The Role of Controlled Chemical Plasma Etching in Advanced Technology Applications

In the past three decades, plasma — the fourth state of matter — has turned out to be a very useful medium for eliminating small amounts of material from an array of substrates in a fast and efficient manner.

In several highly sensitive optoelectronic and integrated circuit packaging applications, plasma processes have been employed to accurately eliminate specific materials from the surfaces of samples. In this article, the theory of chemical etch plasmas, methods to regulate the plasma etch process for these kinds of applications, and a few typical sophisticated technology applications are discussed in detail.

The Chemical Plasma

Generally, the room-temperature gas plasmas are produced in a vacuum chamber. Plasma is generated by pumping the chamber to a pre-set base pressure, followed by introducing a process gas and applying a radio frequency (RF) electromagnetic field to the electrodes in the chamber. This eventually creates a glow-discharge plasma. Several different gaseous species are generated in the plasma. Such species include photons, electrons, ions, neutrals, free radicals, and reaction by-products like ozone. They create a low-temperature and highly active plasma that can rapidly and selectively etch materials.

Regulating this kind of plasma for effective etching is believed to be a balancing exercise. It is important to ensure that the right amount of free radicals (the species that perform most of the work) and ions in and around the region to be etched balances with the RF power input within the system. Usually, the process pressure controls the amount of ions and free radicals, thereby rendering process pressure a highly significant process parameter. However, it is important to choose RF power input so that the highest etch rate is created without over etching or unintentionally impairing other materials or substrates. In addition, the process time parameter should be carefully selected because similar to power and pressure, it can considerably impact the outcome of the process cycle.

The natural selectivity of chemical plasmas gives them an edge over other kinds of plasma. In this context, selectivity can be defined as the propensity of a chemical to react with one substance instead of another. This characteristic is extremely useful in plasma. It gives the chance to modify the plasma etching process to the target substance and prevent undesirable etching of numerous other substances, which can be nearby.

Reactive ion etching, or RIE, is believed to be a type of chemical plasma. In addition to being defined by the above-mentioned parameters and characteristics, RIE is also extremely anisotropic and directional. It has several applications. This article discusses applications that are related to the processing and packaging of optoelectronics and semiconductors.

Applications

Photoresist Removal

Two plasma process applications are included in photoresist removal. The first application involves a homogenous removal of small amounts of resist across the entire surface of a wafer. This is referred to as “descum.” In this example, the etch rate needs to be moderate, and a low-reactivity process gas, like O2, is used. RF power should also be kept low. In order to improve the uniformity of the descum operation, the operating pressure should remain comparatively high and typically from 600 to 1000 mTorr. At low power and high pressure, there is a highly isotropic and even distribution of ions, leading to a highly uniform, moderately rapid etching operation.

Etching features from patterned photoresist is the other plasma application for photoresist removal. In this example, the etching operation should be both quick and anisotropic. The isotropic etch should be able to create highly vertical wall features without any undercutting. Generally, highly reactive process gases or gas combinations like CF4, or a combination of O2 and CF4, are utilized. The pressure is lower than the above-mentioned descum process, whereas RF power is increased to render the plasma as anisotropic as possible.

The increased power offsets the lack of ions to increase the etch rate, and the decreased pressure (usually in the 100 to 200 mTorr range) provides the plasma’s anisotropic nature. Over-etching and non-uniformity are the side effects of this operation. Although process time can handle any over-etching problems, increased uniformity will need a rather close balance of pressure and power. This power and pressure balance will be specific to the material and the geometry of the substrate and may involve some amounts of process development to obtain the desired outcomes.

Glass and Glass-Like Compound Etching

Etching glass or glass-like substances such as single-crystal silicon, SiO2, and Si3N4 are similar to photoresist etching in several ways. The selection of process gas is the main difference. Glass is a non-reactive or a highly stable substance, and as a result, highly reactive process gases like CF4 and SF6 are utilized. Similar to photoresist removal, the degree of uniformity and anisotropy can be governed considerably by power and pressure using the gases described.

Conversely, the etch rate is different from photoresist removal and will differ widely because glass happens to be an amorphous substance and can differ extensively in composition. Therefore, care must be taken when producing etching recipes so that valuable parts are not damaged by unintentional over-etching. Fused-silica optical fiber etching and etching SiO2, Si3N4, and BPSG in integrated circuit failure analysis operations are applications for this type of etch.

