Plasma Technology for Advanced Devices
MERIE and high density plasmas
Slide 1 shows a comparison between low / medium and high density plasmas. Magnetically Enhanced Reactive Ion Etching (MERIE) employs a medium density plasma. Alternative ways to generate low and medium density plasmas are capacitively coupled plasma sources with relatively low exitation frequencies and powers. High density plasma are usually generated by inductively coupled plasma (ICP) sources or sources based on the effect of electron cyclotron resonance (ECR). High density sources operate typically at lower pressures and are characterized by lower ion energies (slide 1).
Typical high density sources for plasma processing have a one order of magnitude higher radical and two order of magnitude higher ion density than medium density plasmas. These properties benefit chemical surface reactions and hence high density plasma sources are the sources of choice for silicon and metal etch (slide 2).
Slide 3 illustrates that for MERIE sources generate conditions under which the etch reaction requires the presence of neutral active species at the surface because the ion densities are too low. For high density plasmas, the theoretically explained by ion driven reactions only. This is most likely the reason why high density plasmas show very high resist erosion rates in oxide etching, the ion flux densities are too high (slide 3).
Slide 4 compares the sources with reaspect to the effects observed on the wafer. High density reactors were introduced primarily because ion density and energy are decoupled. This gives major advantages in gate etching with very thin gate oxides. The drawback of high density systems is the high density of ions, electrons and photons which can lead to enhanced resist erosion and bending, especially 193 nm resist. Dual frequency capacitive systems can operate in the low and medium density regime and are quasi decoupled. They are widely being used in dielectric etching. The degree of decoupling is higher the higher the source and the lower the bias frequency. High density etchers are used when the etch is very chemical (too high a radical density would make the etch hard to control) and when the passivation comes at least partially from the eroding resist, for instance metal etching.