive strength of the polyimide in that similar levels of Cr or low level metal impurity levels with Ar ion beam treatment all result in similar strength cohesive separation when peeled. It was noted that the dose of Ar ion bombardment and resulting damage to the polyimide did effect as-deposited strength. In general low dose bombardment results in higher as-deposited peel strengths . At higher doses no additional de gradation is observed because we are sputter etching the polyimide while propagating a con stant damage zone ahead of the surface being etched. In this study we have chosen conditions of high dose to eliminate run to run variations which can be observed with low dose exposure as well as the enhanced thermal degradation observed at low dose exposure, as will be dis cussed later in this paper.



*30 min, 350 C, no air, H 0 .

β

"30 min, 350 C, F. G .

2

30 30 none 30 10

200 200 none 200 50

Ti Zr Cu Cr Cr

β

Time (min)

Power (W)

Adhesion Layer

RF Sputter Clean

a

Anneal

In Situ Anneal

1

53-68 75-80 0-5 73-78 70-85

Pl-Pl Pl-Pl Cu-Pl Pl-Pl Pl-Pl

0 0 20 35-45 10-30

Ή-Ρ1 Zr-Pl Cu-Pl Pl-Pl mix

Pl-Pl

— — Pl-Pl

— — 70-80 70-80

Pl-Pl

60-70

Peel Strength Failure Peel Strength Failure Peel Strength Failure Interface (g/mm) (g/mm) Interface (g/mm) Interface

As Deposited

Table I. Metal-Polyimide Peel Strengths


FURMAN ET AL.

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Secondary Ion Mass Spectroscopy Να

ΙΟ

2

υ Φ

(Ο

C

Ι

2


b ιο' ζ ω »ζ

20

k

40

U_i

60

L

80

100

80

100

ATOMIC MASS UNITS Να

Cr

10

s

φ

lyimide prior to elevated temperature annealing is essential for the thermal degradation of Ti/polyimide adhesion. At tempt to "dry" sample prior to high temperature annealing fails to eliminate this moisture in duce degradation of the metal/polyimide interface In the case of Cr however, surface pretreatment has been shown to significantly retard the rate at which water absorbed within the polyimide can react with the Cr/rx>lyimide interface . SIMS results with isotopically tagged water are shown in Figure 8 for thin Cr/Cu deposited on polyimide. It should be noted that 0 profile in Figure 8a differs slightly from Figure 5a. This may be due to differences in the oxygen yield at the interface which can be influenced by the metal layer (Cr vs Ti) over the polyimide. Auger analysis indicated no differences in oxygen distributions between samples. Prior to Cr deposition the polyimide was in-situ sputter cleaned for 30 minutes at 200 W for all samples except the one shown in Figure 8(d) which saw a low dose clean of 10 minutes at 50W. Similar to the experiment described previously for Ti, Figure 8(a) represents the na turally occurring O * distribution in the polyimide. Figure 8(b) is observed after the sample is soaked for lhour in H O or exposed to H O vapor for 48 hours. We observe a significant (200x) increase in the O level in the region of Cr/polyimide interface. These results indicate that this region may be capable of gettering water from the bulk of the rx)lyimide. Since no adhesion degradation occurs after the soaking only it is unlikely that significant chemical re action has occurred at this time. In addition, similar distributions are obtained for D 0 sup porting the hypothesis that only absorption and segregation has occurred . If we in-situ sputter clean at high dose as described earlier .expose to H O and anneal we see no significant pene tration or reaction of the O into the Cr.but a significant levelremainingwithin the rxriyimide in the region whose cohesive strength has degraded 25-35%, Figure 8c. These results indicate that the water has reacted with the polyimide resulting in mechanical weakening of this region. This is further supported by the fact that if identical samples are subjected to 'dry* thermal processing no degradation is observed. In addition similar degradation is observed for long exposure to humid environment at elevated temperature ( 1000 hr 85 C 70% RH) . In con trast, if the polyimide surface is exposed to a short sputter clean at low power (50W.10') prior to Cr deposition, significant Cr/polyimide adhesion degradation occurs similar to Ti samples. Figure 8(d) shows the O distribution in a similarly treated sample. In this case 50x levels of Ο are observed in the Cr layer compared to the 30 minute 200W sample. Auger analysis confirmed that this corresponds to significant levels of Oxygen (50-60 % At. ) within the Cr and accounts for the catastrophic failure at the Cr /Polyimide interface. The exact mechanism of the effect of in-situ sputter cleaning on the kinetics of Cr/rx>lyimide adhesion degradation is still a topic of research. Two potential contributor's are the role of trace metals such Cu introduced during RF cleaning and modification of the polyimide during ion bombardment. Experiments to identify their individual and/or synergistic effect on water diffusion across the metal /polyimide interface and their ability to getter and thus minimize the amount of water available for reaction.


