Plasma processing technology is extremely important for manufacturing the very large scale integrated circuits. Because plasma processing is anisotropic, plasma discharge is widely used in deposition, implant and etch. Furthermore, plasma processing is the only technology available for sidewall angle control. What is plasma? A plasma is an ionized gas with equal numbers of free positive and negative charges. The plasma used in the semiconductor industry is a weakly ionized plasma. The electrons are not in thermal equilibrium with ions. Because the plasma discharge are electrically driven and weakly ionized, the applied power preferentially heats the mobile electrons, while the heavy ions efficiently exchange energy by collisions with the background gas. So the electron temperature Te is much greater than the ion temperature Ti for those low pressure processing plasmas. The typical plasma density is 108-1013 cm-3 and the typical electron temperature is Te=1-10 eV for low pressure plasma discharges.
Because the mass of electrons is much less than that of ions, the electron thermal velocity (kTe/m)1/2 is much greater than the ion thermal velocity (kTi/M)1/2. Therefore fast moving electrons will rapidly be lost to the walls or other boundaries and slow moving ions are left behind. Thin positive ion sheaths form near each wall or boundary to ensure charge balance. A electric field pointing from the plasma to the wall or boundary is created within the sheath. Ions that enter the sheath are then accelerated into the wall or boundary. Because of the sheath formation, the energy of ions reaching the wafer surface can be very large and the velocity of ions can be directional. The plasma is divided into the bulk plasma and the sheath. The bulk plasma is quasineutral, whereas the sheath is positively charged since it has relatively very few electrons within it. The sheath region is a relatively dark region because of the lack of electrons. The bulk plasma is also divided into a presheath and an interior region. The presheath is at the outside of the bulk plasma, next to the sheath. The presheath is responsible for most of the accelerations of the ions before they enter the sheath. In general, the ion speed reaches the Bohm speed uB=(kTe/M)1/2 before the ion enters the sheath. The presheath will be of order of an ion mean free path in thickness since this is the distance within which the ions can expect to accelerate freely. In a typical radio frequency (RF) capacitively coupled low pressure discharge, the typical time ion cross the sheath is about micro-seconds. At lower frequency (<MHz), ion cross the sheath in less than one RF cycle, so ion energies can vary very much. At higher frequency (>MHZ), ions take several RF cycles to across the sheath and gain energy over these cycles, so ion energies do not vary much. In practice, the standard frequency used in the semiconductor industry is 13.56 MHz.
Applied Materials
DPS etcher for metal and silicon etch; Etch Centura for metal, silicon and
dielectric etch; Magnetically enhanced HART Centura for silicon etch; eMAX, MxP,
Super e Centura and IPS Centura for dielectric etch.
Lam Research
TCP9400 for polysilicon etch; TCP9600 for metal etch; Exelan and 4520XL for
dielectric etch.
Tokyo Electron Ltd.
TEL Unity series including DRM etcher and Telius series for dielectric
etch.
I proposed a collisionless sheath model based
on Lieberman's collisionless step model. In Lieberman's sheath model, ions
are assumed to respond to the average electric field inside the sheath.
So this sheath model is applied for high frequency RF discharge. In my
model, I use the instantaneous electric field for ion motion. In other
words, ions respond to the instantaneous electric field in my model. I
also got an analytical solution by supplying a oscillating voltage with
DC component Vdc at the powered electrode instead of the oscillating
current used by Lieberman. In the following figures, two simulated ion
energy distributions based on Lieberaman's model and my model are shown
for two different plasma conditions.
By using my own sheath model, I simulated the potential distribution and ion trajectories near the wafer surface for a trench etch. The result is shown in the following figure.
