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2. Experimental

The plating cell consisted of a 1-liter Nalgene beaker to holdthe plating bath, with the anode resting on the bottom of the bea-ker and the cathode being held 45 mm above the anode. An Auto-lab PGSTAT 302N potentiostat/galvanostat was used to control and

ing from an inert platinum anode to a soluble copper anode andadding a resistor in series with the cathode. The cumulative effectof these modications reduced the spike to less than 1% of the totalsignal.

This more robust system was then used to evaluate experimen-tal organic additives for copper plating. Two different short loopmetallization test wafers were cleaved into coupons for theadditive studies. The rst wafer was provided by the College ofNanoscale Science and Engineerings 300 mm line to test thegap-ll and overburden mounding of experimental levelers and

Corresponding author.

Microelectronic Engineering 92 (2012) 9194

Contents lists availab

ic

.eE-mail address: kryan@uamail.albany.edu (K. Ryan).different chemicals and process conditions is needed. Standard ap-proaches to coupon plating involve attaching the coupon to a blan-ket copper wafer and processing it in a commercial plating tool.This process takes an undesirable length of time and requires alarge volume of chemicals. Standard laboratory-scale plating in abeaker, however, does not approximate the dynamic conditionsof an in-line tool [2,3]. We report on the development of an exper-imental methodology designed to ll this gap between researchersand production line, in order to reduce turnaround time and ex-penses in the search for next generation organic plating additives.

uncontrolled plating and pinch-off in the trenches. Several modi-cations were therefore made to the setup to control this spike inorder to improve the reproducibility and reliability of the plating.Fig. 1 demonstrates the reduction in this initial spike. The initialcondition (the blue curve labeled Pt inert anode) was reduced sub-stantially by incorporating a custom Rotating Disk Electrode (RDE)coupon holder to modify the boundary layer thickness. For thisstudy, the rotation speed for the RDE was xed at 1000 rpm. Inaddition, the RDE was inserted into the solution at an angle to pre-vent any air bubbles from being trapped underneath the electrode.Additional performance improvements were achieved by switch-1. Introduction

As local interconnect dimensionsfor effective super-conformal ll ofand more difcult to establish [1].makes full-wafer experiments to scrnology solutions more time-consumiciency, a reliable coupon plating syst0167-9317/$ - see front matter 2011 Elsevier B.V. Adoi:10.1016/j.mee.2011.04.051se the process windowopper becomes tighterighter process windowr next-generation tech-costly. To increase ef-ich allows screening of

monitor our plating conditions to ensure reproducibility. For plat-ing, a 2 cm 2 cm coupon was attached to the cathode by conduc-tive copper tape and masked off with chemically resistantKapton tape to protect the surface. A hole was punched in thistape to expose a 2 cm2 area over the desired pattern on the coupon.

Hot-entry plating was used to protect the copper seed layer;however, a current spike during the initial moments of platingwas observed due to boundary layer formation, which led to rapid,Polyethylene glycolpresented.

2011 Elsevier B.V. All rights reserved.Development of electrochemical copperfor next generation additive selection

Kevin Ryan a,, Kathleen Dunn a, Jobert van Eisden baCollege of Nanoscale Science and Engineering, Albany, NY, USAbAtotech USA, Inc., USA

a r t i c l e i n f o

Article history:Available online 30 April 2011

Keywords:Copper electrochemical depositionDamascene32 nm nodeBeaker plating

a b s t r a c t

To reduce time-to-knowlecopper electrochemical dewhich promote bottom upconditions and functionaliedge feature sizes (sub 50full wafer plating and furtopen source and proprietamance metrics. PreliminarMicroelectron

journal homepage: wwwll rights reserved.position screening methodologies

and costs associated with 300 mm wafer processing a laboratory-scaleition (ECD) system was developed for screening new organic additivesin interconnect trenches and vias. This new setup enables working processends to be identied for experimental suppressors and levelers at leading. These results can then be transferred to the in-line copper plating tool foranalytical testing. This paper presents results of a case study using bothhemistry, focusing on gap ll, throwing power, and mounding as perfor-ta on the transferability of process conditions to the full wafer tool are also

le at ScienceDirect

Engineering

l sevier .com/locate /mee

The second study examined the role of molecular weight onsuppressor effectiveness. For this study, polyethylene glycol(PEG) solutions with three different molecular weights (1000,4000 and 8000) were used as a model suppressor system. Twoexperimental proprietary suppressors codenamed NYH and NYXwere compared to the baseline PEG measurements. Because ofintellectual property considerations the chemical structure ofthese two suppressors must remain unknown. The only informa-tion available about the two compounds is they both have a similarchemical formula and differ only in polymer chain length (NYXbeing twice the molecular weight of NYH). Each suppressors abil-ity to induce bottom-up growth was evaluated by partially llingFig. 1. Current density prole of different implementations of the beaker setup.

