Investigation of the dependence of reactive ion etching of AlSiCu alloys upon film deposition characteristics

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Investigation of the dependence of reactive ion etching of AI-Si-Cu alloys upon film deposition characteristics J Maleham.

Plessey Research (Caswell)

Ltd, Towcester, Northants, UK

Aluminium-silicon-copper alloy films deposited in loadlocked and non-loadlocked sputtering systems with and without substrate bias are etched in a number of commercially-available reactive ion etching machines. Marked differences in etchability are observed depending upon the deposition conditions. The structures of the various films are studied by scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and Auger electron spectroscopy, and the results compared with electrical resistivity and step resistance measurements. It is shown that satisfactory etching can be achieved when films are deposited at high rates in clean vacuum systems, or by using substrate bias to reduce impurity gas levels and minimize the formation of microcracks at substrate steps. If these deposition conditions are not fulfilled it is likely that residual fillets of unetched metal will be left along the edges of steps unless excessive overetching is used to remove them.

Introduction

Dry etching of dielectric and metal films is now becoming established as a means of achieving the precise control of critical dimensions necessary in VLSI pattern replication. In these situations where the minimum lateral dimensions are becoming comparable to the film thickness, wet chemical etching methods are no longer adequate because of their inherently isotropic nature. Plasma etching in chlorine-based chemistries can be used to define anisotropic profiles in aluminium and aluminium-silicon alloys1-22, but copper-containing alloys, used for their enhanced electromigration resistance23-26 are difficult to etch in the plasma-etch mode because of the low volatility of the copper chlorides generated as reaction products of the etching process. The higher energies involved in reactive ion etching (RIE), however, offer the possibility of removing the copper by physical sputtering. Several reports of such processes have appeared recently in the literature 1~4~6~8~9~13~14~16~17~20~22~27, dealing with the plasma chemical and equipment design aspects of the technique. This paper describes the results of etching Al/Si/Cu films in a number of commercially-available RIE machines. The etchability is shown to be influenced strongly by the film deposition characteristics, emphasizing the necessity of treating the deposition and etching sequences as closely inter-related parts of a total processing problem. Experimental

details

The metal films used in this investigation were deposited by planar

magnetron sputtering in three commercially-available systems of different manufacture. Machines A and B were loadlocked, cryopumped systems, C was non-loadlocked, diffusion pumped. The latter was fitted with an RF-coupled biasing facility, capable of inducing a negative dc bias of several hundred volts on the substrate platen2”-32. The films were all 1 pm thick deposited cold. In the case of machines A and B the target compositions were Al/lS% Si/4?/, Cu and Al/l % Si/4% Cu respectively. Materials used in machine C were A1/1.5% Si/4% Cu and pure aluminium. The substrates were oxidized silicon wafers containing photoengraved step profiles typical of those encountered in IC processing, produced by growing 0.55 pm of thermal oxide, photoengraving stripe patterns in buffered HF solution, and depositing 0.4 pm of atmospheric CVD oxide (pyrox). The resulting step profile can be clearly seen in Figure 17. After metal deposition the wafers were patterned with AZ 14505 photoresist, 2.2 pm thick. Etching was carried out in five different batch-processing RIE machines, two non-loadlocked systems of US origin, and three loadlocked machines (two Japanese and one UK). In some cases the etch processes were proprietary, but all used chlorine-based chemistries. The etched films were assessed by a variety of analytical techniques. Scanning electron microscopy (SEM) was used to monitor edge profiles and measure linewidths. Scanning transmission electron microscopy (STEM) provided information on film structure. Electron probe microanalysis (EPMA) and scanning Auger electron spectroscopy (AES) with in situ argon ion-milling were used for transverse and depth profiling, respectively, of 437

J Maleham:

