Reactive ion etching

Reactive ion etching is a dry etching technique used in microelectronic fabrica- tion due to the cleanliness and controllability of the technique. RIE is a technique that uses discharge plasmas for creating the reactive species that will react with the sample. The plasmas are maintained at low pressure so a physical component of the etching due to ion bombardment is also present. The processing equipment is composed of:

• Vacuum chamber. Used for controlling the environment and containing the low pressure cold plasma that promotes the etching reaction.

• Gas control system. Used for controlling the reactants that are used and the fluxes that are insufflated into the reaction chamber.

• Pressure control system. The pumping system is controlled for providing a constant pressure in the chamber for a given gas mixture and inlet flux.

• Discharge system. RIE technique uses cold discharge plasmas for creating the reactive species. The plasma is created by ionization of the gas mixture insuf- flated in the chamber.

Reactive ion etching uses RF discharge plasmas. An RF signal is applied to two electrodes to initiate a discharge and ionize the gases in the chamber. The reac- tive species are prevented to diffuse out of the plasma by the high frequency of the signal, which has to be higher than the mobility of the species in the plasma. A nor- malized industrial frequency, 13.56 M Hz, is used for RIE reactors to ensure that the discharge signal frequency is sufficiently high. Conversely, the degree of ionization in the plasma is controlled modifying the power of the discharge signal.

Autopolarization voltage

One of the main parameters governing the plasma behavior in the reaction cham- ber is the autopolarization voltage VDC. Electrons in a plasma move faster than ions

and arrive faster at the containing surfaces. Thus, the surface containing a plasma, ie. the chamber walls, will be negatively charged with respect to the plasma. A narrow region were this potential drops is formed and it spans several Debye lengths, this re- gion is called plasma stealth. The positively charged species are accelerated through the stealth toward the chamber walls. The voltage difference that builds up at the

stealth is the autopolarization voltage of the plasma. This voltage is self-adjusted to balance the high energy electron and ion fluxes toward the containing surfaces.

In RF discharge plasmas a capacitor is connected in series with the smallest elec- trode, the susceptor holding the samples to be processed. The effect of the decoupling capacitor is modifying the stealth width. The plasma satisfies the equation

V1 V2 = A2 A1 4 (2.1) were Ai and Vi are the area and the polarization across the plasma stealth for

surface i. Consequently, the polarization across the susceptor stealth is much higher than for the other surfaces, directing the reaction toward the sample. The plasma characteristics impose that VDC depends on the reactor geometry (2.1) and the ion-

ization degree, following

VDC ∝

s PRF

p (2.2)

were PRF is the power of the discharge signal and p the chamber pressure. This

relation shows that the polarization voltage, and therefore the acceleration of the reacting species, may be controlled externally with the processing parameters. For more details on plasma processing principles the interested reader may refer to the classical text by Rossnagel, Cuomo and Westwood [88].

Etching process

The plasma contained in the RIE chamber creates reactive species that are formed by the gas ionization. The reactive species are accelerated toward the sample by the above described stealth polarization voltage. Therefore, the sample is subjected to the chemical reaction with the species in the plasma and to physical bombardment by the accelerated ions. The parameters that can be used to control the process are:

• Gas mixture. By controlling the nature and ratios of the gases in the chambers, the amount and chemistry of the reactive species is modified.

• Chamber pressure (p). It is used for controlling the mobility and quantity of the species in the chamber.

• Inlet flux (φ). For a given working pressure the inlet flux determines the resi- dence time of the reactive species in the chamber.

• Autopolarization voltage (VDC). For a given pressure the discharge power is

adjusted for determining VDC and therefore the acceleration of the species.

Adjusting p and VDC the processes can be controlled to promote the chemical or

physical component. A certain degree of sputtering is always present due to the high polarization voltages (≥ 200 V ) but it is useful as it helps desorbing the reaction by-products. The reaction residues have to be volatile or sputtered to desorb in the vacuum chamber and prevent the formation of residues on the etched surface.

Conversely, adjusting p and φ controls the directionality of the etching. RIE is a highly directional technique due to the physical component of the etch. It is actually used for high aspect ratio profile definition. Nevertheless, the chemical component may incorporate a certain degree of isotropic etching. If the residence time of the species is high, the chemical component could contribute to etch along the in-plane direction, depending on the species nature. This feature will be heavily exploded in this thesis, as described in chapter 4.

RIE usage in this thesis

The Oxford Plasmalab µ80 reactor located at the ISOM facilities has been used for this dissertation. The optimization of the etching procedures has been performed for several III-N materials, NCD and sacrificial silicon etching, as discussed in chapters 3 and 4.

The optimization of the following etching procedures will be discussed: • Etching of III-N materials in chlorine and fluorine plasmas

• Etching of NCD in oxygen plasmas • Etching of silicon in fluorine plasmas

The particularities of each of the optimized techniques will be discussed in the following chapters when presenting the research results.