InP is a semiconductor material which has a wide band gap and excellent electron mobility. These qualities make it ideal for the manufacture of optoelectronic devices. One important application is in communications, which is a rapidly-growing sector that is becoming more important as data traffic increases day-by-day.
InP use is key in the production of components suited to high frequencies (which allow for greater volumes of data). The biggest benefits of InP are in the manufacture of laser diodes, where it provides high levels of efficiency alongside price competitiveness. Via design optimization and fabrication, InP lasers provide pure spectra and high optical power, over a large range of temperature. The wavelength range of 1100 nm to 2000 nm, which can be achieved using InP, is best for fiberoptic communications.
Establishing inexpensive protocols for processing InP during the production of InP lasers supports the development of better communication systems to keep up with the constant need for greater data transfer.
This article describes how plasma processing technologies are vital in InP laser diode production by comparing inductively coupled plasma chemical vapor deposition (ICPCVD) with plasma-enhanced CVD (PECVD). The focus is on showing the main qualities of each process so that different processes can then be applied to manufacture efficient and high-performance lasers.
Figure 1. The processing steps involved in the fabrication of an InP laser with those implemented using plasma processing methods highlighted in orange
Core Plasma Processing Technologies for InP Laser Fabrication
Plasma is known to be the fourth state of matter in which there are non-ionized gases, along with cations, electrons, and radicals. Gas ionization requires a lot of energy (usually heat or electrical). The resulting plasma is highly energetic, which makes it ideal for controlled semiconductor manufacture. By adjusting the chemistry – i.e. the plasma composition, or the conditions of processing (such as temperature or pressure) - the properties can be adjusted.
This means the process is versatile enough to accommodate a wide variety of adjustments to produce and to control a large range of surface interactions with extreme accuracy. In addition, the presence of free electrons allows plasma control via electric and magnetic fields within the processing installation. These intrinsic advantages that are present in plasma-based processing have led to the development of many commercial process tools that exploit this technology - many of which are very productive and can be configured freely to fabricate semiconductor devices. The plasma processing tools described in the following sections are especially important in the fabrication of InP lasers.
PECVD – Desirable Film Properties for a Wide Range of Materials
The PECVD process is based on the reaction of ionized gases to result in their deposition as a film or coating from a plasma state onto a substrate.
Figure 1 shows the chief components of the PECVD platform. The gas mixture enters the vacuum chamber by being pumped through a showerhead type of gas inlet (located in the top electrode). With the application of RF power, plasma is formed. The film is deposited as radicals and ions from the plasma, on the surface of the substrate, and the residual material is removed by pumping. The operating pressures for this process are usually between 0.5 and 1.0 Torr.
PECVD is useful when coating many materials, such as SiOx and SiNx. The latter two are valuable in masking the succeeding etching or passivation deposition processes during the fabrication of InP. The result is films of high quality throughout the substrate with high density and the least possible pinholing. This ensures that the properties of the material, such as the refractive index and the hardness, can be tightly regulated. The processes involved in PECVD usually occur at temperatures between 90 oC to 650 oC, with the substrate (used for processing InP) operating at temperatures of about 300 oC.
Figure 2. PECVD enables the deposition of high quality films at relatively low working temperatures
ICPCVD – Plasma Deposition at Low Temperatures with Minimal Substrate Damage
The process known as ICPCVD uses ICP instead of electrically produced plasma as in PECVD. As Figure 3 shows, inert gases enter the vacuum chamber at the top and become plasma during their passage through the powered coil. There is another point just above the substrate that feeds reactant gases that trigger film formation into the chamber, as well as dopants (if required). It is possible to introduce RF biasing over the whole substrate table so that film characteristics can be fine-tuned. The processes usually occur between pressures of 1-20 mTorr, which is much less than that required for PECVD.
The use of ICP causes a high-density flux to form at the surface. As a result, the films are high in quality and form at much lower temperatures than could be done using PECVD processes. The temperatures at which ICPCVD operates lie between 20 oC and 400 oC in most cases, but the substrate is kept at about 150 oC or below. This is an important advantage, especially when working with Si-based materials. The reason for this is that there is less damage to the substrate because the ions have low energies at the time of deposition (and/or the plasma is farther away from the substrate surface). In general, the use of ICPCVD results in good surface coverage and is much preferred because of the high conformality, especially when the surface has deeper features.
