Abstract:

This chapter shows the surface morphology evolution of the deposits from different plating solutions, and reveals the different growing process of the deposits. Diamond pyramid structures are discovered on some low temperature deposits. These diamond pyramid structures consist of pure copper. The morphology of the electroplated copper film on the substrate side includes numerous small voids scattering across the surface of the deposit, with relatively large cracks at some portions. No voids are in the deposit on the solution side. In the electroless copper deposits, the granular particles developed on the heat-treated deposit are the most intensive.

Key words

electroless copper

glyoxylic acid

surface morphology

plating

scanning electron microscopy (SEM)

2.1 Introduction and surface morphology of the substrate

The objective of this chapter is to examine the surface morphology evolution of the electroless copper deposits during the plating process. With the changes of the ingredients in the plating solution with different reducing agents and the plating condition, the deposit may grow on the epoxy substrate in a different way, which may further affect the properties of the deposits. The chemical composition of the deposits is analysed.

The scanning electron microscopy image in Fig. 2.1 shows that the substrate before electroless copper plating has a rough, eroded like surface with many dimples, providing good adhesion of the subsequent copper film on the substrate.

2.2 Formaldehyde high temperature solution deposits

This section and the next five will show the surface morphology of eight typical electroless copper deposits. The names for the eight deposits originate from their plating solution compositions as given in Table 1.2.

With 5 minutes plating in the high concentration solution (Fig. 2.2(a)), only a small amount of the deposit appears on the substrate and the surface morphology of the substrate is still clearly presented. After 15 minutes plating (Fig. 2.2(b)), a large amount of the deposit appears both inside and at the edge of the dimples, indicating a similar plating rate in both parts. The deposit starts to cover the dimples but the edges of the dimples can still be distinguished. For the 60-minute deposit (Fig. 2.2(c)), the deposit both inside and at the edge of the dimples continues to build up at a similar rate and forms an uneven surface. The substrate is fully covered with copper deposits, but showing some signs of morphology of the substrate. For the 120-minute deposit (Fig. 2.2(d)), there is a much smoother surface. Copper deposits both inside and on the edge of the dimples fuse together, fully covering the surface of the substrate. Some features of the substrate can still be seen from the surface of the deposit. Some gaps are visible between deposit clusters.

After a circuit is made, a photo resistant coating is required to protect the circuit. This further coating process involves a curing stage at a temperature of around 140 °C. The heat treatment results in no significant change to the surface of the deposit (Fig. 2.2(e)).

After 5 minutes plating (Fig. 2.3(a)), in the lower concentration solution, the deposit starts to build up more from the inside of the dimples and the edge of the dimples is clear and sharp. After 15 minutes plating (Fig. 2.3(b)), the copper deposit is on the edge of the dimples, as well as inside. However, compared to Fig. 2.2(b), less deposit is on the edge of the dimples and copper tends to build up more from the inside of the dimples. For the 60-minute deposit (Fig. 2.3(c)), the whole surface of the substrate is covered with copper deposit. Less features of the dimples are shown from the surface except for some sharp edges. Compared with Fig. 2.2(c), the surface of the deposit is much smoother, when plated using the low concentration plating solution. For the 120-minute deposit (Fig. 2.3(d)), the substrate is fully covered with few features visible on the surface as a result of the substrate morphology. One such feature is some gaps between clusters of copper deposit. The overall deposit surface is smooth and integrated. The 140 °C treatment (Fig. 2.3(e)) does not make a significant difference.

2.3 Glyoxylic acid high temperature solution deposits

For the deposit of 5-minute plating in the high concentration solution (Fig. 2.4(a)), the surface shows basically similar features to those of the surface of the epoxy substrate. Deposit is inside the dimples but little is at the edge of the dimples. For the 15-minute deposit (Fig. 2.4(b)), the copper deposit starts to fill in the dimples. The edge of the dimples is not covered with solid deposit inside. Compared with Fig. 2.2(b), by using glyoxylic acid, the copper deposit tends to grow more from the inside of the dimples. For the 60 minute deposit (Fig. 2.4(c)), the copper deposit has grown further from the inside of the dimples, overwhelming the edge and fusing into large and smooth clusters. Some of the edges of the dimples are covered by these clusters. On the 120-minute deposit (Fig. 2.4(d)), a smooth surface with relatively rough grains is obtained. The round particles are the random copper deposits in the solution attached to the surface of the sample. The large amount of hexagon shaped grains are around 2 microns in diameter. The increased size of grains may be due to the reduction of the concentration of the chemicals in the plating solution in the last stage of plating, which reduces the plating rate and enables the grains to grow more extensively. Some voids on the surface are possibly due to the mismatch at the joint point among clusters during the growth. The heat treatment (Fig. 2.4(e)) does not cause significant change in surface morphology against the as-plated one.

Although the concentration of the copper salt in the solution is low, due to the addition of the reducing agent and high plating temperature, the low concentration plating solution decomposes very easily. Copper is plated on the surface of the substrate but also everywhere in the solution. The consumption of the chemicals due to unwanted deposition greatly affects the plating rate. At 5 minutes, copper is on both the inside and at the edge of the dimples (Fig. 2.5(a)).

After 15 minutes plating (Fig. 2.5(b)), slightly more copper deposit builds up from inside of the dimples providing more coverage on the surface of the substrate. However, due to the reduction of the plating rate mentioned above, the difference between the surface of the 15-minute and 5-minute deposit is not significant. In the 60-minute deposit (Fig. 2.5(c)), more deposit is on the surface but it is not fully covered by copper. Dimples can still be observed. The loose copper deposit debris may come from the individual deposit particles in the solution. The decomposition of the plating solution seriously affects the plating rate, resulting in less deposit plated on the substrate. The plating reaction ends in around 60 minutes.

2.4 Formaldehyde high concentration low temperature (FHCLT) solution deposit

After 5 minutes plating (Fig. 2.6(a)), copper is on both the edge and the inside of the dimples of the substrate, indicating a similar plating rate at both parts of the substrate. The result of this is that deep holes form originating from the dimples on the surface of the substrate (magnified image in Fig. 2.6(a)).