Elimination of the current spike was achieved by successive improvements,including an RDE, a soluble anode and a series resistor.

92 K. Ryan et al. /Microelectronic Engineering 92 (2012) 9194Fig. 2. Cross-sectional SEM images of 50 nm trenches plated in (a) beaker setup. (b)300 mm plating tool (after CMP). The voids in the beaker-plated sample indicatethat further improvements are needed; however, the ll in 60 nm trenches andlarger was identical.suppressors. This coupons plateable area included a SEM bar withtrenches ranging from 50 nm half-pitch up to 5 lm at 1:1, 1:3 and1:5 spacing in a low-k dielectric. The second wafer was a SEMA-TECH quick cleave design that contained trenches that were0.25 lmwide by 1 lm deep. To prevent oxidation of the seed layerbefore the coupon could be plated; unused portions were storedunder liquid nitrogen, providing an oxygen-free atmosphere andlow thermal energy. Thermal cycling tests (down to 77 K and backto room temperature) were conducted to conrm that this extremeenvironment did not damage the seed or dielectric.

Two sets of experiments were run to demonstrate the utility ofour new setup for screening additive behavior. The rst examinedthe effectiveness of new levelers to produce a at overburden. Forthis set of experiments, the SEM bar samples were plated with a650 nm overburden. An FEI Nova NanoSEM 600 dual beam instru-ment was used to cut a cross-section and image the structures andmeasure any mounding that occurred.

Fig. 3. Cross-sectional SEM images of 90100 nm trenches demonstrating overburden mguide to the eye. (a) 0.01 mL/L (b) 0.1 mL/L (c) 1 mL/L. Mounding decreases with increathe SEMATECH trenches with a cutoff charge of 0.5 Coulombs fora nominal thickness of 90 nm of copper on a 2 cm2 area.

3. Results and discussion

To assess the transferability of our bench-top results with thosefrom a full-wafer tool, coupons were plated using the identicaladditive package in both systems. The ll results for 50 nm half-pitch trenches are shown in Fig. 2. The beaker plating is not iden-tical at the 50 nm half-pitch, with a few voids present that are notobserved in the sample plated in-line. However, the differences arenot gross, and disappear altogether in trenches with half-pitcheslarger than 60 nm. Therefore, although more tool development isnecessary to achieve full parity with in-line plating in the smallestfeatures, we believe that general behavioral trends can be reliablyidentied for transfer to the full-wafer tool.

The leveler experiments evaluated the effectiveness of severalexperimental additives as a function of concentration and trenchwidth. Fig. 3 presents an example of these results for levelerLK0816. In particular, cross-sectional images of the 90, 95 and100 nm trenches show mounding decreases with higher concen-trations of leveler in the solution, as expected for a good leveler.A quantitative analysis for each leveler is underway to allow directcomparison of different molecules. General trends in behavior,however, have identied reasonable process windows for transferto the in-line tool for further analysis.

The suppressor study rst examined the effect of molecularweight on the performance of the open chemistry suppressorPEG. Fig. 4 shows the cross-sectional images of PEG 1000 andPEG 8000 both at 1 mL/L concentration in the bath. The largermolecular weight of PEG more effectively suppresses the growthof copper on the trench sidewalls and top elds. This resulted ina faster bottom up ll rate inside the trenches which will ulti-mately promote a void free ll.

This is demonstrated explicitly by the histograms in Fig. 5,showing the superior bottom up growth and throwing power (ratioof sidewall growth divided by bottom up growth) for PEG 8000 at1 mL/L. These results correlate well with the ndings of Wei-Pingounding as a function of concentration of leveler LK0816. Dotted line added as asing concentration.

ic EnK. Ryan et al. /MicroelectronDow et al. in their studies of molecular weight on PEG [4]. Thistrend is expected to hold at higher PEG concentrations, althoughfurther work is needed for conrmation.

