Abstract
TSV(Through Silicon Via) is a promising interconnection for the next generation smartphone, driving assistance and medical care system because of its high speed image processing and low energy consumption. Conventional TSV electrodeposition requires 15 minutes to 1.5 hour due to the application of a low current of less than 10 mA/cm2. Two shapes of vias have been used, columnar and conical shapes. Columnar shape via has acute edges at via mouth and the current lines concentrates at these edges, hence introducing void. In conical shape via, the acute angle at via bottom concentrates the current lines, hence improving bottom up filling. Using conical vias, we are able to electrodeposit a 6 μm diameter and 25 μm deep TSV via within 5 minutes. A very high on-current of 90 mA/cm2 is able to be applied without any void formation.
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TSV is a promising interconnection for the next generation smartphone, driving assistance and medical care system because of its high speed image processing and low energy consumption. Columnar shape via has acute edges at via mouth and the current lines concentrates at these edges, hence introducing void. Conventional TSV electrodeposition requires several 15 minutes to 1.5 hour due to the application of a low current of less than 10 mA/cm2. This long time electrodeposition prevents TSV realization.
K.Kondo have electrodeposited a 10 μm diameter and 70 μm deep via within 35 minutes.1,2 O.Lhun, A.ARadisic and P.M.Verecken have used a partially covered Ta cap layer and 15 minutes was required to electrodeposit the via.3,4 By using the same commercial additives, a 5 μm diameter and 40 μm deep via has electrodeposited in 1.5 hours.5 H.Kadota6 have used a pulse current with a long off-time of 1.0 sec at a current density of 10 mA/cm2. It takes 90 minutes to electrodeposit the 10 μm diameter and 70 μm deep via. T.P.Moffat has electrodeposited donut shaped via using Cl and poloxamine additives at a potential of −0.650 V SSE7 with 17 minutes. Hence a further time reduction to several minutes is demanded.
Another issue to preventing TSV realization is that TSV consumes part of the device area. With RIE(Reactive Ion Etching), our conical-shaped via forming from the back side of a device, the device consumption can be avoided. This is because the smaller conical shape bottom areas are facing device side.
Experimental
Two via shapes have used, i.e., columnar and conical shapes. A 0.1 μm tantalum and 3 μm copper layers have sputtered. The times required to perfectly electrodeposit these vias have determined by evaluating the maximum current without voids. Figure 1 shows a cross section of a 30 μm diameter and 50 μm deep conical via(a) and of a 6 μm diameter and 25 μm deep conical via(b). For the Fig. 1a via, the electrodeposition time has compared to the columnar via of 30 μm diameter and 50 μm depth. For the (b) via, the electrodeposition time has compared to the columnar via of 5 μm diameter and 30 μm depth. By gradually increasing the current density and observing the void in the cross section, the maximum current density without voids gives the minimum electrodeposition time. We further have compared the minimum electrodeposition times to the electrodeposited columnar and conical vias. For the 6 μm diameter and 30 μm deep conical via(b), the columnar via of 5 μm × 30 μm has compared. This columnar via is already known to electrodeposit within 25 minutes.8
Figure 1. Cross sections of conical vias of 30 and 6 μm diameters. (a) 30 μm diameter (b) 6 μm diameter.
The basic bath consists of 200 g/L CuSO4 · 5H2O, and 25 g/L H2SO4. HCl, SPS ( (bis (3-sulfopropyl) disulfide, PEG) polyethylene glycol) and a leveler. A rotating disk electrode has used for the electrodeposition. The current source is an HA-151 potentiostat/galvanostat (Hokuto Dneko). The TSV via silicon substrates have attached to the cathode of the rotating desk electrode. The speed of the rotating disk electrode is 1000 r.p.m. Phosphorous copper is used as the anode. The current wave has controlled by an HB-21 pulse generator (Hokuto Denko). The periodic reverse current waveform(PR current) has used for the electrodeposition. The PR current consists of three periods of Ton(On-time), Trev(Reverse time) and Toff(Off-time)(Fig. 2). The on-time is 200 ms, the reverse time is 10 ms and the off-time is 100 ms. The ratio of irev/ion is set equal to two. All the TSV samples have electrodeposited at room temperature. Prior to the electrodeposition, oxygen gas have bubbled through the solution for 30 minutes.8
Figure 2. Periodic reverse current waveform.
The via cross sections have prepared by polishing. The via silicon substrate has initially molded using a photo solidification resin and polished with emery papers and 0.3 μm Al2O3 colloidal silica. The silicon via cross sections have been observed by FESEM(Hitach-4300). We have examined the void in the cross sections by gradually increasing the on-time current, ion. The maximum ion without a void gives the minimum electrodeposition time.
