Ultrafast lasers find industrial and medical uses

A new generation of compact and rugged ultrafast lasers has spurred development of commercial and industrial applications.

A new generation of compact and rugged ultrafast lasers has spurred development of commercial and industrial applications.

Commercial and industrial applications for lasers usually develop after the laser design passes a minimum threshold of reliability, compactness, and ruggedness. In the past, ultrafast lasers were delicate and maintenance-intensive systems that were unsuitable for the rigors of an industrial environment. Recent advances in laser design have significantly changed this picture, however (see Fig. 1). As discussed at the 1999 Conference on Commercial and Biomedical Applications of Ultrafast Lasers (San Jose, CA; Jan. 28-29), the development of a new generation of compact and rugged ultrafast lasers has led to OEM applications in noncontact thin-film metrology, as well as significant potential in medicine and materials processing.

Ultrasonic thin-film metrology
In the semiconductor industry, the manufacture of a single microchip may require the deposition of more than 40 separate metal layers on a single silicon wafer. The thickness of these layers must be accurately controlled and monitored during the manufacturing process to ensure that the final product operates according to design specifications. For example, the thickness and uniformity of each current-carrying layer help define its resistance-capacitance time constant, which is a factor in determining the frequency at which the chip will operate. Consequently, deviations from the expected film thickness can severely degrade performance.

Traditional optical techniques such as reflectometry and ellipsometry are not used for measuring and monitoring metal-film thickness because such films are usually opaque. Historically, metal-film thickness has been estimated from sheet-resistance measurements made using four-point probes. This inherently inefficient and costly method involves touching the surface of the wafer on which the film is deposited, thereby requiring that measurements be made on sacrificial witness wafers rather than on the patterned wafers on which the chips are created.

Picosecond ultrasonic metrology is a noncontact, nondestructive technique that makes measurements directly on patterned wafers.1 In this technique, a 200-fs pulse from a modelocked Ti:sapphire laser is split into a "pump" pulse and a "probe" pulse. The pump pulse is focused onto a spot that is less than 10 µm in diameter on the surface of a metal film. This pulse is partially absorbed within a volume close to the surface of the film and sharply raises the temperature of the volume by a few degrees (based on a typical pulse energy of 0.1 to 1 nJ). Rapid thermal expansion within this volume creates a sound pulse that travels from the surface toward the interior of the film. For a single metal layer deposited on a substrate, this pulse travels until it reaches the substrate and is partially reflected back toward the surface.

Upon returning to the surface, the sound pulse changes the optical reflectivity of the film by approximately one part in 105, depending on the film material and the pulse energy. This change in reflectivity is detected by the probe beam, which is focused onto the same spot as the pump pulse. Changing the optical path length of the probe pulse by means of a delay line mounted on an adjustable mechanical stage means its arrival can be delayed in time relative to the pump pulse. The reflected part of the probe beam is detected by a photocell, and its intensity is recorded as a function of pump-probe delay time (see Fig. 2). The film-layer thickness can be determined from the product of one-half of the echo time and the velocity of sound in the film material.

Since 1997, Rudolph Technologies (Flanders, NJ) has supplied picosecond ultrasonic thin-film metrology systems incorporating an ultrafast Ti:sapphire laser from Coherent Inc. to major microchip manufacturers. Says George Collins, director of marketing at Rudolph Technologies, "These systems are currently being used in the manufacture of logic (central processing unit), memory, and application-specific chips such as for digital-signal processing."

A unique advantage of ultrasonic metrology for manufacturers is its ability to measure layers that may be buried beneath several other opaque layers. Because in many metal-deposition processes wafers are not removed from the vacuum system between layer depositions, there is no way to measure each layer in isolation. However, picosecond ultrasonics offers an efficient way to measure all layers simultaneously at the end of the multilayer deposition process.

Because the generated sound pulse has to travel round-trip through each layer and back up through all the layers preceding it before it can be detected, the echoes from each layer are displaced in time by the round-trip time through that layer. Consequently, by analyzing the first series of echoes one is able to determine the properties of the top layer while each successive series of echoes reveals properties of a lower layer.

