Substrate bias surface characteristics

These surface modifications were performed in "pure" micro-wave (2.45 GHz, "single-mode") or in combined microwave/ radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions. 

Fig. 3 Effect of substrate bias on the surface characteristics of PVC and PC treated by Ar in MW/RF "dual-frequency-mode" plasma.

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

Interestingly, electrochemical processes are also evident in certain two-electrode STM experiments performed in air. It is well known that water is absorbed on surfaces exposed to humid environments [48,49]. When such circumstances arise in combination with certain bias conditions, me conventional two-electrode STM exhibits some of the characteristics of a two-electrode electrochemical cell as shown in Fig. 4 [50-53]. This scheme has been used for modifying surfaces and building devices, as will be described in me last section of mis chapter. In a similar vein, it has been suggested mat a two-electrode STM may be used to perform high-resolution SECM for certain systems mat include insulating substrates such as mica [50]. 

In this study, we control the film growth solely by substrate surface processes, by varying Ug and/or Tg, without affecting the bulk plasma parameters. This is possible when a third electrode, used as the substrate holder, is placed in the plasma system as shown in Fig. 1. A small amount of RF power delivered to this electrode results in a bias potential Vg which controls bombardment of the growing films by low energy ions. If the area of this third electrode is substantially smaller than that of the main RF electrode, its presence does not appreciably influence the plasma characteristics this has recently been confirmed by actinometric optical emission spectroscopy (8). 

RALEIGH I wish to make the point that if you want to use a solid-electrolyte cell to measure diffusion coefficients in this way, you don t necessarily have to measure the double layer capacitance and electron redistribution characteristics in the electrolyte. When you apply a d.c. bias that fixes a significant activity of the diffusant at the substrate surface, double-layer charging should be completed fairly soon. If you plot the cell current versus and get a significant linear... 

---