Optoelectronic Thin-Film Characterization

Compound semiconductor technology is not new. Devices based on GaAs, the most mature of the III-V technologies, have been used for more than 30 years, mostly in aerospace and military applications. In these applications, their ability to operate at very high frequencies and power despite extreme temperatures and radiation was required. However, in the past 10 years or so, GaAs devices have come to be used in high-volume commercial applications such as cellular phones and other wireless applications, which require operating frequencies that are difficult to achieve with silicon-based devices. The growing market for GaAs optoelectronic devices is adding to this demand, and GaAs manufacturers are now ramping up production and increasing the standard wafer size from 100 to 150 mm.¹

As with the current increase in silicon wafer size from 200 to 300 mm, the larger GaAs wafer presents both opportunities and risks. If the risks are properly managed, significant cost savings can be obtained through this more than doubling of the number of chips on a single wafer. Metrology tools will be required to control the new processes in 150 mm GaAs manufacturing. Once large-scale production begins, metrology will also be required to provide early detection of costly process deviations and misprocessing. Because a 150 mm GaAs wafer, after epitaxial growth of the device structure, can cost about 60 times more than an equivalent Si wafer,² reducing or eliminating test wafers and detecting or preventing errors will be critical for manufacturers that successfully transition to 150 mm GaAs production.

One of the fastest growing segments of the optoelectronics market is vertical-cavity, surface-emitting lasers (VCSELs). These devices convert electronic signals to optical signals by producing a narrow laser beam that can be switched on and off electrically, and can easily be coupled to optical fibers with an efficiency of up to 80%.³ They also have many other advantages over their edge-emitting competition, including smaller size, greater efficiency, and a short cavity that allows the beam to be turned on and off without an external modulator. Finally, unlike edge-emitting lasers, VCSELs can be tested while still on the wafer, allowing bad devices to be eliminated before costly packaging.

The market for VCSELs is predicted to triple over the next three years, reaching 3.4 billion by 2004. GaAs-based VCSELs operating at 850 nm all but replaced standard transmitters in short-haul local area networks within the first two years of commercial availability.⁴ InP-based VCSELs are now on the cutting edge of technology, but will have the capability to operate at 1310 and 1550 nm, which are the low-loss transmission windows for optical fibers. Once commercially available, these devices are expected to be hugely successful in the marketplace for mid- and long-range transmission applications. For manufacturers to successfully compete in this market, they will need metrology tools that can characterize the GaAs- or InP-based materials accurately and reproducibly.

High-volume optoelectronic production will require new metrology solutions. The non-contact, non-destructive picosecond ultrasonic laser sonar (PULSE) thin-film measurement technology (see "PULSE Technology," this page), currently being used to effectively characterize and monitor copper and other opaque films in silicon-based technology, also has the characteristics needed for the optoelectronics market. All measurements shown in this article were taken with Rudolph Technologies'Meta PULSE-II metrology system. While a variety of applications are possible, this paper will concentrate on these critical applications: GaAs metalization and VCSELs.

Metalization is the foundation of the process that establishes an electrical contact with each device terminal and connects the individual devices by means of metal wires to form circuits. Although metalization schemes in silicon-based and compound semiconductor devices display some similarities, GaAs metalization processing is significantly different. GaAs forms no stable oxides that can be used for isolation. In addition, As dissociates from the GaAs substrate at temperatures above 400°C, restricting the thermal budget. This and other processing considerations require that GaAs metalization be formed at temperatures below 100°C, precluding many of the higher-temperature processes common in silicon-based devices. The effect of these complications is that the advantages GaAs materials offer in terms of higher electron mobility and lower power consumption may be lost due to poor metalization. Also, poor metalization can increase resistance, resulting in slow device speed and degraded performance.

GaAs metalization uses materials seldom encountered in silicon-based manufacturing. Aluminum, the old standard for silicon-based contacts, cannot be used because it readily forms eutectics such as AlGaAs.² Gold (Au), despite its expense, is the most commonly used contact metal. Au is an excellent conductor, has a high resistance to oxidation and, being a noble metal, reacts with few other materials. Other common interconnect materials compatible with III-V semiconductors include platinum (Pt), nickel (Ni) and germanium (Ge).

Perhaps of more interest is metrology for multilayer metal stacks. In the new 150 mm fabs, these stacks may be deposited by cluster tools that lay down several layers in the single process tool. These cluster tools not only reduce the total time required for processing, but also reduce the possibility of interlayer contamination and the chances of breaking the brittle wafers. However, cluster tool processing requires a metrology tool that can simultaneously measure the multiple layers of the contact film stack after deposition.

	Ni/Ge/Au STACK THICKNESS   1. Measurement from a Ni/Ge/Au stack on GaAs. The thickness of all three layers was simultaneously determined as 177 Å of Ni, 142 Å of Ge, and 505 Å of Au. The graph on the left shows the acoustic signal (black) and the best fit simulation (red).

The processes involved in VCSEL manufacturing are complex. A VCSEL's active region is sandwiched between highly reflective mirrors called distributed Bragg reflectors. These mirrors are comprised of alternating layers of two materials having different indices of refraction to form a stack having multiple opaque layers grown using either metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). The slightly different indices of the individual layers reflect a very narrow range of wavelengths back into the cavity, stimulating laser emission at only one specific wavelength. To do this, it is important to deposit films in both the active region and the mirror structures within tightly controlled process ranges. The thickness and uniformity of the Bragg reflectors is critical to ensure that their peak reflectivity occurs at the lasing wavelength.

	2. A 43-point uniformity map of an InGaAs layer on an InP substrate with a 0.5 mm edge exclusion.

A key factor to consider when producing VCSELs is the planarity of the layers. Non-planar layers can cause improper alignment of the mirrors, resulting in defective devices. Therefore, the deposition process must be carefully monitored for uniformity across the wafer. The newest processes being developed for VCSELs involve InP wafers that can be used to produce 1310 or 1550 nm devices. A thick layer of InGaAs is commonly used as a base for the mirrors and the active region that will be deposited over it. A uniformity map of such a layer on an InP substrate is shown in Figure 2. The 43-point map shows the film is thinnest nearest the notch and gets progressively thicker as you move away from the notch. This information was acquired in <5 min and, because the measurement was non-destructive, the wafer could be sent on for further processing. Picosecond sonar metrology can also provide fast, non-destructive, on-product process control metrology of other steps in the VCSEL manufacturing process.

Picosecond sonar has been shown to have the ability to characterize and monitor certain processes in compound semiconductors. As the optoelectronic market grows, high-volume manufacturing is becoming the norm, and competition will demand ever tighter process control to maintain high yields and profits. In this environment, metrology is critical to success. Picosecond sonar is unique among metrology techniques because it is able to non-destructively measure opaque films and film stacks on product wafers.