For years the majority of platinum resistance thermometers were manufactured with wire wound technology. The temperature-sensitive parts of the resistance elements were thin platinum wires and production was very labor intensive.  As customer requirements moved the market toward smaller physical dimensions and higher resistance values, the resistance wires needed  to become significantly thinner. Wire wound technology was limited in its ability
to produce these advanced  platinum resistance elements.

In response, a manufacturing technology now known as thin film was introduced in the early 1970's. In this process
the standard measuring wire was replaced by layers of platinum 1 to 2 m thick on a flat substrate composed of AL2O3 . Although cathode vaporizing proved to be the best processing method, more R&D work was necessary to comply with the international industrial standards DIN EN 60751 (temperature coefficient of 0C, 100C of 3.85*10-3K-). 
To establish a defined resistance value within a small area on a thin platinum film, a pattern on the layer must be determined. This can be achieved by the defined removal of platinum. The resulting configuration, whether striated
or folded in two dimensions, is called a meander.

Laser Trimming vs. Sputter Etching

Sputter etching permits all resistance elements to be built simultaneously on a large substrate. This method is suitable for a mass production process rather than the complex individual structuring required by laser trimming, and is also the preferred method for resistance elements of higher ohmic values (e.g. 500 and 1000). Due to the larger meander structure of these elements, the laser trimming process requires significantly longer machining time.

On the other hand, laser trimming is more flexible because modifications can be made on short notice. This technology
is well suited to the development of new structures and custom designs. Small production lots can be produced at lower cost.

Photo 1. In this continuous laser cut through a platinum surface 1-2 mm thick, the side areas appear wave-shaped and not well defined. This is a result of the transition that occurs during the vaporizing and heating of the platinum. (200 x magnification).

Laser Trimming

A light beam of high, dense energy is produced with a solid-state laser operating in short bursts or shots. An optical device focuses the beam on an area ~20 m dia. The time is short and local concentration of the energy capacity in the focus is extremely high. During this period, the platinum vapors begin to liquefy in the area of the laser burst in a manner similar to combustion but without reaching a liquid state.The continuous isolation cut is achieved by a precise lateral motion of the laser beam within a predefined speed and a pre-defined overlap of the laser burst. (see Photo 1).

To produce the meander-shaped resistance pattern with a defined resistance value, separation cuts must be made against neighboring resistance elements and connection areas for the electrical contacts must be created. Guiding the laser beam along the complex path necessitated by these operations requires a special motion system controlled by a programmable processor.

Photolithographic Sputter Etching

The etching method (see Photo 2) designed as an alternative to the laser trimming, entails removing the platinum during structuring through a selective etching process whose principal steps are:

1. A photosensitive lacquer several m thick is deposited on the platinum-coated substrate.

Photo 2. After the sputter etching process has removed most of the platinum, the remaining material is formed into a smooth and very defined meander pattern. (200 x magnification.)

2. The photo lacquer is exposed through a glass mask carrying a metallized meander structure and placed  on the platinum layer.

3. The exposed areas are then removed using a process-specific spray developer. The remaining lacquer is hardened by annealing. This lacquer structure is geometrically shaped, precisely depicting the structure design on the platinum layer.

4. The unit is sputter etched in a vacuum device (i.e., removal by ion bombardment).  In this process the exposed platinum is etched down to the substrate and the lacquer is simultaneously removed in the same ratio as the platinum. All that remains on the substrate is the platinum structure under the lacquer layer. After a lacquer-stripping process, the untrimmed resistance is complete.

Note that the laser trimming process as shown in Photo 1 creates side areas on the meanders that appear wave shaped and less defined.  There are also molten residue and elevated rims due to the transition between vaporizing and heating the platinum area.  These characteristics by no means diminish the quality of the laser trimmed resistance, nor do they have any negative effects on long-term stability or the isolation resistance between the meander lines.

Photo 3. The entire surface of this laser trimmed element is covered with platinum. The pad area for wire lead connection is cut out on the lower third of the substrate and the center portion of the meander is created specifically for the resistance value. Isolation is provided by a laser cut around the perimeter. (50 x magnification.)

Photo 4. Platinum remains only in the required areas of a photolithographically processed element. The active area is centered on the substrate, creating a wider area of isolation for improved electrical resistance stability. The contact areas for the pad have additional material for optimum weld adhesion. (50 x magnification.)

Photo 5. A screen printing process is used to cover the active resistance area of this photolithographic processed element with a glass coating. The lead wires are welded to the pads by thermal compression. Additional fine trimming can be achieved by laser cut through the exposed loops prior to the application of the strain relief over the lead wires and the remaining active platinum. (50 x magnification).

Photos 3 and 4 show structures made by the two different processes.  These samples do not have the connecting leads or the protective glaze that will be added later.  On the laser trimmed element in Photo 3, the short fine-trim cuts can be seen at the bottom left and right of the meander.  Most of the structure's surface, to the left and right of the meander, is taken up by the contact areas to which the leads will be welded.   These areas are separated from the edge of the chip by a laser cut, and by double cuts from the meander.  In the lower third of the element in Photo 4 can be seen the resistance pattern consisting of a vertical primary meander and seven additional trim loops.  Here, fine-trimming the resistance value is achieved by laser cuts through the loops.

In Photo 5, loops 1, 5 and 7 are next to the step-shaped digital trim. An additional continuous fine-trim runs through a laser cut in a closed area of the platinum layer (below leads at left).  From the picture, it can be seen that all superfluous platinum areas are removed, especially between the contact areas and the rim of the chip.  While this step improves electrical resistance stability, particularly in the border area, it significantly reduces the area available for the contacts.  As a result, all contact areas are given an additional bonding pad to ensure maximum weld adhesion. The lateral dimensions of the rectangular glass ceramic insulation layer, covering the meander by means of screen printing, are significantly reduced.  The border areas of the chip remain metal free.