Using Controlled Chemical Plasma Etching for Advanced Technology Applications

Over the past thirty years, the fourth state of matter, which is plasma, has become an extremely useful means of removing small quantities of material from a wide range of substrates efficiently and quickly. Plasma processes have been used in a number of highly sensitive integrated circuit packaging and optoelectronic applications in order to precisely remove particular materials from sample surfaces. This article discusses the theory of chemical etch plasmas, how to control the plasma etch process for these applications, and some typical advanced technology applications.

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The Chemical Plasma

The room-temperature gas plasmas employed are usually generated in a vacuum chamber. To generate plasma, the chamber is pumped to a pre-set base pressure, process gas is introduced and a radio frequency (RF) electromagnetic field is applied to the electrodes in the chamber producing a glow-discharge plasma. In the plasma, many different gaseous species are produced. These species include ions, free radicals, electrons, photons, neutrals and reaction by-products such as ozone. These species make a highly active, low-temperature plasma that can etch material quickly and selectively.

Controlling this type of plasma for effective etching is considered a balancing exercise. The correct amount of ions and free radicals (the species that do most of the work) in and around the area to be etched must balance with the RF power input into the system. Generally, the amount of free radicals and ions is governed by the process pressure, thus making process pressure an extremely vital process parameter. On the other hand, RF power input must be selected so that the highest etch rate is formed without over etching or inadvertently damaging other substrates or materials. The process time parameter must also be selected with care because it, like pressure and power, can immensely affect the outcome of the process cycle.

One main advantage that chemical plasmas have over other types of plasma refers to their natural selectivity. In this sense, selectivity is defined as a chemical’s propensity to react with one substance rather than another. This is a very useful characteristic in plasma. It provides the opportunity to customize the plasma etching process to the substance of interest and not lead to unwanted etching to various other substances which can be in close proximity.

Reactive Ion Etching (RIE) is considered to be a type of chemical plasma. It is characterized not only by the characteristics and parameters mentioned above, but also RIE is very anisotropic and directional. It has a number of applications, this article discusses applications specific to optoelectronic and semiconductor processing and packaging.

Applications

Photoresist Removal

Photoresist removal comprises of two plasma process applications. The first application is uniform removal of small quantities of resist over the whole surface of a wafer. This is called “descum”. In this case, etch rate is required to be moderate, and a low-reactivity process gas, such as O2, is employed. It is also necessary to keep RF power low. Operating pressure must remain relatively high and usually from 600 mTorr to 1000 mTorr in order to increase the uniformity of the descum operation. At high pressure and low power, an extremely isotropic and uniform distribution of ions exists thus achieving a highly uniform, moderately rapid etching operation.

The other plasma application for photoresist removal is etching features from patterned photoresist. In this case, the etching operation must be fast and also particularly anisotropic. The isotropic etch must generate extremely vertical wall features with no undercutting. As a rule, greatly reactive process gases or gas combinations such as CF4 or a mixture of CF4 and O2 are used. Pressure is lower than the descum process discussed above, while RF power is increased, in order to make the plasma as anisotropic as possible.

The decreased pressure (generally in the 100 to 200 mTorr range) provides the anisotropic nature of the plasma, and the increased power compensates for the lack of ions in order to increase the etch rate. The side effects of this operation are non-uniformity and over etching. Process time is capable of managing any over etching issues, but increased uniformity will require a rather close balance of power and pressure. This pressure and power balance will be material and substrate-geometry specific and may take a small amount of process development in order to achieve satisfactory results.

Glass and Glass-like Compound Etching

In a number of ways, etching glass or glass-like substances like Si3N4, SiO2, and single-crystal silicon is similar to photoresist etching. The major difference is process gas selection. Glass is an extremely stable or non-reactive substance. Consequently, highly reactive process gases like SF6 and CF4 are used. Analogous to photoresist removal, the degree of anisotropy and uniformity can be controlled to a large extent by pressure and power using the gases mentioned.

