Results of TSV-CMF with More Stacked Chips

3.4 Results of TSV-CMF with More Stacked Chips

It is worth noting that by vertically stacking more chips, the stopband bandwidth of the TSV-CMF can be enhanced without increasing the chip size. Fig. 3.15 shows the concept of inserting one more chip (Chip B) between the interposer and the original chip (Chip A). TSVs and bumps are employed in Chip B to interconnect Chip A above and interposer below.

(a) (b) Fig. 3.15. The configuration of the proposed TSV-CMF with multiple stacking chips

(two chips stacked on an interposer as an example). (a) The 3-D view of the structure.

(b) The cross-section view (yz-plane) of the structure.

Fig. Fig. 3.16 shows the CM transmitted coefficients |Scc21| for the cases of 2 stacked chips (chip A and B with interposer) and 1 chip (chip A with interposer). It is seen that there are two transmission zeros, one is around 11 GHz and the other is around 22 GHz, for this 2-chip case. The stopband, which is defined by |Scc21| < -10 dB, is from 9.2 GHz up to 30 GHz. Compared with the 1-chip case, the fractional bandwidth is significantly increased from 20 % to 106 %.

The reason is that the effective length of the TSV #1 – TSV #5 is doubled by inserting one more chip (Chip B) in the stack, which will increase the equivalent inductance of L1 and L2 for the simplified LC model. The effective C1 will also rise slightly due to the coupling of TSVs and metal lines. As introduced in (3.6), the two transmission zeros will be shifted lower if the L1 and C1 are with larger values.

Fig. 3.16. The simulated CM response of proposed TSV-CMF with two stacked chips.

0 5 10 15 20 25 30

Besides, by routing more in the metal layers on Chip B, inductance L1 can be enhanced more than inductance L2. As explained in Section 3.1.3, the separation of two transmission zeros will decrease when the value of L1/2L2 is increased. As a result, the parameters of Case 2 in Table 3.1 can be realized. The transmission zeros are located closer and, as a result, 10-dB suppression for CM is achieved in a wide band, as shown in Fig. 3.16. By leveraging the parasitic effect of TVSs, which are the inherent parts for 3-D ICs, the TSV-CMF can be miniaturized in horizontal size and enhanced in common-mode stopband without degrading the differential-mode signal integrity.

Because the fabrication cost for stacking more chips is much higher, this part is demonstrated only in full-wave simulation.

The improvement of CM suppression can be also observed in time-domain. All of the three structures (reference board, Case 1, and Case 2) are under the same differential PRBS excitation with the same skew. It is obvious that from Fig. 3.17, the output CM noise after Case 2 is 80 mV (peak-to-peak), which is much smaller compared with the reference board (240 mV) and Case 1 (140 mV). That the suppression is so significant is because the CM noise converted from the DM signal is always broadband, so the bandwidth of CM suppression plays an important role. Here, the benefit from stacking chip is proved again.

3.5 Summary

In this chapter, a TSV-based common-mode suppressing filter is proposed to solve the noise coupling and EMI/RFI problems in 3-D ICs. By taking the vertical interconnects as part of design, the L/C parasitics of the TSVs can be beneficial for CM suppression. In addition, since some of the circuit elements have been implemented with vertical interconnects, the area for RDL lines can be reduced. This TSV-CMF is based on a differential second-order T-model circuit and can contribute two TZs for common mode. By analyzing the circuit model, design methods of the CM stopband are also proposed. Both the TZs can be controlled by choosing the suitable combination of lumped inductance and capacitance. To validate the proposed concept, a TSV-CMF is implemented in an ABF-coated silicon interposer process with a compact size of 0.72 Fig. 3.17. CM suppression in the time domain of the TSV-CMF.

mm². The CM stopband is located at 15 GHz with an FBW of 20%. Good agreement among equivalent circuit simulation, full-wave simulation and measurement can be observed. In addition, the performance of the proposed TSV-CMF can be further improved with an FBW of 106% by using more stacked chips on the interposer.

As listed in Table 3.5, the proposed TSV-CMFs are compared with other state-of-the-art CMFs, including DM cut-off frequency (insertion loss of 3 dB), available CM stopband (|Scc21| < -10 dB), fractional bandwidth (FBW), and electrical size (normalized to λg which is the wavelength in dielectric at the frequency of lower bound of the CM stopband). It is evident that the proposed TSV-CMFs have the highest DM bandwidth and the most compact sizes with the help of TSVs. In addition, the stopband can be further enhanced by using multiple stacked chips. These results clearly indicate the advantages brought by the 3-D implementation in TSV-CMF.

Work Process DM cut-off (GHz) ^*2

CM stopband

(GHz) ^*3 FBW Area

(λg2)^*4

[32] PCB > 7 1.65 ~ 5.2 104% 0.0034

[33] PCB 9 1.9 ~ 8.9 130% 0.0073

[34] PCB 8.5 1.6 ~ 4.3 92% 0.0129

[36] PCB > 10 4.2 ~ 5.0 17% 0.6054

[41] GIPD 9 4.0 ~ >15.0 > 116% 0.0025

Case 1 ABF > 25 13.7 ~ 16.8 20% 0.0050

Case 2^*1 ABF 30 9.2 ~ 30.0 106% 0.0023

*1: Simulated results by full-wave tool

*2: Defined by |Sdd21| > -3 dB

*3: Defined by |Scc21| < -10 dB

*4: λg is the wavelength in the dielectric at the lower bound of the CM stopband

Table 3.5. Performance comparison with state-of-the-art CMFs.

Chapter 4

pter 4 A Common-Mode Filter with Three Alterable and Designable Transmission Zeroes

Common-mode filters have at least one designable CM transmission zero (TZ) which can provide a higher suppression level in certain frequency range compared with the ferrite chokes. To improve the performance further, the designers have been trying to generate more transmission zeroes to form a wider CM stopband. For example, in Chapter 3, the 2-TZ design (Case 2) has a much larger FBW (106%) compared with that of the 1-TZ design (Case 1, 20%). These benefits can also be seen in the previous work [31] – [34]. Moreover, if multiple TZs are designed closer, the CM suppression level can be even higher. For instance, in [40], a CM stopband with insertion loss of 20 dB is formed.

However, the analytic equation of the CMF with multiple zeroes is always a high-degree polynomial or a transcendental function, which makes the analytic solution difficult to be found, or even not existent. Due to this constraint, there is almost no easy method to alter the transmission zeroes in the previous works. That is, except for frequency scaling, we can hardly control and design the TZs for common mode.

In the real cases, sometimes the EMI may be a broadband issue; then a common-mode filter with a wide suppressing range is needed. However, if sometimes the radiation is strong in only few bands, or the wireless circuits (as victims) are only operated in the certain frequency range, a design with narrow-band but deep suppression will be necessary. Hence, in this work, a common-mode filtering circuit with three alterable and designable transmission zeroes is studied and proposed. By analyzing the circuit model, the analytic form of one of the transmission zeroes can be derived. Then a convenient method will be introduced to alter the other two transmission zeroes.