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compositional variations. Electrical resistance measurements on test pattern structures (see below) in conjunction with linewidth data gave sheet resistivity and step resistance values. Results (a) SEM evaluation. In the first series of tests pure aluminium and Al/Si/Cu samples deposited in machine C (without substrate bias) were etched in all five RIE systems. The pure aluminium films etched satisfactorily in all cases, with anisotropic edge profiles and residue-free etched field regions. Figure 1 is typical of these films. The alloy samples, on the other hand, showed much greater variability. In three cases etching was incomplete, the surface being characterized by a dense residue which inhibited further etching, as seen in Figure 2. The other two samples, exemplified by Figure 3, showed little evidence of residues, but in all five cases a continuous fillet of unetched metal which electrically shorted adjacent tracks was left along the edges of oxide steps, as can be

Figure 1. Pure Al film deposited 8 pm

in machine

C (no bias). Gap width (upper

portIon)

Figure 2. Al/M% S/4% Cu film deposited in machine residues and step fillet. Gap width 8 pm. 438

C (no bias). Heavy

Figure 3. Al/lSD/, fillet.

S/4%

Cu film deposited

in machine

C (no bias). Step

seen in both micrographs. A truly anisotropic etching process would be expected to require longer to clear the edges of steps because of the effectively greater metal thickness, but whereas in the case of the pure aluminium films a 30:; overetch was sufficient for this, the alloy films in some instances still retained a fillet even after a 200”’ overetch. In a se&d series of tests AliSiiCu films from all three sputtering systems were etched under identical conditions in one of the loadlocked etchers. This equipment has been described in detail e1sewhere’4.22. As before, samples sputtered in system C without bias left a heavy residual film and step fillets. similar to Figure 2. Significantly, however, samples from the same deposition system sputtered under a - 300 V bias etched cleanly with no residues or fillets (Figure 4), as did the samples from the two loadlocked deposition systems A and B. Bias-sputtered specimens which had been annealed in dry nitrogen for 30 min at 400-C after deposition and etched for the same length of time as the other wafers left residual fillets and discrete particulate residues (Figure 5). These samples also showed lateral erosion at intervals along the etched edges (Figure 6). Similar erosion effects have been

Figure 4. Al/1.5”, $4 1 ’ ; Cu film deposited No fillet or residues.

in machine

C

(- 300 V bias).

J Maleham: Dependence of reactive ion etching

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Figure 7. Sheet resistivity and step resistance of Al/Si/Cu films. Track width 8 pm. Film thickness 1 pm. Films annealed 30 min at 4OWC. Deposition systems A and B loadlocked; system C non-loadlocked.

Figure 5. A1/1.50/,Si/4”/, Cu film deposited in machine C (- 300 V bias). Annealed 30 min at 400°C before RIE. Residual fillet and residues.

wet-etched results are quoted, as the residues on the RIE samples made measurement meaningless). The wafers were measured before and after a 30 min anneal in dry nitrogen at 400°C. There was no significant change except for the non-bias deposited Al/Si/Cu samples from system C, where both parameters were some 30% higher before anneal. (c)STEM evaluation. Specimens were prepared from unpatterned alloy films from machines B and C, by local chemical etching of the silicon substrate to leave the metal film supported on a membrane of the underlying oxide. Both bias and non-bias sputtered wafers from machine C were used, and in all three cases both as-deposited and annealed specimens were examined. Figure 8 is a micrograph of the as-deposited non-bias sputtered film from machine C. It shows a uniform structure with grain size of approximately 0.2 pm and no evidence of phase segregation. The lighter area represents the base of an oxide step where the

Figure 6. Same film as Figure 5. Lateral edge erosion.

although in that case the extent of erosion was less in annealed than in as-deposited specimens. As a check on this second set of tests, the experiment was repeated using one of the non-loadlocked etchers. The results were similar in all cases to those of the loadlocked system. reported previously2’,

