There are multiple parameters that one can control during the RIE process. The process itself is complicated and there are many ways to affect the etching results, for example, selectivity, directionality, and etching rate to name but a few. The RIE process development is generally an experimental process, where one will adjust pa-rameters until the desired outcome is reached. In most RIE applications one would change the following parameters: flow rate of reactive gas, radio frequency (RF) power, ICP power, chamber pressure, substrate temperature, and what gases are used in the process. [11–13,15] Modifying these parameters, are used for optimiza-tion for RIE processes. In some processes, one would also modify the ICP power parameter. [29,30]
2.3.1 Gas flow rate
The flow rate of the reactive gas or gases is one of the more important parameters as it highly affects the etching speed, along with RF power. Flow rate describes the amount of reactive gas per minute introduced into the chamber, usually measured in standard cubic centimeters per minute (sccm). The higher the flow, the more reactive atoms will be in the chamber, and with suitable chamber pressure, on the sample surface. In other words, there is a direct correlation between etching speed and the gas flow rate. Too high flow rate, however, can lead to short dwelling time
in the chamber, when the pressure is constant. It should be noted, that we need to adjust the RF power along with an increased flow rate. Otherwise, the generation of ions is decreased, which reduces the etching rate. If the flow rate is too low, the supply of radicals on the surface decreases, which also slows the etching rate. [13]
2.3.2 RF Power
Adjusting the RF power along with the flow rate directly alters the etching rate and selectivity of the etching process. Increasing RF power increases the probability of ionization, which usually leads to an increased etching rate. Increased RF power also makes the process more directional, hence it can increase selectivity when used in moderation. When RF power is increased, the amount of ion sputtering in the process increases, thus etching the resist with an increased rate. If the RF power is too low, the plasma might not ignite properly, hence affecting the plasma stability.
[13]
Increasing the RF power increases the self-biasing in the system. This self-biasing field increases the directionality of the process, as it increases the amount of ion sputtering in the process, thus increasing the etching rate. However, too much ion sputtering has its downsides. It can eat away the mask layer, especially if there is just a polymer mask. [13]
2.3.3 Chamber pressure
Chamber pressure in RIE is usually around 10−3–10−1 Torr and it mainly affects the mean free path of the particles inside. If the chamber pressure is low, it improves the anisotropicity of the etching process as the particles do not collide with each other as much in the chamber. Lower chamber pressure, on the other hand, increases the etching rate by moving volatile gases from the substrate surface faster as the pumping speed is higher to keep the higher vacuum state. However, too low chamber pressure can reduce plasma stability or prevent plasma from igniting. In higher chamber pressures the etching becomes more isotropic as the mean free path of ions is reduced. [12,13]
2.3.4 Temperature
As the sample is under a continuous bombardment of ions for extended periods, the surface temperature will increase. This might affect the anisotropicity of the RIE
etching as the temperature change will affect the etching rate of the process. This can be controlled by cooling the substrate. For example, cooling can be achieved by having helium gas circulating under the electrode, on which the sample is laying. [13]
Depending on the duration of the etching process, the surface temperature of the substrate can reach 100-200 degrees centigrade. This kind of temperature can start to soften the polymer resist layer on top of the substrate. This is problematic, as when the resist starts to soften, it generally distorts the mask or increases the etching rate of the mask. Both effects can be detrimental to the end product. For-tunately, this can be overcome by using other inorganic materials in which melting temperature is significantly higher than the surface temperature, or by cooling the substrate. [13]
2.3.5 Gas composition
Gas composition plays a major role in the etching process when there are multiple gases used. The ratio of different gases in the etching process affects the etching rate of the mask and the substrate. This technically determines the selectivity of the process. In the analogous structures, this ratio of gases also affects the shape of the structure as this depends on the selectivity and isotropicity of the etching. In the RIE system, the gas composition is controlled with the gas flow rate. Therefore, the gas flow rate and the ratio of different gases are the most important parameters during this work. [12,13]
Many different gases can be used to etch inorganic and metallic surfaces. When etching silicon, one can use compounds that include fluorine, chlorine, or bromine.
These radicals can be introduced with many different compounds, for example, using fluorine in carbon tetrafluoride form (CF4) or in sulfur hexafluoride form (SF6) to etch single crystal silicon. If one would want to use chlorine to etch silicon, boron trichloride (BCl3) can be used. [12,13]
Although these gases can be used by themselves to etch materials, one will gener-ally combine them with oxygen to enhance desired etching properties. For example, adding oxygen with CF4 reduces the recombination of radical gases, which in term increases the etching rate of silicon. Argon gas, on the other hand, can be used to stabilize the plasma process by introducing more electrons to the plasma. Argon also ionizes to Ar+ ions which participate in the ion sputtering during the process.
Argon is used, for example, when etching with SF6. SF6 is an electronegative gas,
so it consumes free electrons from the plasma during the process as it ionizes. Too much argon in the process can lead to unwanted amounts of sputtering and dilution of the gas composition, which can inhibit the chemical etching of the process. [12,13]
Chapter III
Process overview
In this chapter, the processes used in the practical part of this thesis work are dis-cussed. Generally, the process would start by designing a pattern for the desired purpose. This pattern is then produced via one of the aforementioned patterning methods onto a wafer. Then the process generally proceeds to the pattern trans-ferring, where one would etch the pattern from the resist layer onto the substrate.
However, in this work, the samples were already patterned. Hence, this work is focused on optimizing the etching process and the characterization of the etching product. The samples used in this work had a single blazed–style grating structure.
In this chapter, we will describe the handling of the samples during the whole pro-cess. Then we will discuss how the RIE process was conducted during this work.
Finally, we will discuss the characterization method used in this work.