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l

lt

xt

2

2

n

2

xl

2

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CONCLUSIONS SIMS has been shown to provide information on trace impurities and isotopic distrib utions applied to the study of metal/polyimide adhesion. Combined with Auger and me chanical results a more precise understanding of the effects of thin film of processing on adhesion has been obtained. In the case of Titanium .very good adhesion to sputter cleaned rxrtyimide in the as-deposited condition was observed. However, on subsequent thermal ex posure at 350 C for 30 minutes in either N or forrning gas the adhesion degrades to zero. This degradation is a result of the reaction of the water absorbed in the polyimide with the Ti during e

2



310

METALLIZATION OF POLYMERS

DEPTH (μη\) Figure 8. SIMS depth profiles of Cr/thin Cu films on polyimide: (a) as deposited, showing O* from ambient humidity exposure (normalized for isotopic abundance); (b) exposed to H 0 and (c) exposed to H 0 and annealed in forming gas at 350 * C (200 W, 30 min RF) and (d) exposed to H 0 ' and annealed in forming gas at 350 (50 W, 10 min RF). 8

18

18

2

2

8

β

2


22.

FURMANETAL.

Secondary Ion Mass Spectroscopy

311

annealing to form an oxygen rich Ti layer (most probably a form of titanium oxide) at the Ti/polyimide interface. Similar results are observed for Zirconium . In the case of chromium where the reaction kinetics appears slower than Ti and Zr ,we have found that surface pretreatment of the rxjlyimide prior to Cr deposition can significantly alter the rate of metal/ polyimide degradation. Details asto the role of metallic impurities and a modified polyimide layer in retarding water reaction with the metal/polyimide interface are still under study. ACKNOWLEDGMENT We would like to thank C.Parks for his help with SIMS measurements.


REFERENCES 1. M.Terasawa, S.Minami and J.Rubin, Int. J. Hybrid Microelectron., vol. 6, 1983, pp. 607-615. 2. T.Watari and H. Murano, IEEE Trans. Compon. Hybrids Manuf. Technol., CHMT-8 , No. 4, 1985, 462-467. 3. N.J.Chou and C.H.Tang, J. Vac. Sci. Technol. A, vol.2(2), 1984, pp.751-755. 4. P.S.Ho, P.O.Hahn,J.W.Bartha,G.W.Rubloff, F.K.LeGoues and B.D.Silverman,J.Vac. Sci. Technol. A, vol.3(3), 1985, pp.739-745. 5. F.Ohuchi and S.C.Freilich, J. Vac. Sci. Technol. A, vol.4(3), 1986, pp.1039-1045. 6. W.E.Vanderlinde, P.J.Mills, E.J.Kramer and A.L.Ruoff, J. Vac. Sci. Technol. B, vol. 3(5), 1985, pp.1362-1364. 7. P.Bodo andJ.E.Sundgren, Sur. and Int. Anal., vol.9, 1986, pp.437-440. 8. E.Sacher and J.R.Susko, J. Appl. Polym.Sci.,23 , 1979, pp. 2355-2364 RECEIVED May 16, 1990


MetalPolyimide Interfaces Characterized by Secondary Ion Mass

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