CU FILM DEPOSITION FROM SUPERCRITICAL CARBON DIOXIDE FLUIDS FOR 3D-IC THRU VIA FORMATION. Masahiro Matsubara and Eiichi Kondoh

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1 CU FILM DEPOSITION FROM SUPERCRITICAL CARBON DIOXIDE FLUIDS FOR 3D-IC THRU VIA FORMATION Maahiro Matubara and Eiichi Kondoh Mechanical Sytem Engineering, Univerity of Yamanahi, Kofu, , Japan. 1 INTRODUCTION A miniaturization proceed, new metal-wiring (interconnect) technologie are required for the fabrication of high-performance LSI. One of the crucial technological target i the formation of MEMS-baed thru ilicon via (TSV) of 3D IC. Cu electroplating i the mot popular of the depoition technologie currently under invetigation, but it plating technology doe not eem to be able to perform defect-free Cu filling in high-apect via hole with ufficient tep coverage. Technology for depoition in upercritical fluid i becoming of crucial interet a a replacement for electroplating becaue of the excellent penetration capabilitie of upercritical fluid. Supercritical carbon dioxide (cco 2 ) demontrate a unique combination of ga and liquid propertie above the critical point (304 K, 7.2 MPa). Additionally, it ha a high a denity a liquid, a high a diffuivity and a low a vicoity a gae, zero urface tenion, and olvent capabilitie. Becaue of thee unique propertie, cco 2 impregnate and deliver a metal precuror diolved into cco 2 into high-apect-ratio gap, trenche, and TSV. The metal depoited on thee topographie demontrate good gap-filling and tep coverage. Technologie uing cco 2 are very attractive for the fabrication of new metal wiring. We developed a flow-type depoition proceor deigned to enable the independent control of depoition parameter and long-term depoition in which a precuror diolved in cco 2 i continuouly upplied to a reactor while the depoition characteritic of the film are tudied [1]. Thi paper report the Cu depoition characteritic of cco 2 uing Cu(dibm) 2 in our developed flow-type reaction ytem. 2 EXPERIMENTAL A chematic diagram of our flow-type reaction ytem i hown in Figure 1. Liquid CO 2 cooled at 0 O C wa preurized uing a high-preure pump to above the critical point of CO 2 and wa upplied to a H 2 mixing unit deigned to enable the admiion of a low-preure (max. 1.5 MPa) ga to a high-preure fluid, typically continuou 10 MPa. A precuror, copper(ii)bi-diiobutylmethanate or Cu(dibm) 2, wa diolved in acetone and upplied continuouly to a hot-wall reaction chamber via a per-heating ytem that heat CO 2. Depoition

2 preure wa kept contant uing an automatic preure controller. Cu(dibm) 2 wa purchaed from Kojundo Chemical Laboratory Co., Ltd. (Sakado, Japan) and ued a received. The ubtrate ued were TaN-coated Si wafer coupon with gap-fill tet pattern 10 µm in dia. x 90 µm in depth. Before depoition, Au thin film, nom. 50 Å in thickne, wa puttered to the wafer in order to generate nuclei for Cu depoition. Film thickne wa meaured uing a Dektak 3 urface profiler. Film heet reitance wa meaured by a four-point probe method uing a HIOKI 3239 Digital Hiteter. The depoition condition were a follow: temperature O C, preure 10 MPa, hydrogen concentration 0.4 and 1.2 mol%, precuror concentration ppm, acetone concentration 6 mol%, and depoition time 1-8 hr. Figure 2 how the dimenion of the reaction chamber. The thin channel arrangement enabled one-dimenional flow. The ubtrate wa poitioned at the midpoint of upper heat in a facedown manner. thermotatic chamber (40 ) Cooling unit Mixing unit Pre-heating ytem Reactor Automatic preure controller Pump CO2 cylinder Pump Heater Subtrate Separator H2 cylinder Precuror + acetone reervoir Figure 1. Schematic diagram of the flow-type reaction ytem deigned to enable long-term depoition. A precuror diolved in acetone wa continuouly upplied to the reaction chamber. 1 mm 60 mm 8 mm Wafer (facedown) Figure 2. Schematic diagram of the reaction chamber with a built-in heat block. A ubtrate i poitioned at the midpoint of the heat block and faced toward the flow.

