Sunday, September 19, 2010

CNT Loudspeakers

This seems to be something straight out of science fiction, but they've actually done it. Some smart researchers in China have figured out how to create an entire speaker out of carbon nanotubes. A piece of carbon nanotube thin film could be a practical magnet-free loudspeaker simply by applying an audio current through it. These nanotech loudspeakers (merely 10s of nm thick) can be manipulated into any shape and size while exhibiting flexibility, transparency, and stretchability. They can be mounted onto room walls, ceilings, pillars, windows, flags, and clothes without much area limitations.

In 2002, a group of Chinese researchers developed a technique for creating nanotube yarns up to 30 cm long. They drew the yarns out from super-aligned arrays of CNTs (I guess you can call this nano-knitting). Super-aligned CNT arrays differ from ordinary vertically-aligned CNTs in that their alignment is far superior to that of ordinary CNT arrays. This is important for continuous thin films, or ribbons (composed of parallel pure CNTs); in that they can can be drawn from super-aligned arrays in the solid state. These thin films are transparent and conductive, with aligned CNTs parallel to the drawing direction.

Three years later, the same group of scientists successfully synthesized super-aligned CNT arrays on 4-inch silicon wafers. One such wafer is capable of being transformed into a continuous thin film with dimensions of 10 cm wide by 60 m long (yes, sixty meters).

In 2007, Dr. KaiLi Jiang, the head of the research group and associate professor of physics at Tsingua University in Beijing, had discovered that a piece of a CNT thin film can emit sound by applying an audio frequency current through it, with the interesting of effect of the sound frequency being double the current frequency. This is attributed to the thermoacoustic effect. The alternating current periodically heated the CNT thin films, resulting in temperature oscillation. Said Dr. Jiang: "The temperature oscillation of the thin excites the pressure oscillation in the surrounding air, resulting in sound generation."

Jiang also said that the thermoacoustic effect has been studied for more than 200 years, has led to the invention of thermoacoustic engines and even loudspeaker driven refrigerators.

What remained obscure in the scientific literature is the use of alternating current in thermoacoustic generation. Jiang and his team believed they were the first to discover this effect, but they were beaten by Arnold and Crandall in the late 19th century. A & C used ultra-thin foils made of platinium to feed the current through, which then generated a very weak thermoacoustic effect; too weak for practical use. A & C claim that the sound efficiency is inversely proportional to the heat capacity per unit area (HCPUA) for the material studied. For 700 nm thick platinum foil the HCPUA value is 260 times weaker than for CNTs at the same power input.

The loudspeakers were fabricated by placing the as-drawn CNT thin film on two electrodes, forming a simple loudspeaker. Several thin films were placed together so as to increase the loudspeaker area. The films could be formed into arbitrary shapes or placed on arbitrarily curved surfaces to make loudspeakers with special functions.

Carbon nanotube thin film loudspeakers. (a) The CNT thin film was pulled out from a super-aligned CNT array grown on a 4 inch silicon wafer and put on two electrodes of a frame to make a loudspeaker. (b) SEM image of the CNT thin film showing that the CNTs are aligned in the drawing direction. (c) A4 paper size CNT thin film loudspeaker. (d) The cylindrical cage shape CNT thin film loudspeaker can emit sounds to all directions, diameter 9 cm, height 8.5 cm. (Reprinted with permission from American Chemical Society). 

The CNT loudspeakers exhibit volumes and frequencies that are quite pleasing to the human ear.   When connected to a simple amplifier, the CNT thin film speaker shares all the functions of a voice-coil loudspeaker, plus the bonuses of no magnet, and no moving parts.  In contrast to conventional loudspeakers, they are stretchable, transparent, and flexible.  CNT loudspeakers also don't vibrate and are quite durable;  they can work even if part of the thin film is torn or damaged.  The possibilities are endless for CNT loudspeaker applications.  E.g., laptop computers where the current audio system is replaced by simply placing a transparent loudspeaker over the screen itself, or for tomorrow's mobile devices such as the Nokia Morph.

The team hopes to develop real commercial products with CNT loudspeakers.  This article is two years old, so progress on this front is currently unknown.  An update will appear here as soon as I hear of it. 

Sunday, September 12, 2010

Results and Discussion

Figure 3 shows a Light Point Defect (LPD) map for the first wafer used in the study. The wafer has typical edge defects, but the LPD map doesn't show noticeable differences among the various anneal process conditions (zones). This implies that any modification to the wafer surface that might have occurred during the laser anneal is below the detection threshold of the defect channel.

However, the SURFimage wafer map (see Fig. 4) shows distinct visual  differences among the different process zones on the wafer.  The average haze from each process zone was plotted against the LSA temperature.

The figure above shows wide channel haze versus process temperature.  It also shows strong correlation between the scattering signal and the anneal temperature. 

Previous studies have shown a good correlation between the so-called power spectral density (PSD) and surface roughness as measured by laser scattering.  Based on these studies and on this work, the haze results indicate that the wafer surface is modified and roughened by the LSA process. Therefore, the surface roughening increases with LSA temperature.

The authors then reviewed the different zones by SEM to better understand the surface features on the annealed wafer.  The results in Fig. 6 show increased surface topography and modification for the zones annealed at higher temperatures.

 More annealed wafers (Fig. 7) show similar correlation between LSA temperature and SP2 (?) haze, which shows that this technique is repeatable. 

The figure above also shows the raster pattern on the wafer.  This technique is also effective at capturing within wafer variations in the surface morphology.  A wafer that was annealed at T-75ÂșC was scanned in high-sensitivity mode on the SURFscan SP2.  The SURFimage map (Fig. 8) is effectively able to detect annealing variations within the wafer via changes in the local haze.

The second map shows the wafer binned into low, medium, and high haze regions for ease of view.  These results show the ability of SURFimage to provide full-wafer surface information at industrial scale throughputs.

Following this study is planned AFM analysis of the different LSA zones in order to obtain a direct quantitative correlation between the measured surface roughness and haze.  Also, the amorphous wafer defectiveness and surface morphology will be also correlated to inline product wafer inspection results.  At 45 nm, surface morphology requirements become increasingly strigent.  The results here can establish SPC limits for production monitoring of the LSA process. 


As ICs shrink to nearly molecular scales, understanding and characterizing the impact of process variations on wafer surface conditions and identifying potential surface damage becomes critical.  UV laser scattering technology enables full-wafer surface monitoring with sub-nm vertical resolution and high throughput.   This technique can be quite sensitive to small variations in LSA process temperatures that wouldn't be detected by standard defect-monitoring methods; we got ourselves a powerful new tool for process development and monitoring in a fab production environment.