Matthew Thorum

Advanced metallization scheme for 3×50µm via middle TSV and beyond

An advanced Via-Middle TSV metallization scheme is presented, featuring a high conformal ALD oxide liner, a thermal ALD WN barrier, an electroless NiB platable seed and a high throughput copper ECD filling process. Because of the high conformality of the WN barrier and NiB seed, these layers can be deposited very thinly, reducing the cost significantly, while still guaranteeing continuous barrier/seed layers all along the TSV sidewall till the bottom of the TSV. This metallization scheme has been successfully processed on 3 m diameter and 50 m deep Via-Middle TSVs, showing void-free copper fill in FIB cross-sections. Good adhesion between liner, barrier and seed was maintained during the processing, including the post plate anneal which is present to reduce the copper pumping. The polishing of the NiB seed and WN barrier has been optimized in order to limit the recess of these layers at the top of the TSV. Electrical results are shown, proving the maturity of this new TSV middle process scheme. Low leakage current in both accumulation and depletion mode is measured. The flat band voltage is low as well, indicating little ionic contamination in the liner oxide. The intrinsic integrity of the liner and the ability of the barrier to prevent Cu diffusion from TSV to silicon has been verified, using the established controlled I-V method. High field acceleration factors are extracted, ensuring good reliability of this advanced and scalable 3×50 TSV middle module.

Advanced integrated metallization enables 3D-IC TSV scaling

Innovative solutions have been developed to address the challenges of through-silicon via (TSV) metallization with small sizes and high aspect ratios. We demonstrate an advanced metallization scheme including conformal film depositions of metal barrier and seed with excellent sidewall coverage to achieve void-free Cu fill in small-size (10 to 1 μm) TSV with high aspect ratio (10:1 to 20:1). In addition, it reduces the field metal thickness to significantly lower the costs of metallization and subsequent CMP. TSVs fabricated using this new process integration scheme exhibited higher breakdown voltage and lower leakage current than those made with the conventional PVD barrier seed. No degradation in performance was observed after 400°C annealing and thermal cycling. The improved performance is attributed to the formation of pinhole-free metal barrier layer with excellent sidewall coverage.

Reliability study of liner/barrier/seed options for via-middle TSV's with 3 micron diameter and below

In high aspect ratio TSVs, the step coverage (conformality) of liner, barrier and seed is critical for both the integration and reliability. If the conformality of a deposition technique is improved, the required thickness to be deposited on the field of the wafer can be reduced. Consequently, less material needs to be removed by CMP on the field, which reduces the manufacturing cost. In this paper, the reliability of two liner/barrier/seed options, which were successfully integrated into via-middle TSVs with a diameter of 3 micron and an aspect ratio (AR) of 17 is investigated. Both controlled ramp rates (IVctri) as well as standard Time Dependent Dielectric Breakdown (TDDB) at 100°C were employed as electrical testing methods to investigate the dielectric and barrier reliability properties of the studied systems. The first studied system consists of a non-conformal CVD O3 TEOS liner, an ALD TiN barrier and a PVD Cu seed. The second studied system employs a conformal ALD liner, a thermal ALD WN barrier and an ELD NiB seed. Both studied systems show excellent reliability properties. Scalable highly conformal liners are more sensitive to local field enhancement at the high fields applied during highly accelerated tests which are far above normal operation conditions. Their performance at lower fields, however, still meets standard reliability specifications.

An alternative approach to backside via reveal (BVR) for a via-middle through-Silicon via (TSV) flow

An alternative scheme has been developed to combine three major backside via reveal (BVR) processes, including (a) wafer polishing, (b) Si recess etching, and (c) wet clean, into an integrated wet etch process to replace the high cost chemical-mechanical planarization (CMP) and dry etching steps. The process combines two steps on a single-wafer platform: (1) bulk Si etching chemistry with high etch rate (>10 μm/min.) to replace the CMP or polishing and (2) selective Si etching chemistry (Si: SiO2 ~ 1800:1) to replace the Si recess dry etching step. Using this process, Si thickness uniformity can be significantly improved (for 20 μm Si removal), resulting in a lower variation in step height of through-Si via (TSV) protrusion across a 300 mm wafer. The overall cost is significantly lower than CMP plus dry etching. After the integrated wet etching process, passivation layers of low-temperature silicon nitride and oxide were deposited on the backside, followed by CMP to planarize the wafer and expose the Cu nails. The film adhesion is very good without showing any film delamination or peeling. This new integration scheme is robust with a wide process margin and provides cost savings over the conventional BVR flow.

Thermo-mechanical behavior of copper TSV and the effect of alternative metal liners

Large thermal stress of copper in through-silicon vias (TSVs) threatens the reliability of 3D integrated circuits (3DICs). Understanding the thermomechanical behavior of Cu in TSVs will help to control defect formation. In this paper, Cu protrusion and microvoid formation were studied under various anneal conditions. Also, the effect of alternative barrier/seed layers on these defects was investigated using atomic layer deposition (ALD) WN and electroless deposition (ELD) NiB as an alternative barrier/seed metallization scheme. Higher anneal temperature and longer anneal time resulted in increased protrusion. The TSVs with a WN/NiB liner showed less Cu protrusion compared to TSVs with a standard PVD Ta/PVD Cu liner under the same anneal conditions. The optimum anneal temperature for minimal defect formation was somewhat higher for WN/NiB liners. Large protrusion caused by over annealing increased microvoid size in the Cu, potentially degrading the reliability of the TSV.

Continuity and reliability assessment of a scalable 3×50μm and 2×40μm via-middle TSV module

An advanced TSV metallization scheme, featuring a high conformal ALD oxide liner, a thermal ALD WN barrier, an electroless NiB platable seed and a high throughput copper ECD filling is presented. Because of the high conformality of the WN barrier and NiB seed, very thin layers can be deposited, reducing the manufacturing cost significantly, while still guaranteeing continuous barrier/seed layers all along the TSV sidewall to the bottom of the TSV. 3 × 50μm via-middle wafers, processed with this metallization scheme, are further processed through the thinning module, by using temporary bonded carriers, the backside passivation module and a copper RDL module by using a semi-additive process. The TSV resistance is measured between the 5μm thick RDL copper layer at the back side and the METPASS aluminum layer at the wafer front side. Low spread and high yield is obtained on the resistance data distribution of both single kelvin and daisy chain structures. The same metallization scheme is successfully scaled to a 2μm diameter and 40μm deep via-middle module. The conformal deposition of the barrier and the seed layer enables further scaling down to aspect ratio 20:1 through silicon vias with 5μm pitch, still ensuring the void-free bottom up copper fill by electroplating. The integrity of the liner/barrier system against Cu diffusion from TSV to silicon has been verified using the established controlled I-V method. Field acceleration factors, extracted in both copper-confined and copper-driven regime, indicate good TDDB reliability of this advanced 2 × 40μm TSV middle module.