However, unlike photoresist removal, etch rate will differ extensively due to the fact that glass is an amorphous substance that can differ widely in composition. Care must be taken when evolving etching recipes not to damage valuable parts by inadvertent over etching. Applications for this type of etch include fused-silica optical fiber etching and etching BPSG, Si3N4 and SiO2 in IC failure analysis operations.

Polymer Etching

Etching polymers can be exceedingly simple or exceedingly challenging. The challenge springs from the fact that polymeric substances are extensively varied in their makeup. For example, polypropylene encompasses several hundred different compounds of polymer, all of which conform to the characteristics essential to polypropylene. The smallest change in UV stabilizer or plasticizer makes developing a single, all-encompassing etching recipe almost impossible as there is usually no common starting point. The technique for etching polymers refers to the use of a mixture of process gases.

Oxygen and tetrafluoromethane (CF4), when combined together for use in plasma etching, generate the oxyfluoride ion (OF-). The oxyfluoride ion is a powerful etching agent for polymeric substances. This ion is particularly adept at cutting the carbon-carbon molecular bonds in the polymer backbone and removing the molecule quickly.

One application of polymer etching refers to hole boring in polyamide when the polyamide is sandwiched between two, conductive sheets of metal. The holes are effortlessly made using a ratio of 80% oxygen and 20% CF4 at high power and low pressure. Time will rely on the polyamide make up and the depth, but low pressure will insure straight, clean sidewalls in the hole. Selectivity will insure that only the polymer present in the whole is etched. The metal around should be unharmed by the etching operation because of the selectivity of the etching process.

Another application refers to the etching of polyamide in bulk form such as off a wafer. This, again, is similar to etching photoresist in that the process pressure is set comparatively high for good uniformity, and the power is increased in order to speed etching rate. Additionally, this operation is like etching glass since the composition of the polyamide can be varied, thus causing unpredictable etch rates. A good starting point for process recipe development is generally moderate power and high pressure.

Optical fiber cladding etching is the final application in the discussion of polymer etch. Optical fibers comprise of a fused silica core, which is about 100 µm thick surrounded by polyurethane cladding usually another 125 µm thick. The application here is to etch the cladding completely off a particular section of the fiber in order to expose the fused silica core without damaging it. An especially isotropic plasma is needed to uniformly treat all the fiber optic strand. Thus, pressure should be near 500 mTorr with high power. Time is the most vital parameter in this application, because, even at 90% O2 and 10% CF4, the CF4 can damage the silica core and diminish strand pull strength. The fiber optic stand must be etched only long enough in order to remove the cladding.

Bleedout Removal

During epoxy dispensing operations, the substrate material causes a small quantity of the epoxy to wet out over its surface or the amount of dispensed epoxy is excessive. This problem is mainly critical when subsequent wire bonding is needed. The epoxy residue can contaminate wire bond pads causing poor bond strength or wire bond “pull-up” which is the complete failure of the wire bond.

An argon, oxygen and argon, or argon and hydrogen plasma is used around the 200 mTorr to 250 mTorr pressure range to remove the epoxy contamination. Argon is an inert gas, but it can be extremely effective in removal of the epoxy bleedout via pure bombardment using Ar+ ions. The bombardment cleans the surfaces of the wire bond sites by ablating the epoxy and leaving behind a pristine metal surface.

The inclusion of a chemically reactive agent such as hydrogen or oxygen simply increases the reaction rate. The amount of reactive gas can differ, but generally, no more than 30% by volume is added. There is one caveat however: addition of oxygen can lead to oxidation of silver-filled epoxies thus turning them black. This oxidation is nothing more than the surface tarnishing of the silver in the epoxy, and it does not affect the epoxy’s potential to conduct electricity or heat.

Recipe Selection

Below is a chart for selecting the proper process gasses for etching process recipe development.

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%

 

Nordson MARCH

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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