Polymer Etching

Etching polymers can be either extremely difficult or extremely easy. The difficulty comes from the fact that polymeric substances differ widely in their makeup. For instance, polypropylene covers many hundred varied compounds of polymer, all of which comply with the properties that are important to polypropylene. A single, all-encompassing etching recipe would be almost impossible to develop even if there is the slightest change in UV stabilizer or plasticizer. This is because most often, there is no common starting point. The method for etching polymers refers to the use of a combination of process gases.

When tetrafluoromethane (CF4) and O2 are mixed together for use in plasma etching, they form the oxyfluoride ion (OF), which is known to be a strong etching agent for polymeric substances. In particular, this ion is not only adept at cutting the carbon-carbon molecular bonds in the polymer backbone but also adept in removing the molecule rapidly.

One polymer etching application refers to hole boring in polyamide when the polyamide is packed between two conductive metal sheets. Furthermore, the holes are easily made using a ratio of 20% CF4 and 80% O2 at low pressure and high power. While time will depend on the makeup and the depth of the polyamide, low pressure will ensure clean and straight sidewalls in the hole. Selectivity ensures that etching is done only to the polymer present in the hole. The surrounding metal should not be damaged by the etching operation due to the selectivity of the etching process.

There is another application that refers to the etching of polyamide in bulk form, for example, off a wafer. Again, this is analogous to etching photoresist in that the process pressure is set relatively high to achieve excellent uniformity, and the power is boosted to accelerate the etching rate. This operation is also similar to etching glass because the polyamide’s composition can be changed, thus leading to unforeseeable etch rates. Usually, high pressure and moderate power are considered a good starting point for process recipe.

The final application in the discussion of polymer etch is optical fiber cladding etching. Optical fibers include a fused silica core, which has an approximate thickness of 100 µm enclosed by polyurethane cladding with a typical thickness of 125 µm. Here, the application is to etch the cladding completely off a specific section of the fiber so as to expose the fused silica core without impairing it. For uniform treatment of all the fiber optic strands, a specific isotropic plasma is required. Therefore, pressure should be close to 500 mTorr with high power. In this application, time is the most important parameter because even at 10% CF4 and 90% O2, the CF4 can still reduce the strand pull strength and damage the silica core. The fiber-optic stand should be etched sufficiently long such that the cladding is removed effectively.

Bleedout Removal

At the time of the epoxy dispensing operations, either the amount of dispensed epoxy is in excess or the substrate material causes a small amount of the epoxy to wet out across its surface. Only when subsequent wire bonding is required, this issue becomes mainly significant. Wire bond pads can be contaminated by the epoxy residue, leading to poor bond strength or even wire bond “pull-up,” which indicates total failure of the wire bond.

Therefore, to remove the epoxy contamination, an argon plasma, an argon and hydrogen plasma, or an argon and oxygen plasma is utilized around the 200 to 250 mTorr pressure range. Since argon is an inert gas, it can be highly effective in removing the epoxy bleedout through pure bombardment using Ar+ ions. To clean the surfaces of the wire bond sites, the bombardment ablates the epoxy and leaves behind a pristine metal surface.

The reaction rate is simply increased by the inclusion of a chemically reactive agent like oxygen or hydrogen. Although the quantity of reactive gas can vary, not more than 30% of the volume is added. However, there is one caveat — the addition of oxygen can oxidize silver-filled epoxies and thus turn them black. This oxidation is purely the surface tarnishing of the silver in the epoxy and does not have an impact on the potential of the epoxy to conduct heat or electricity.

Recipe Selection

The following chart can be used for choosing the right process gases for the development of the etching process recipe.

Substance Process Gases Mixtures
Photoresist O2
O2 + CF4
100%
80% + 20%
Polyimide O2
O2 + CF4
100%
80% + 20%
Polyuethane O2
O2 + CF4
100%
80% + 20%
Single Crystal Silicon CF4
CF4 + O2
SF6
SF6 + O2
100%
(80% - 92%) + (20% - 8%)
100%
(80% - 90%) + (20% - 10%)
Silicon Oxide (SiO2) CF4
CF4 + O2
C2F6
CF3H
C3F8
100%
(80% - 92%) + (20% - 8%)
100%
100%
100%
Silicon Nitride (Si3N4) CF4
CF4 + O2
SF6
CF3H
NF3
100%
(80% - 92%) + (20% - 8%)
100%
100%
100%
Epoxy Bleedout Ar
Ar + O2
Ar + H2
100%
(90% - 70%) + (10% - 30%)
(90% - 70%) + (10% - 30%)
Tungsten CF4 + O2 (70% - 92%) + (30% - 8%)
GaAs CH4 100%

This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.

For more information on this source, please visit Nordson MARCH.

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