化合物半导体的选择性蚀刻

The term etching is used to describe any technique by which material can be  uniformly removed from a wafer, or locally removed as in the transfer of patterns during  fabrication of a microcircuit. The basic mechanism of wet chemical etches of  semiconductors is the formation of an oxide, or oxides on the surface, and the subsequent  dissolution of the oxidized products by either acids or bases. Wet chemical etching of  any material can be thought of as a sequence of five steps:

1) Transport of the reactant to the surface 2) Adsorption of the reactant 3) Reaction at the surface 4) Desorption of reaction products 5) Removal of reaction products away from the surface

Many etchants have been reported for GaAs, however, very few of them are  isotropic. This is because the surface activity of the Ga and As faces is quite different.  The As face, terminated on arsenic, has two unsatisfied bonds per atom. Consequently, although some reconstruction occurs in the surface layer, it is still more reactive than the  Ga face, and thus etches at a faster rate. As a result, most etches give a polished surface  on the As face. The Ga face etches much slower, because it has no unsatisfied bonds, and  tends to show up surface features and crystallographic defects. Almost any combination  of oxidizer and oxide-dissolver will operate as an etchant. In most GaAs etchants the  oxidizer is H2O2. The acid is generally one of the more common ones, such as H3PO4,  HNO3, H2SO4, HCl, or C6H8O7. Ammonium hydroxide-hydrogen peroxide mixtures are  also popular because of the controlled etch rate of ~3000Å/min for a 1ml NH4OH:700ml  H2O2 solution. There are also mixtures that exhibit strong anisotropy.

Electrons also drift at a much higher velocity than ions. As electrons gain kinetic  energy from the electric field, their effective temperatures increase above the gas  temperature. While the temperature of the atoms and molecules in the gas remains near  ambient, electrons can attain high average energies, typically 1-10 eV, corresponding to  an effective electron temperature of 104 -105  K .(49-51) This energy is transferred to the gas  by collision processes in which ions and highly reactive species are created. It is this  property of the plasma that allows high-temperature type reactions to occur at low  ambient temperatures and permits the use of temperature sensitive materials such as  organic resist masks for etching.

The large difference between electron and ion mobilities also creates a sheath near  the powered electrode. Since the coupling capacitor suppresses dc current components,  electrons can accumulate at the electrode surface which assumes a negative dc voltage  superimposed on the time-average AC potential. The powered electrode reaches a "selfbias" negative voltage, VDC, with respect to ground. Similarly, when an electrically  isolated surface (such as an insulating substrate isolated from ground by an insulating  film) is in contact with the plasma, it must receive equal electron and ion fluxes at steadystate. Following the same reasoning as above, the isolated surface must acquire a negative potential with respect to the plasma to retard the motion of electrons and  enhance the motion of ions to equalize the fluxes of both carrier types. The potential of  the isolated surface with respect to ground is referred to as its floating potential, VF. As in  the case of the grounded walls and the powered electrode, the isolated surface is  surrounded by a sheath of reduced electron concentration. The sheaths are typically a few  millimeters thick.

Fig1

Etch processes are judged by their rate, selectivity, uniformity, directionality,  surface quality, and reproducibility. While typical wet etch techniques exhibit excellent  selectivity and allow processing in large batches, they suffer from the inability to transfer  pattern sizes smaller than about 1µm with the degree of fidelity and control required for  the manufacture of VLSI/ULSI devices. Because wet etching is isotropic, the horizontal  pattern dimensions must be much larger than the thickness of the film to be etched due to  the inherent undercutting. These problems are solved by replacing isotropic wet chemical etching with dry etching techniques. Other advantages of dry over wet etching are the  reduced chemical hazard and waste treatment problems, and the ease of process  automation and tool clustering.Since most dry etch processes use a low pressure gas  in the form of a plasma to provide etchants, dry etching has become synonymous with  plasma etching. There are, however, non-plasma etch techniques (such as photo-chemical  and vapor etching) that are receiving increased attention and fall in the category of dry  etching. While a wide variety of phenomena may play a role, etching mechanisms can be  grouped into four basic categories, as shown in Figure 3.

Fig3

The samples used in this study included GaAs substrates grown by liquid  encapsulated Czochralski (LEC) which were undoped, semi-insulating (100) orientation  and undoped, 5000Å thick Al0.22Ga0.78As layers grown using triethylgallium (TEG),  arsine (AsH3), and dimethylamine alane (DMAA) in a Metal Organic Molecular Beam  Epitaxy system.

Most of the etching was performed at room temperature (21°C), but temperaturecontrolled baths were used for activation energy determination over the range of 0-90°C.  All etching experiments used a two part etch mixture. First, anhydrous citric acid crystals  were dissolved in deionized water (DI H2O) at the ratio of 1g C6H8O7:1ml DI H2O. The reaction is endothermic and hinders quick dissolution, so the citric acid is mixed with  deionized water at least one day in advance to ensure complete dissolution and room  temperature stability. Approximately fifteen minutes before conducting any material etching, the liquid citric acid/water mixture (considered as one part C6H8O7 in this thesis)  was mixed with 30% hydrogen peroxide at a given volume ratio (x parts C6H8O7 to 1 part  H2O2 by volume). The fifteen minute delay is used to allow the etchant to return to room  temperature, if any temperature changes occur due to mixing.