3 3 RESULTS AND DISCUSSION For long-term depoition, the precuror mut be upplied continuouly. Liquid precuror, uch a Cu(I)(hexafluoroacetylacetonate)trimethylvinylilane or Cu(hfac)tmv, eem uitable for thi purpoe, but they are troubleome in handling becaue they react eaily with oxygen, which change their tructure. Although copper(ii)bi-(hexafluoroacetylacetonate), Cu(hfac) 2, ha good olubility in cco 2 (around 35.2g/L), thi precuror i not acceptable due to it incluion of the environmentally unfriendly fluorine. Cu(dibm) 2 ha a coniderable low olubility of only 3.32 g/l and therefore cannot be ued directly a a precuror in depoition. In our experiment, Cu(dibm) 2 wa diolved in different organic olvent and upplied to the reaction ytem. In order to determine the efficiencie of different olvent, Cu(dibm) 2 wa diolved in olvent uch a ethanol, propanol, hexane, acetone, and cyclohexane. Figure 3 how the olubility of Cu(dibm) 2 in olvent of volume 1 ml. Hexane, acetone, and cyclohexane had good olubilitie; cyclohexane in particular diolved Cu(dibm) 2 excellently. Depoition from cco 2 uing thee olvent were carried out one at a time. Figure 4 how film thickne and (111)/(200) relative intenity. Acetone demontrated thicker and better microcrytallinity, with a relative intenity higher than that of any other olvent. The reulting bet olvent wa thu acetone. 250 ] g200 [ m ) 2 m150 ib (d u C f 100 o y ilit b 50 lu o 0 ethanol Propanol Hexane Acetone Cyclo- 200 ) m160 n ( 120 e n 80 k i c 40 h t 0 Cyclohexane Acetone Non olvent Hexane Hexane (111)/(200) relative intenity Figure 3. Solubility of Cu(dibm) 2 in olvent uch a ethanol, propanol, hexane, acetone, and cyclo-hexane. Figure 4. Acetone wa thicket and had the highet (111)/(200) relative intenity. Acetone i thu the bet olvent. Figure 5 how the dependence of temperature between the inlet and midpoint of the reaction chamber on the pre-heating ytem temperature. The temperature difference between the inlet and midpoint decreae a pre-heat temperature increae and above 150 O C, inlet temperature nearly reached the midpoint temperature. The pre-heating ytem mut heat the chamber to lightly above 150 O C. Temperature of 180 O C and above are not acceptable becaue they would initiate depoition in the pre-heating ytem and thereby conume the precuror.

