By Phil Garrou, Contributing Editor
The 2013 IEEE IEDM was held in WDC the 2nd week of Dec. Let’s take a look at some of the key 3DIC presentations there.
Chamdrasekaran addressed challenges in future memory manufacturing for both front end 3D NAND and back end 3DIC stacking. He does not see any of the newer memory technologies making inroads against conventional DRAM or NAND in the next decade.
Micron proposes that “…3D integration faces several equipment and fundamental technology challenges. Bonding dicing and packaging equipment have not evolved to the same level of control capability as front end equipment technologies.” In terms of fundamental technologies they see challenges in thin wafer handling, thermal budget management and stress constraints.
New techniques will be required to test the silicon interposers with TSV and 3D devices keeping test costs low. Ability of probe technology to handle finer pitch TSV tips, eliminate probe created scrub mark defects, and handling of thinned wafers are all viewed as significant challenges.
3D stacking also introduces intra die defects due to several processing steps, which needs to be detected and understood. Thinned die and multiple materials with different thermal expansion coefficients also create thermo-mechanical problems that can lead to test and probe issues.
Doug Yu and the TSMC packaging group described the integration of Rf chip and array antennas using FO-WLP packaging technology.
Low dielectric constant and thick substrate are two essential factors for a wide-bandwidth patch antenna. The band for 60 GHz system in US is from 57-64 GHz, which can be achieved with mold cmpd. (MC) dielecric constant of 4 and MC thickness of MC, h1, is chosen to be 300 μm
LTCC and PCB substrate are currently used for integration of mm-wave antenna with RF chip but there are power consumption issues. The high power dissipation results from interconnect losses from chip to antenna through bumps or balls. “Antenna-on-chip” is other proposed solution to reduce the signal loss, but silicon is lossy and degrades the antenna efficiency.
The TSMC antenna structure is depicted in the figure below. It consists of two RDLs sandwiching mold compound. One is for patch radiator with the size of w × l = 890 × 1020 μm2 on the top of MC and the other one is for feeding structure embedded in the polymer on the bottom of MC as shown in the figure.
This FO-WLP array antenna integrated technology has been approved for millimeter wave system applications. High gain array antennas of 14.7 dBi, low-loss interconnects of 0.7 dB and small form factor of 10 × 10 × 0.5 mm3 can be achieved with the technology. So called “InFO-WLP” is reported to be an excellent technology for system scaling of low power millimeter wave applications on high-data-rate wireless communication.
Comparison of power savings, parasitics and system performance/size is shown below.
Paul Franzon presented on applications and design styles for 3DIC. When comparing 2.5D interposers vs 3DIC stacking . While 3DIC technology offers a substantial advantage in interconnect power efficiency and bandwidth density (effective wiring density * bit rate), their thermal flux is higher, complicating cooling. On the other hand, interposers add more cost than just TSV processing, which will keep them out of lower cost markets such as mobile. Two big advantages for interposers are that they reduce the need for power/ground feedthroughs
and physical standards. This was a significant barrier to the adoption of Wide IO but can be overcome through adoption of appropriate standards.
Aoki of Hitachi and collegues at ASET described activities on IC stacking using vias last / backside. Compared with the “via-middle”, the “backside-via-last TSV” can reduce process cost because TSV revealing is unnecessary. The via-last TSV process has two main advantages: first, modification of the BEOL process is unnecessary and, second, reliability concerns such as the “copper extrusion (pop-up) problem” do not arise.
ASET studied a process flow using the thinning after bonding approach. This is the process used by Tezzaron. With this method, temporary bonding is not necessary, so process cost should be reduced.
The process flow of thinning-after bonding is shown below . First, copper bumps embedded with polymer are formed on three wafers. Co-planarization of the copper/polymer surface by CMP is used to form a flat bonding surface. The step height between the copper bump and polymer was kept to below 50 nm across the entire wafer surfaces. The wafers (“LSI-wafer 1” and “Si-IP”) are then subjected to face-to-face (F2F) bonding. Copper-copper bonding was achieved by applying hydrogen-radical cleaning. Next, the bonded wafer is subjected to wafer thinning and backside-via-last TSV processes Total-thickness variation (TTV) of the thinned wafer was kept to around 1.4 μm. A backside-via-last TSV was connected to copper/low-k interconnects. The diameter and length of the TSV were respectively 7 and 25 μm. After that the backside-via-last TSV processes, the bottom wafer “LSI wafer 2” is directly bonded to the previously bonded wafers in back-to-face (B2F) configuration. A three-layer-stacked CMOS wafer was successfully fabricated by the above-described processes.
To maximize interconnect resources, contact of the TSVs with the copper/low-k interconnects should be restricted to the lowest interconnect level possible. Cross-sectional SEM images of the stack is shown below.
Fabricated devices exhibit high TSV yield of at least 99.2% and low TSV capacitance (about 40 fF). The transmission performance of the TSVs is 15 Tbps/W. Copper/low-k damage is reportedly negligible after via-last TSV formation. TSV-contact wiring needs only two interconnect levels. The estimated KOZ is up to 2 μm from a TSV because of low silicon stress (less than 50 MPa).
Koyanagi’s group at Tohoku Univ reported on “Reliability Issues Related to TSVs.”
Mechanical Stress Induced by TSVs
It is known that a compressive stress is induced in the Si substrate next to Cu-TSV. Mechanical stresses decrease as the TSV size decreases whereas they increase as the TSV spacing decreases.
Cu Pop-up from TSVs
Cu extrusion (pop-up) occurs at Cu-TSV surface when Cu-TSVs are annealed at higher temperature. Cu extrusion increases as the TSV size increases and the annealing temperature increases.
Cu Diffusion from TSVs
It is very important in order to suppress Cu diffusion from TSVs that a barrier metal layer is uniformly formed with a high step-coverage within Si trenches before Cu electroplating for Cu-TSV formation. Step-coverage of barrier metal depends on the size and aspect ratio of Si trench.
Minority carrier lifetime was seriously degraded by Cu diffusion from Cu TSVs as the blocking property of barrier layer in TSV is not sufficient.
Reliability Issues in Thinned Wafer
Cu diffusion from backside surface of the surface of Si substrate is more significantly influenced
as the Si thickness is reduced. A dry polish (DP) treatment produced a superior extrinsic gettering (EG) layer to Cu diffusion at the backside.
DRAM Retention Degradation by Si Thinning
The retention characteristics of DRAM cell are degraded depending on the reduction of the chip thickness. The retention time of DRAM cell in the 20-μm thick chip is dramatically shorted by approximately 40% compared to the 50-μm thick chip.
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