(b) Sheet resistivity and step resistance. The concept of step resistance and the method of measuring it have been described previously33. Briefly, the resistances of two identical adjacent metal meander tracks are compared, one on a flat surface and the other crossing orthogonally over a series of substrate steps. The extra resistance introduced by the stepped structure is a useful measure of the metal step coverage. The resistance of the plain meander can be used to calculate the sheet resistivity of the metal. Figure 7 tabulates the results for the wafers processed in the second series of tests described above, using an 8 pm wide meander pattern. An identical set of wafers delineated by conventional wet chemical etching gave almost identical results, after applying a correction for the loss oflinewidth of the latter. (In the case of the non-biased Al/Si/Cu deposition in system C the

Figure 8. STEM of A1/1.5%/4% Cu film from machine C (no bias), as

deposited. Grain size 0.2 pm approx. 439

J Maleham:

Dependence

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thinner underlying oxide (0.4 pm) causes less signal attenuation than the thicker (0.95 pm) field oxide. The structures of the other two as-deposited films were similar, except that the grain size of films from machine B was approximately 1.0 pm Figures 9 and 10 show the structures after annealing of films

Figure 9. STEM of Al/l ‘)ASi/49/, Cu film deposited 30 min anneal

in machine

Figure 10. STEM of Al/lS’,; Si/4Y0 Cu film, deposited ( - 300 V bias). after 30 min anneal at 400 C. 440

B, after

at 4OO’C.

in machine

C

from machine B and bias-sputtered films from machine C. respectively. Segregation of a separate phase has clearly occurred during heat treatment. In the case of machine B the precipitates were of a similar size or smaller than the matrix grains (i.e. 0.4-1.0 pm); those from the bias-sputtered films. although generally smaller than this, were usually considerably larger than the host metal grains. The composition of the precipitates was established by in siru energy-dispersive X-ray analysis to be strongly copper-rich compared to the background matrix. They appear to be located below the surface of the film. probably nucleating at the oxide interface, since the same specimens observed in the SEM mode revealed only a very faint structure. Similar studies have recently been reported elsewhere34, where precipitates from an Al/4”,, Cu film were identified by high resolution AES measurements as theta phase Al,Cu, which formed at temperatures as low as 200 C. The non-bias sputtered films from machine C after annealing showed precipitates similar in size to those of Figure 10. The biassputtered films showed a tendency for preferential nucleation of precipitates at the step edges, as can be seen in Figure IO, but this did not apply in the other two cases. (d) EPMA evaluation. Specimens from the same three sources as in the STEM studies were used for this evaluation, both asdeposited and annealed. A 20 kV electron beam (1 pm spot size) was scanned across the surface at right angles to the oxide steps. and the silicon and copper signals detected. At this beam energy the effective penetration is approximately 2 pm, and therefore the silicon signal derives predominantly from the oxide and silicon substrate. A square-wave output is therefore generated, the peaks corresponding to the enhanced substrate contribution from underneath the thin oxide regions. This provides a convenient marker for locating the steps. Figure 11 is a trace from an as-deposited non-bias sputtered film from machine C, and is typical of all the as-deposited films. Superimposed upon the silicon profile is a continuous copper signal with minor perturbations, corresponding to a mean copper content of approximately 47, (wt.), in close agreement with the nominal target composition. Figure 12 shows the result of annealing this film. and Figures 13 and 14 are the annealed profiles for bias-sputtered films from machine C and films from machine B, respectively. In agreement with the STEM results. it can be seen that the large Cu peaks in Figure 13, resulting from the large Cu-rich precipitates, correlate strongly with the oxide steps (cf. Figure 10). The occurrence frequency of large peaks in the other two cases agrees with the incidence of large ( - 1 ,um) precipitates and shows little correlation with step edges.

