Summary of Factors Affecting Metallization/Substrate Adhesion (金属膜と基板の接着に影響を与える要因のまとめ)

In Tecdia's products, a pivotal part of the proper functioning of a capacitor is the adhesion of the metallization to its substrate. Without good adhesion, the metallization on chips will easily peel off the substrate.

As a result, many of Tecdia have Ti or TiW used as the bottom layer, known as the adhesion layer. Shown below is an example of a Ti/TiW-Pt-Au layer on a substrate.



picture 1


How can adhesion be improved? Below are the main factors that affect the adhesion between a metallization and its substrate.


Metal to substrate bond (金属膜と基板の結合)

Each metal has their own characteristic ability to bond with the substrate. Metals can form bonds with the oxygen atoms found in the ceramic of the substrate. For example, Ti can form bonds with the oxygen contained in the AlO2, forming either Ti2O3 or TiO2. How favorable it is to form a bond depends on the free energy of bonding, ΔG . A negative free energy of bonding implies that the bonding is favorable, while a positive free energy implies that the bonding is unfavorable.



Free energy of bonding numbers of Ti and Au can be compared below. (T=298K)



Ti2O3: -1434 kJ/mol

TiO2: -883 kJ/mol

Au2O3: 75.5 kJ/mol


From these numbers, we can see that the bonding reaction for Ti is extremely favorable, while the Au-O bonding is an extremely unfavorable reaction. This is expected, as Au is known to be very difficult to oxidize.


Film stress(膜応力)

A lot of Tecdia’s metallizations are formed through the physical vapor deposition process (PVD), also known as sputtering. Sputtering metals on a substrate results in stress being generated, causing the entire substrate to curve. Depending on the curvature direction, stress can be compressive(-) or tension(+). Below is an image of both phenomenon:



picture 2


Having large stress causes the metallization to easily peel off from the substrate. Stress is composed of the following two components:



1.Thermal stress(熱応力)

This is caused by the difference in thermal expansion coefficients between the metallization and the substrate.



2.Intrinsic stress(固有ストレス)

This is caused by the structure of metallization when it is sputtered. This is a function of the sputter conditions. For example, low sputter pressure results in compression, while high pressure will result in tension.



To improve the adhesion between the metallization and the substrate, care must be taken in not only choosing the correct adhesion layer for the metal, but also in selecting the proper sputtering conditions to minimize the overall film stress. We work tirelessly to find the optimal metallization conditions to ensure that our products have strong metal/substrate adhesion, as well as attaining other physical and electrical properties can satisfy the demands of our customers.