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Customer Driven Technology

Producing analogue and mixed-signal components often involves a tricky “trimming” process to tune resistor structures and create the desired performance. Trimming machines often have several components and consist of an unhappy compromise solution. ESI has produced a system that is independent of the prober and tester, allowing the process to provide for smaller die sizes with even higher yields

Semiconductor manufacturers are constantly looking to economise production by increasing parts per wafer and yields or downsizing operations. Wafer design and device sizes are constantly being improved and consequently pushing the boundaries and capabilities of the capital equipment manufacturers.

One example is Electro Scientific Industries (ESI) recently launched tool for laser-trimming on silicon. The process of laser trimming and circuit tuning of analogue and mixed-signal devices involves three distinct areas - probing, testing and trimming. The steps have to be performed on each element for the effective manufacture of quality products. In contrast to the number of testers and probers available to customers for testing wafers, past trim systems have integrated the probing and trimming functions. This meant that customers who wanted a specific prober or tester could not choose a trim system of their choice that best served their needs or vice versa. Instead they were often faced with making some form of sacrifice with regard to the optimisation of their production lines. It soon became apparent that these advances in wafer design were making the trim process difficult and maintaining cost effective and reliable manufacture was becoming increasingly more complex.

A concept challenged

The trim systems available on the market have generally been produced by companies whose area of expertise was in laser trimming. They had to incorporate probing technology into their own core competence in order to offer a complete and viable solution to the customer base. During the early days this was accepted as the best way forward but this concept is now being challenged by probing, testing and wafer design improvements. It became apparent that the design and flexibility of this kind of equipment design was experiencing difficulties in keeping up with the advances in wafer design with regard to parts per wafer, wafer sizes and pin counts.

The systems available were not always offering the best expertise and technology in all three of the trim process elements. This issue was highlighted by a number of existing ESI customers who had installed trim solutions from a variety of manufacturers. Customers enquired about a product that could overcome these process obstacles and allow greater productivity.

ESI has a longstanding history of more than 30 years as a laser systems manufacturer with core competencies in laser-material interaction, lasers and optics. However, as pin counts increased, it was evident at ESI that there was a need for companies that specialised in the probing to work alongside ESI to provide the “whole” product of most value to customers.

The objective was clear - ESI was to design and manufacture a system that met customers' modularity needs and gave them greater flexibility in their production. ESI took the decision to recognise that their core competence was the laser element of trimming and set about designing a solution that would not only meet their customers' needs but would also be commercially viable.

Modular solution

The recently launched Model 2100 system provides a totally new approach to thin-film laser trimming. It is based around a modular concept, giving manufacturers the ability to specify probers and testers that are more suited to their needs. Although ESI's R&D teams had been researching the concept for some time, it was after partnering with ESI's teaching customers and defining their requirements that the project development began. Fundamentally, the goal was to provide a laser trimming solution that could be interfaced into existing or other manufacturers' testers and probers by using industry standard tools for linear, mixed signal and sensor trimming needs. By selecting the prober and tester of their choice, semiconductor manufacturers are able to optimise their process by having access to key technology developments in each of the component areas.

From ESI's perspective the advantage is clear - by concentrating solely on the tuning element of the process, they were able to incorporate major technological advances that may not have been compatible with a more integrated system. An example of this is the inclusion of their patented 1.3µm laser wavelength process. The addition of this feature dramatically reduces, or virtually eliminates, laser induced performance shift of a device during the functional trimming process.

At this point, it is worth mentioning that the concept of this modular approach to the trimming process does not represent another step or manufacturing element in the production of devices. All too often process improvement solutions are introduced that require not just capital outlay but, in many cases, additional steps in the manufacturing process itself. ESI's system is designed to replace existing trimming solutions that have become dated as a result of technology advances.

Functional trimming

The functional trimming of thin-film resistors on silicon has been a production process for more than 20 years and has been well documented during that time. For most of this period, tuning was done using arc lamp-pumped lasers operating at 1.064µm. More recently, advances in laser design has resulted in the use of more reliable 1.047µm diode-pumped lasers. However, over this time frame, the process of trimming devices either built on silicon or with silicon chips integrated in it has been made difficult because of performance shifting within the devices caused by the laser.

This performance shift meant that the processing system and the tester had to wait a sufficient time interval after each laser pulse, or series of pulses, in order for the shift to dissipate and the measurement process to then be carried out accurately. In certain cases, the entire device could ‘latch-up' in response to the laser pulse. Under these circumstances, the power supply to the device had to be disconnected for a period of time in order for the device to return to its normal operating state. These obvious problems meant that trimming with laser wavelengths of 1.064 or 1.047µm was usually slow and complex.

Although it was widely known that laser induced photoelectric response (Figure 5) was the major contributor to the performance shift, it was thought to be unavoidable and therefore needed to be worked around via circuit layout and elaborate trim algorithms. Consequently, many promising circuit designs could not be laser tuned since the long trim cycle required made their manufacture uneconomical. The answer to this issue turned out to be an alternative laser wavelength that is well absorbed by both thin-film and thick-film resistor materials but is “invisible” to silicon based structures.

The introduction of this new laser wavelength means that manufacturers also have far greater flexibility in the circuits that can be tuned and increased yields now that the performance shift is not such an issue.

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