Figure 3. ICPCVD enables low temperature deposition with minimal substrate damage
Selection of the Optimal Processing Technique for Each Step of Fabrication
Only once the various plasma processing techniques are properly understood can they be selected and used at different phases of InP manufacture. In Figure 1 the process flow during the fabrication of an InP laser can be seen. The emphasis is on finding the best plasma processing technique for each stage. Generally, however, the InP Mesa/Ridge etch is one of the most important techniques, as it determines the attributes of the waveguide formed as a result, which, in turn, defines the quality of the final laser product. By looking at example performance data, the following sections focus on how to select the best plasma process and ensure high performance. The data here comes from laboratory experiments, proof of performance studies and manufacturing process data that used Oxford Instruments platforms for specific applications.
Fabrication of the InP Mesa/Ridge Mask
The InP Mesa/Ridge mask is typically made with either SiNx or SiOx. Less faceting occurs within SiNx than SiOx, which means the profile has a geometrical right-angled shape, and the rounding is significantly less. Thus, this material is the usual choice. The term ‘faceting’ refers to the erosion of the top corners of a masking pattern at a higher rate than the planar surfaces, which could cause mask quality deterioration. Whichever is used, photoresist is used to pattern the mask, and is removed before etching is performed on the InP through the SiNx or SiOx. The remaining mask is then removed as the last step.
Mask deposition techniques are chosen predominantly on the basis of the film quality and then the deposition rate. Dense, high-quality film has good selectivity during the subsequent etching phase, and this can be quantitatively measured using wet etch rates (WERs) assessed in standard conditions with potassium hydroxide or buffered HF (or BOE, buffered oxide etch).
One more aspect to be considered is the need to avoid hydrogen while deposition is proceeding. In some situations, the InP stoichiometry is changed in the presence of hydrogen, which impacts device performance. To prevent this, ammonia is not used as a deposition gas – this removes the risk of hydrogen evolution during the process. This is practiced universally.
In Table 1 (below), the differences in operating conditions are shown for PECVD and ICPCVD (used for mask deposition). PECVD takes less time and is generally preferred unless the application has temperature criteria. In such situations ICPCVD can be used because it operates at lower temperatures, which more than compensates the small lowering of deposition rates. Both of these can be used with low hydrogen process chemistries as needed.
Table 1. PECVD delivers high SiNx/SiOx deposition rates but requires higher operating temperatures which can be a limitation for some devices
|Recommended temperature range
||90 °C - 650 °C
||20 °C - 400 °C (higher temperatures possible)
|SiNx - LF power, He addition, power pressure SiOx - SiH4 power
||All film types, process conditions, e.g. ICP power, pressure
||600 - 3000 m Torr
||2 - 30 m Torr
||20 - 1000 W
||150 W - 2000 W
||7 - 1200 nm/min
||6 - 150 nm/min
Figure 4. Wet etch rates highlight the higher quality of SiNx films (upper graph), relative to SiOx analogues (lower graph), and the ability of ICPCVD to deliver high quality films at low processing temperatures
Figure 4 shows the comparison between WERs when either SiOx or SiNx is used for PECVD or ICPCVD, respectively. Film quality is always better when SiNx is used, whichever the technique. Between the techniques, however, ICPCVD always produces better films compared to PECVD and the film performance is not so sensitive to temperature. In all situations, processing temperature improves film quality, but this is less so with ICPCVD, when SiNx is used because, at very low temperatures, this combination leads to films of excellent quality.
This mask is usually etched with RIE due to the low cost and the low level of difficulty in this application, which does not require etch rates to be sharply defined. In Figure 9, the outcome of RIE use in etching an SiN mask is shown, using a SF6-CHF3 process with a PlasmaPro 100 for RIE. The vertical feature is sharp and vertical, which plays a major role in creating an InP profile that is vertical during the next step of etching.
The final step in plasma processing during InP laser fabrication is to deposit a passivation layer with the accompanying etching, so that the electrical contacts may implemented as necessary. Here again the usual materials preferred are SiNx or SiOx, for the reasons mentioned in the mask deposition section. The technology used in this step is either PECVD or ICPCVD, depending on the application, since any restriction based on operating temperature leads to a preference for ICPCVD. At this stage, the film quality is now associated with device protection, as films of high density are more resistant to water entry. This allows higher voltages to be applied before the film breaks down.
Figure 5. RIE can be used, along with photoresist, to fabricate a high quality SiNx mask for etching of the InP Mesa/Ridge
To fabricate the best waveguides and gratings for high performing InP laser diodes, a range of plasma processing methods must be taken into consideration, with the most suitable one selected depending on the processing requirements. Now available in sets, there are many sophisticated and adaptable tools for plasma processing which allow for efficient deposition, masking and etching during manufacture. This makes it possible to control features and their attributes with precision.
Understanding how these tools provide benefits as well as limitations helps manufacturers select the right suite to fabricate smoothly vertical profiles with a high degree of precision, without compromising on high throughput or cost-effectiveness.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
For more information on this source, please visit Oxford Instruments Plasma Technology.