4 240 ) 220 ( re 200 tu p e ra 180 m te midpoint inlet pre-heater temerature ( ) ) 4000 m n ( 3000 e n k2000 ic h t1000 ilm F Depoition time (h) Figure 5. Temperature difference between inlet and midpoint of the chamber decreae above a pre-heater temperature of approx. 150 O C. Figure 6. Depoition time dependence of copper film thickne over 1-8 hr at 240 O C, 10 MPa, with H 2 and Cu(dibm) 2 concentration of 1.2 mol% and 57 ppm, repectively. The depoition time dependence of copper film thickne i hown in Figure 6. Depoition wa carried out at 240 O C, 10 MPa, and H 2 and Cu(dibm) 2 concentration of 1.2 mol% and 57 ppm, repectively, and depoition time varied from 1 to 8 hr. Film thickne increaed with depoition time, and a good upward linear relationhip wa oberved in the range of 1-8 hr. The film growth rate wa 6.95 nm/min. The film thickne after 8 hr of depoition wa 3.2 µm. A long-term depoition wa performed uing our developed flow-type reaction ytem to enable the continuou upply of a precuror over a long period of time and to allow the independent and precie control of depoition parameter uch a depoition temperature, time, preure, and the concentration of H 2 and the precuror. High depoition rate uch a thoe around 30 nm/min are thu conceivable if thee parameter are adjuted to optimum condition. High growth rate and thicker film are critical to MEMS proceing deign. Figure 7 how the temperature dependence of the copper film growth rate between 200 O C and 260 O C. Depoition condition were 10 MPa, 60 min, precuror concentration 57 ppm, and H 2 concentration of 0.4 ml% and 1.2 mol%. The calculated activation energie for the H 2 concentration were 0.38 ev for 0.4 mol% and 0.41 ev for 1.2 mol%. The activation energie obtained in thi tudy were lightly lower than the value we reported previouly (approx ev [2]; ee dahed line in Fig. 7). In our previou tudy, copper(ii)hexafluoroacetylacetonate wa alo ued a a precuror, but without the addition of a co-olvent. The lowering of activation energy oberved in thi

5 3.5 3 Cu(dibm)2, H2 0.4% Cu(dibm)2, H2 1.2% Cu(hfac)2, H2 1.2% Cu(hfac)2, H2 0.8% ) 2.5 Ṙ (Ġ 2 ln /T Figure 7. Temperature dependence of film growth rate between 200 O C and 260 O C, 10 MPa, 60 min, Cu(dibm) 2 concentration of 57 ppm, and H 2 concentration of 0.4 mol% and 1.2 mol%. tudy i aumed to be the effect of the co-addition of acetone a a olvent. Freetanding film obtained from a long-term depoition are hown in Figure 8. The depoition condition were: 240 O C, 10 MPa, 57ppm Cu(dibm) 2, 1.2mol% H 2 ; the left film wa depoited over 4 hr and the right over 8 hr. The copper film depoited on the heat block and later removed wa hiny, howed good uniformity, and wa approx µm in thickne. The overall film wa about 40 mm in length and 8 mm in width. 90µm 240 8h 240 4h Figure 8. Freetanding thick Cu film obtained from a long-term depoition of 4-8 hr. 40 mm length x 8 mm width x µm thickne. Figure 9. A cro-ectional SEM image of Cu filling in via with 10 µm dia. x 90 µm depth. The inet how a cloe-up of a via. Figure 9 diplay a cro-ectional SEM image of Cu film depoited inide via 10 µm in diameter and 90 µm in depth. The inet how a cloe-up of a via with 2 µm of film (in thickne) depoited at the top, but the film thickne depoited on the wall of the via decreaed a depth increaed. Depoition wa carried out under the following condition: 240 O C, 8 hr, 10 MPa, 57 ppm Cu(dibm) 2, and 1.2 mol% H 2. Adjuting the depoition parameter to optimum

6 value caued the growth rate to increae even more, and thu the excellent filling of gap in via everal µm in diameter now appear to be a poibility. 4 CONCLUSION We tudied the depoition characteritic of Cu film from cco 2 uing Cu(dibm) 2 diolved in acetone and long-term depoition. Our flow-type depoition proceor wa deigned to enable the independent control of depoition parameter and long-term depoition, wherein a precuror diolved in acetone i continuouly upplied to a reactor chamber. Reult of our tudy indicate that a long-term depoition wa uccefully completed and that a thick film wa obtained. Through the optimization of the depoition parameter, we expect to enable further increae in the growth rate and the creation of thicker film. We believe that it will be poible to enable the formation of MEMS-baed thru via of 3D IC uing our proceor. 5 REFERENCES [1] M. Matubara and E. Kondoh, 39 th Autumn Meeting of Society of Chemical Engineering Japan, (Sep. 2007) [2] E. Kondoh and J. Fukuda, J. Supercritical Fluid, in printing (accepted manucript became available online on 27 December 2007).