Electroless Metallization of Dielectric Surfaces
Authors: Dubin Val M , Gindilis Andrei L , Walton Barbara L , Bauers Sage R , Albrecht Amy , Norelli Kevin , Johnson David C 
Published: February 2020

Electroless plating is a conformal and low cost deposition method that can improve the critical parameters of 3D Through-Si Vias (TSV) interconnects. It enables conformal barrier/seed layer deposition to improve the scalability and decrease the cost of TSV interconnects by replacing PVD barrier/seed method, which has the disadvantages of higher cost and poor step coverage. Another major application of selective electroless metallization technology includes transparent metal mesh for flexible displays and touch screens to replace expensive and brittle ITO.

Activation of dielectric surfaces with catalytic nanolayers. Dielectric surfaces need to be activated prior to electroless plating. SiOX-NH2-Pd and TiOX-Pd activation methods were developed and characterized to achieve control growth of self-assembled catalytic nanolayers containing Pd nanoparticles. Characterization was performed using XRR, profilometry, FTIR, ellipsometry, SIMS and HR SEM. Thickness of the assembled nanolayers was measured to be < 30 nm on glass and silicon substrates. The characterization of the properties (morphology) and composition of self-assembled silane coupling agent with catalytic monolayers containing Pd nanoparticles was performed by using XRR, Profilometer and SIMS, respectively, to achieve low surface roughness (RMS <2 nm) and high uniformity (<10%, 1 Sigma) with low trace impurities (<100 ppm) of the substrate coating.
The thickness of assembled catalytic nanolayers including SiOX-NH2-Pd and TiOX-Pd was measured by using XRR, ellipsometry, profilometry and HR SEM. The thickness ranges from 2 nm to 10 nm for SiOX-NHX nanolayers and from 15 nm to 30 nm for TiOX nanolayers. The surface roughness of catalytic nanolayers was measured by using XRR and profilimetry. The thickness uniformity measured at 9 points by profilometry was < 10%, 1 Sigma for SiOX-NHX and for TiOX while the surface roughness was < 2 nm for both types of nanolayers. Composition of nanolayers and trace impurities in nanolayers were characterized by FTIR and SIMS. FTIR data showed OH and CH absoption peaks in catalytic nanolayers. Composition of nanolayers (Ti, Si, Pd, CH, NH, O) were also qualitatively characterized by SIMS with no detectable trace impurities.

Electroless metal deposition on catalytic nanolayers. Electroless NiWP (NiWBP) and CoWP (CoWBP) films (down to 30 nm thick) were deposited on catalytic nanolayers with surface roughness < 1 nm and thickness uniformity <10% @ 1 Sigma. The film properties were studied by using Profilometer, EPMA, XRD, DSC and HR SEM. Crystallization temperature was achieved of over 350 C for electroless Ni(Co) alloys. The concentrations of P and W in electroless plated films were over 12% and 2%, respectively. Electroless Co(Ni)WP(B)/Cu barrier/seed layers are effective barriers against copper diffusion with low surface roughness and high adhesion to the substrate.
Conformal Ni alloy (Co alloy)/Cu layers were deposited on catalytic nanolayers to form barrier/seed layers for Cu interconnects. Selective and conformal (>95% step coverage) deposition of electroless Ni(Co)alloys/Cu was obtained inside TSV (< 5 µm size and > 10:1 aspect ratio) if photosensitive TiOX-Pd catalytic nanolayers were used. 5-20 μm wide Cu (up to 2 μm thick)/Ni (up to 0.1 μm thick) patterns were deposited to form copper patterns (redistribution layers, copper mesh et al). The adhesion of electroless plated metal to dielectric surface was measured to be over 65 MPa by using pull test.

Plating bath metrology. Bath metrology was developed to control the quality and replenish the plating bath for both metals (Ni, Co, W, Pd) and reducing agents (hypophosphite, DMAB) in electroless plating baths (Ni, Co) with precision/tolerance (p/t) < 0.3. Standard solutions (LCL, Target, UCL) were prepared and linear calibration curves were obtained. UV-VIS analysis of Co was performed at 530 nm while UV-VIS analysis of Ni was performed at 390 nm with dilution in DI water. To analyze Pd, PdCl2 solution was diluted in IPA. To analyze tungstate, three solutions were used including solution #1: 12% SnCl2 in HCl, solution #2: 25% KSCN in DI water, and solution #3: 1.4 mL Solution #1 + 0.1 mL Solution #2. UV-VIS Absorbance data were taken at 398.5 nm. Hypophosphite and DMAB reducing agents were analyzed by using titration.