Results
Electrodeposition of columnar via of 30 μm diameter and 50 μm depth
Figure 3 shows the cross sections of the columnar via electrodeposited at ion = −5 mA/cm2, −10 mA/cm2, −15 mA/cm2. The bright dark portion in the SEM micrograph center is the electrodeposited via and dark portions on both sides of the via is the silicon. At −5 mA/cm2, a 120 minute electrodeposition time has required. For −10 mA/cm2, 60 minute and for −15 mA/cm2, 40 minute times have required. For both −5 mA/cm2 and −10 mA/cm2, no voids are present. On the contrary, at −15 mA/cm2, voids form at the via bottom(indicated by the arrow). Sixty minutes at −10 mA/cm2 is the minimum electrodeposition time.
Figure 3. Cross sections of columnar via of 30 μm diameter. (a) −5 mA/cm2 120 min (b) −10 mA/cm2 60 min (c) −15 mA/cm2 40 min.
Electrodeposition of conical via of 30 μm diameter and 50 μm depth
Figure 4 shows the cross sections of the conical via electrodeposited at ion = −5 mA/cm2, −10 mA/cm2, −15 mA/cm2. At −5 mA/cm2, a 100 minute electrodeposition time is required. For −10 mA/cm2, 50 minute and for −15 mA/cm2, 35 minute times are required. For −5 mA/cm2, −10 mA/cm2 and −15 mA/cm2, no voids are present. Additional high currents should be applied for this conical via.
Figure 4. Cross sections of conical via of 30 μm diameter. (a) −5 mA/cm2 100 min (b) −10 mA/cm2 50 min (c) −15 mA/cm2 35 min.
Figure 5 shows the cross sections of the conical via electrodeposited at ion = −26 mA/cm2, −28 mA/cm2, −30 mA/cm2. At −26 mA/cm2, a 20 minute electrodeposition time is required. At −28 mA/cm2, 20 minute and for −30 mA/cm2, 18 minute times are required. For both −26 mA/cm2 and −28 mA/cm2, no voids are present. On the contrary, −30 mA/cm2 forms voids at the via bottom(indicated by the arrow). Twenty minutes at −28 mA/cm2 is the minimum electrodeposition time. A reduction in the electrodeposition time of 67% is obtained by changing from a columnar to conical via with the same 30 μm diameter.
Figure 5. Cross sections of conical via of 30 μm diameter. (a) −26 mA/cm2 20 min (b) −28 mA/cm2 20 min (c) −30 mA/cm2 18 min.
Electrodeposition of columnar via of 5 μm diameter and 30 μm depth
Figure 6 shows the cross section of a columnar via of 5 μm diameter and 30 μm depth. This columnar via is already known to be electrodeposited within 25 minutes at ion = −5.5 mA/cm2 and irev/ion = 6.9
Figure 6. Cross sections of columnar via of 6 μm diameter. −5.5 mA/cm2 25 min.
Electrodeposition of conical via of 6 μm diameter and 30 μm depth
Figure 7 shows the cross sections of conical via electrodeposited at ion = −80 mA/cm2, −90 mA/cm2 and −100 mA/cm2. At −80 mA/cm2, a 10 minute electrodeposition time is required. For −90 mA/cm2, 5 minutes and for −100 mA/cm2, 4 minutes. At −100 mA/cm2 a void forms at the via bottom (indicated by the arrow). Five minutes at −90 mA/cm2 is the minimum electrodeposition time. A reduction in the electrodeposition time of 75% is obtained by changing from a columnar to conical via with a 6 μm diameter.
Figure 7. Cross sections of conical via of 6 μm diameter. (a) −80 mA/cm2 10 min (b) −90 mA/cm2 6 min (c) −100 mA/cm2 4 min.
There are two reasons which make possible to apply the high ion of −90 mA/cm2 for the conical via of 6 μm diameter. One is due to the concentration of current lines at the conical via bottom and may improve bottom up filling. The other is the formation of a recirculating vortex of Cu(I)thiolate, the accerelator, at the conical via bottom due to the electrolyte flow.8–14
Conclusions
TSV(Through Silicon Via) is a promising interconnection for the next generation smartphone, driving assistance and medical care system because of its high speed image processing and low energy consumption. Two shapes of via have been used, columnar and conical shapes.
- (1)To electrodeposit a columnar via of 30 μm diameter and 50 μm depth, 60 minutes is required at −10 mA/cm2. For the conical via of 30 μm diameter and 50 μm depth, a shorter time of 20 minutes is required at −28 mA/cm2.
- (2)To electrodeposit a columnar via of 5 μm diameter and 30 μm depth, 25 minutes is required at −5.5 mA/cm2. For the conical via of 6 μm diameter and 30 μm depth, an extremely shorter time of 5 minutes is required. A very high on-current of 90 mA/cm2 is able to be used without void formation.