With a clear picture of the effect of molecular weight on thesuppression power of PEG, a similar analysis was pursued usingNYH and NYX. Fig. 6 shows the partial ll results for both suppres-sors at a concentration of 3 mL/L and demonstrate an improvementin bottom up ll as compared to PEG at the same concentration. As

Fig. 4. Cross-sectional SEM images of partial ll for (a) PEG 1000 at 1 mL/L. (b) PEG8000 at 1 mL/L. The lower molecular weight polymer has approximately equalsidewall and bottom-up growth, while the larger molecule promotes betterbottom-up ll.

Fig. 5. Suppressor performance of PEG as a function of molecular weight. (a)Bottom up growth measurements. (b) Throwing power measurements. The highestmolecular weight (PEG 8000) performed best at 1 mL/L; this trend is expected tocontinue at higher concentrations but must be conrmed by additionalexperiments.gineering 92 (2012) 9194 93in the case of PEG, the higher molecular weight polymer sup-presses sidewall growth and promotes bottom-up ll. A completepicture of bottom up growth and throwing power is displayed inFig. 7 at each concentration tested. Although the throwing powerof both NYH and NYX improves for higher concentrations, the lar-ger molecular weight polymer outperforms the lower with greater

Fig. 6. Cross-sectional SEM images showing partial ll analysis for experimentalsuppressors (a) NYH at 3 mL/L. (b) NYX at 3 mL/L. While both moleculesoutperformed PEG, the trend with molecular weight held true for both open andproprietary chemistries. The larger molecule (NYX) demonstrated superior sup-pression capabilities than the smaller molecule (NYH).

Fig. 7. Suppressor performance of experimental molecules as a function ofmolecular weight. (a) Bottom up growth measurements for NYH and NYX. (b)Throwing power measurements for NYH and NYX. As with PEG, the larger moleculeis a stronger suppressor.

throwing power and narrower distribution as well. While theseobservations hold true throughout the range of concentrationsstudied, they are only preliminary results; future work will deter-mine whether there is a critical molecular weight or concentrationwhere bottom up ll begins to degrade.

4. Conclusion

To date the beaker setup has proven robust and reliable inrevealing trends that can guide experiments in the full-wafer plat-ing tool. Key to this was implementation of hot entry and elimina-tion of the initial plating spike to produce a consistent andrepeatable plating process. These precise controls become extre-mely important as the trench and via widths continue to shrink,narrowing the process window for effective void-free ll. Two dif-ferent methodologies were presented to pre-screen for organic lev-elers and suppressors for copper deposition. New levelers wereevaluated by measuring the overburden mounding above theSEM bar at varying concentrations in the plating bath, with level-ing depending strongly on concentration. For suppressors thegap-ll properties were examined directly by plating 50 nmhalf-pitch trenches. Strong dependence on molecular weight was

demonstrated for both the model suppressor PEG, and the experi-mental suppressors NYH and NYX.

To move conditions in the beaker setup closer to what is seen onthe full-scale plating tool, boundary layer analysis will be per-formed. The limiting current at different spin speeds will be mea-sured and used to calculate a theoretical value for the boundarylayer thickness using the Levich equation [5]. Additional researchand development is underway to evaluate any remaining differ-ences between the results from coupon and full-wafer plating.

Acknowledgment

This work has been supported by Atotech USA.

References

[1] Michael West et al., AIP Conf. Proc. (683) (2003) 504513.[2] Lyndon Graham et al., J. Electrochem. Soc. (1498) (2002) C390C395.[3] Paul H. Haumesser et al., Copper seeding and lling: New strategies for the

32 nm node, 211th ECS Meeting, May 2007.[4] Wei-Ping Dow et al., J. Electrochem. Soc. 152 (11) (2005) C769C775.[5] Bill Q. Wu et al., J. Electrochem. Soc. 152 (5) (2005) C272C276.

94 K. Ryan et al. /Microelectronic Engineering 92 (2012) 9194

Development of electrochemical copper deposition screening methodologies for next generation additive selection1 Introduction2 Experimental3 Results and discussion4 ConclusionAcknowledgmentReferences