"Ultrasonic metrology`s ability to directly measure patterned wafers instead of just witness wafers, as well as its combination of fast measurement, nondestructive testing, and simultaneous measurement of multiple layers, translates into enormous cost savings for microchip manufacturers," says Collins.

Materials-processing applications
Historically, mechanical machine tools, electron-beam (electrical discharge machining) tools, and conventional laser tools based on carbon dioxide (CO2), Nd:YAG, copper-vapor, or excimer technology have been used to process materials. But each of these technologies is limited either in precision or by the type of materials it can comfortably process. In the case of most conventional lasers, thermal stress is introduced to the material that often leaves permanent damage within a heat-affected zone. Ultrashort (<10 ps) laser pulses, on the other hand, can process virtually any material (diamond, silicon carbide, titanium carbide, stainless steel, teeth, living tissue, or high explosives) with precision and minimal collateral damage.

Researchers at Lawrence Livermore National Laboratory (LLNL; Livermore, CA), led by physicist Brent Stuart, have shown that amplified ultrashort pulses deposit laser energy into the electrons of materials on a short time scale compared to the transfer time of this energy to the bulk of the material (either by electron-phonon coupling or thermal diffusion).2 This process forms a critical-density plasma that expands away from the surface, thereby achieving high ablation efficiency with negligible energy deposition to the remaining material either by heat or shock. Numerous groups worldwide are actively exploring both the fundamental science and practical implementation of these advantages of ultrafast machining.

A dramatic example of the advantages of machining with ultrafast lasers is the safe cutting of high explosives such as LX-16 and pressed TNT. Femtosecond pulses remove approximately 3 µm of material per pulse and impart negligible shock to the explosive. Analysis of waste products reveals no thermally induced ignition. Pulses of 600-ps duration, on the other hand, quickly melt and partially ignite high explosives (see Fig. 3).

Medical Applications
Potential medical applications for ultrafast lasers are numerous and include heart and spinal surgery. In some diseased hearts the vascular system develops blockages that prevent oxygenated blood from reaching the heart muscle. These blockages are not always surgically repairable. In such cases a procedure called transmyocardial revascularization (TMR) is used to deliver oxygenated blood to the heart muscle. In this technique, numerous small-diameter holes are drilled through the heart muscle to connect it directly to the inner heart chambers that contain oxygenated blood. These holes spur the formation of new vessels which help increase blood flow in the ischemic heart muscle

In the past, TMR has been conducted using CO2, holmium, and excimer lasers. However, researchers at LLNL have shown that there are significant advantages to using the output of an amplified ultrafast laser for this application.3 "We have found that we can cut with shorter than 10-ps pulses from a Ti:sapphire laser," says John Marion of the LLNL team. "[We can] drill tiny holes of less than 150 ?m in diameter without thermal or mechanical damage to surrounding tissues."

Ultrafast lasers also appear ideally suited to spinal surgery, where extreme precision is required to remove bone mass that is impinging into nerve openings. Currently, mechanical drilling is used for this purpose. However, an ultrafast laser coupled with a diagnostic system invented at LLNL has proven more precise in cutting bony mass without disturbing soft nerve tissue. This new system analyzes the luminescence of the plasma generated by ablation of tissue. By monitoring the calcium signal, it determines whether the laser is cutting bone (high signal) or nerve (low signal). When the calcium signal drops, the laser is instantly shut off to prevent the surgeon from accidentally cutting nerve tissue. Both clinical treatments and diagnostic techniques based on ultrafast laser pulses are likely to emerge in the near future. o

REFERENCES

  1. 1. R. J. Stoner et al., Proc. SPIE 3269, 104 (1998).
  2. 2. B. C. Stuart, H. T. Nguyen, and M.D. Perry, "Femtosecond Laser Materials Processing," Proc. SPIE 3616 (1999).
  3. 3. J. E. Marion and B-M. Kim, "Medical Applications of Ultrashort Pulse Lasers," Proc. SPIE 3616 (1999).

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