(e) Scanning AES evaluation. Figures 15 and 16 are AES depth profiles of as-deposited films from machine C, bias-sputtered and non-bias sputtered respectively. The area scanned was approximately 20 x 20 pm. In both specimens there was a ‘pile-up’of silicon and copper at the oxide interface. After annealing the profiles were similar except for a substantial reduction in these interface peaks, in agreement with the results of a study already quoted34. The non-bias sputtered specimens showed an unexpectedly high concentration of oxygen throughout the thickness of the film. The films from machine B had uniform silicon and copper profiles as-deposited. but after annealing the copper showed a tendency to migrate from the surface to the oxide interface.

J Maleham: Dependence of reactive ion etching

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J Maleham;

Dependence

of reactive ion etching

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J Male-ham: Dependence 100

of reactive ion etching

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Discussion of results

The use of substrate bias to improve the purity of sputtered films by preferential resputtering of absorbed gases has been reported previously28*30. This would account for the differences in sheet resistivity of the alloy films tabulated in Figure 7. Films A and B were deposited at relatively high nett rates (1430 and respectively) in clean (loadlocked) systems, 1000 A min-’ whereas films C were deposited at 500 A min- ’ in a nonloadlocked system. It will be noted, however, that the resistivity of the bias-sputtered films C is still significantly higher than that from machines A and B, where the resistivity is close to that of bulk Al. If the mechanism for this resistivity increase is grain boundary oxidation 30.39 however, it is not obvious why the same effect is not observed in’the pure Al films. Bias sputtering is also used to improve step coverage30~3193s-3s. Figure 17 depicts the coverage of an Al/Si/Cu film deposited without bias in machine C, and delineated by wet etching. A microcrack can be clearly seen extending an appreciable distance down towards the oxide interface. Figure 18 shows an identically processed structure, but with a -300 V bias applied during deposition. A small cusp is still evident at the metal surface but the penetration is insignificant. (Also evident is a distinct faceting of the metal profile similar to that which occurs during ion milling.) Prior to undertaking this study it was postulated that the very high step resistance and residual fillets after reactive ion etching these unbiased alloy films were due to precipitation of excess silicon and/or copper at the microcracks, since the same effect was not observed with pure Al films. The STEM and EPMA results

Figure 18. SEM of 1 pm thick A1/1.5% Si/4% Cu film deposited in machine C (-300 V bias) over oxide step. Wet etched. No microcrack.

described above, however, indicate that copper is not responsible since there was little evidence of any copper segregation in the asdeposited films, and even after an anneal the film with the most evident segregation of copper to the oxide step (bias sputtered film from machine C) still etched with only a small residual fillet (Figure S), probably composed of Al,Cu crystallites similar to those visible in the field regions of this specimen. It is likely, however, that precipitates of Si form in grain boundaries associated with the microcrack. Like copper, silicon diffuses sufficiently rapidly through aluminium, as can be seen from the AES profiles of Figures 15 and 16, to precipitate at grain boundaries at the oxide interface during the course of the deposition. It will also tend to segregate at the ‘free’ surface of a microcrack, and an increase in step resistance with increasing silicon content in unbiased films sputtered in this system has already been reportedJ3. In a situation where grain boundary oxidation can occur the resistivity could be further increased by the formation of SiOz. In bias sputtered films, the tendency for step segregation of 443

J Maleham:

Dependence

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ion etching

silicon is reduced because of the self-healing of incipient microcracks. It is also probable that some of the silicon is immobilized as SiOZ due to oxygen ion bombardment. since it has been observed that bias sputtered alloy films exhibit pitting in contacts to silicon substrates at sintering temperatures where non-biased films do not.

Conclusions The etchability of Al/Si/Cu films in reactive ion etching systems is strongly influenced by the film deposition characteristics. Where conditions are such that microcrack discontinuities tend to form at substrate steps, especially in systems containing significant background pressures of oxygen or water vapour, residual fillets of unetched metal are likely to be left at these steps. Conversely, films in which step segregation is minimized can be etched satisfactorily in several commercially-available machines.

Acknowledgements This work is published with the permission Plessey Research (Caswell) Ltd.

of the Directors

of

References

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