Four basic mechanisms are encountered for the removal of material from its surface under low pressure conditions:
I. Sputtering
II. Pure chemical etching III. Ion energy driven etching IV. Ion inhibitor etching.
A basic overview of each of the process characteristics is shown in Figure 33.
Figure 33 – The Four Basic Etching Mechanism at Reduced Pressure
Sputtering is an unselective, anisotropic etching process that is strongly dependent on the incident angle of the ion (244). Maximum etching yield is generated at the normal to the
Page | 99 surface with little effect at a grazing angle, achieving nearly vertical etching (216). This mechanism depends on the energy imparted by the incident ion. Ions need to break the binding energy of the surface atoms, resulting in a rate of one ion for atom removal. Sputtering is particularly useful for removal of involatile target atoms, but by its nature is very slow (244).
Pure chemical etching (PCE), species are supplied from gas phase reactions in the plasma (244)
. This process is inevitably highly chemically dependent and exhibits isotropic profiling (216)
. Isotropic interaction describes the etchant atoms arriving in a near uniform angular distribution through the reduced pressure. Conformal etching of the surface is attainable with these conditions. This state remains unless a particular resistant crystal orientation exhibited, and the etching preference is developed (244). Subsequently etch rates are high with a substantial flux of etchant species, but the limiting factor is dependent on the complex chemical interactions at the surfacing leading to the etchant product.
Ion enhanced energy driven etching, delivers both the etchant species and ions to the working surface (244). Effective blending of these treatments offers a superior etching process greater than sputtering and pure chemical etching individually (293). Investigation into this etching mechanism shows that it is predominately chemically etched, with a reaction rate determined by the ion bombardment (244). Etching rate increases in coordination with increased ion energy above a threshold energy of a few volts. Volatile etching products are produced as they are for pure chemical etching, combined with bombarding ions possessing high energy. These ions propagate through the medium with a highly directional angular distribution promoting highly anisotropic etching. Ion contribution to the processes may have a reduced selectivity, so careful consideration of the etching characteristics are required to develop a balance between anisotropic and selectivity (244) (294). The associated mechanism for etch product formation is not well understood and comprehension of this is given by an empirical model. Ion enhanced inhibitor etching involves the delivery of the etchants, energetic ions and inhibitor precursor from the plasma discharge (244) (283). The inhibitor molecules absorb or deposit on the substrate surface forming a protective layer or polymer film. Specific selection of the etchant material is taken in order for the chemical etch process to occur at high etching
Page | 100 rates in the absence of ion bombardment or inhibitor layer (281) (282). Ion bombardment prevents the inhibitor layer from developing or eradicates its current formation, revealing the surface for the chemical etchant. Regions that avoided the ion flux protect the surface from chemical exposure due to the remaining inhibitor layer. Inhibitor precursor molecules deposit to form polymer films on the surface, as interaction of ions expose an area for chemical etch, the side walls are re-deposited with the inhibitor to produce anisotropic etch condition (288). Careful selection of precursors, gas mixtures and conditions allow the etching process to occur without significant defects, becoming synonymous with the ion enhanced etching technique (244) (295).
5.7 Chemical Framework of Semiconductor Etching
Carrier gasses for establishing the plasma discharge that deliver the etchant source creates a complex configuration based on etch rate, selectivity to the mask & under layer and anisotropy. In conjunction with the introduced materials, the carrier gas becomes dissociated in the plasma generally into more reactive species, aiding or prohibiting the etching process. Flamm (296) summarised the chemical constituents and the dissociated products formed from the carrier gas;
Saturates: CF4, CCl4, CF3Cl, COF2, SF6 …etc. Unsaturates: CF, CF2, CF3, CCl3 …etc.
Etchants: F, Cl, Br, O …etc. Oxidants: O, O2 …etc. Reductants: H, H2 …etc.
Page | 101 It is found that these species are utilized for the etching process, reacting in the gas phase or upon the sample surface. Generally encountered are the following reactions;
In the case of particular substrate compositions a situation is encountered where the unsaturates become an etchants for example SiO2;
Three bodied reactions are experienced are at low pressures;
( ) ( )
These reactions may not be as important in the gas phase, becoming significant during surface interaction or at higher gas pressures. If oxidants or reductants (O2 or H2) enter the feed gas or exist with the gaseous composition, further reactions can arise;
Principally the overriding process parameter for the etching mechanism is the ratio of etchant to unsaturate flux at the substrate surface, with a high ratio leading to isotropic etching while a low ratio may permit film deposition (216). An intermediate phase is experienced if a controlled level is maintained; promoting the ion inhibitor process whereby sidewalls are coated and the trench is excavated (288). This anisotropic regime may be induced by the introduction of etchants (Cl2, Br2) into the feed gas mixture to enhance the etchant/unsaturate ratio or by introducing oxidants that incinerate the unsaturates (244) (296). Contrary to this is the suppression of this ratio as it is utilized to promote the protection of sidewall. Inert gases may added to the feedgas to control the electrical discharge of the plasma and substrate temperature, dilute the etchant mixture or to adjust the gas-phase chemistry through invoking
Page | 102 mechanism such as Penning ionization or excitation (250). Alternative substitutions involved in the gas mixture can rupture the protective oxide layers and to remove contaminants on the sample.