Diablo Industries Inc.
Thin Film Circuit Design Guidelines
Diablo Industries has been producing thin film sputtered and etched circuits for 30 years. The following guidelines give a basic overview of the process, and answer some of the more common questions. There are many hints and suggestions here that will help the designer be as efficient and practical as possible when creating a new design. Like many things in life, the key to practical thin film is to be reasonable and to not overkill with conditions that are not required.
If you can possibly let us take a look at your design early in its life, we may be able to suggest slight modifications that can save you money and make your design much easier to manufacture.
Thin film technology is a microcircuit fabrication technique whereby thin metal films are vacuum deposited and electroplated onto ceramic substrates and then patterned and etched to create conductor lines, passive devices, and electrical interconnects. Metal films are applied via a process called sputtering, which is a physical vapor deposition (PVD). Sputtering deposits films atom by atom, and this is probably the reason why the technology has been given the name "thin film." Metalizations can be fully sputtered or electroplated to full thickness after sputtering.
The substrate material is chosen for its electrical, mechanical, or thermal properties. Since virtually all thin film substrate materials are ceramic-based, they are drilled or machined using lasers or diamond cutting tools.
Added features available on thin film circuit devices are plated-through holes, non-plated-through holes, etched spiral inductors, sheet resistors, edge wrap-arounds, air-bridge interconnects and filled vias.
There are five (5) substrate materials that are commonly used in the fabrication of thin film circuits. Each material occupies its own niche where it is used with much success.
Alumina - 99.6% aluminum oxide (Al2O3) is a white ceramic that is used in the majority of thin film applications because of its low cost and desirable electrical properties. Generally, as-fired alumina with a nominal 3-4 u-inch finish is used for applications below 18 GHz, either 1-2 u-inch as-fired or polished alumina is for higher frequency or more critical applications. As-fired alumina with a 1-2 u-inch surface finish is available from Coors Ceramics and offers a performance close to polished, but at a much lower cost. The 3-4 u-inch material accounts for about 65% of thin film work, 1-2 u-inch another 20%, and polished the remainder. Using 3-4 u-inch as a baseline cost, 1-2 u-inch is about three times the cost and 1 u-inch nearly 20 times the cost. 96% alumina is very inexpensive, but finds very little use in the microwave world, primarily due to its higher loss and lower level of consistency.
Beryllium Oxide – 99.5% BeO is a white, granular ceramic used in high power applications because of its high thermal conductivity. It costs twice as much as polished alumina and becomes toxic if it is ground or otherwise turned into powder or dust and subsequently inhaled. It should not be exposed to high temperatures. When used in a normal manner and not “ground”, it is perfectly safe to use. It is still used in many applications, although whenever possible, AlN should be considered because it is non-toxic. In addition to its toxic properties, Brush Wellman is the only remaining manufacturer of this material and often struggles to keep up with the demand. This has led to large price increases and extended deliveries, with little chance of a reversal. Still, if there is a need to remove high amounts of heat through a substrate, it works well.
Aluminum Nitride - AIN is a grayish brown ceramic also used in high power applications because of its high thermal conductivity. It is roughly equal in cost to polished alumina, does not have the toxicity of BeO, but has very distinct chemical incompatibilities during thin film processing that make processing it more difficult. Because AlN is gradually replacing BeO in so many applications, there are more manufacturing sources and it is slowly dropping in cost.
Fused Silica - chemical composition SiO2 looks like glass and is usually polished to optical quality. It is used in high frequency applications because of its low loss and excellent signal propagation capability.
Barium Titanate - is used in varying formulations to provide a high dielectric constant material that has good mechanical strength. There are now several suppliers of this material, thus it is becoming less expensive and more readily available.
Thin film metalization schemes usually consist of two, three, or four layers. The first layer is the adhesive or resistor layer. This layer serves as the glue layer that holds the metalization onto the substrate surface. Commonly used adhesive layers are titanium-tungsten (Ti/W) or chromium (Cr). Ti/W accounts for 90% or more of the work because it is far more resistant to diffusion during heating than Cr.
When resistors are required on the circuit, a thin layer of tantalum nitride (TaN) is deposited below the adhesion layer directly onto the substrate. This layer is deposited to a specific sheet resistance, usually 50 or 100 ohms-per-square, and will be photo defined to form resistors in a circuit. In unusual cases, sheet resistances as high as 250 ohms-per-square can be used but tend to be harder to control as the film becomes very thin. When possible, it is advised to use either 50 or 100 ohms-per-square.
Non-Solderable Metal Systems
The next layer in the sequence, when a circuit does not require solderability, is the top layer, usually gold (Au). Au is typically deposited to 125-150 u-inches. Au can be 100% sputtered to the final thickness, or a base layer sputtered to 10-15 u-inches and the remainder plated. The majority of the films are done by the combined sputtering and plating process as it is more efficient than 100% sputtering. These two systems, TaN- Ti/W- Au for resistor circuits or Ti/W-Au for conductor circuits, are by far the most common. Both of these systems are used in manufacturing thin film microcircuits where either epoxy attach or eutectic attach with gold-tin (Au/Sn) or gold-silicon (Au/Si) is used in combination with wire bonding to build the final circuit.
Solderable Metal Systems
If the assembly of the circuit dictates the use of solder, a solderable interlayer must be used under the gold to prevent it from being leached away. Copper can be used in a system that does not require high temperature assembly such as Cr-Cu-Au or Ti/W-Cu-Au. In both cases the copper is typically deposited to a thickness of 80-150 u-inches and the top layer of gold to 50-100 u-inches. Both of these systems have been used for many years to manufacture high reliability aerospace systems, but suffer the disadvantage of high temperature resistance. If the circuit is required to see much over 200C for any period, the copper layer will diffuse through the gold rendering the gold un-bondable. Another drawback is that it cannot be used in a system with the integrated TaN resistor layer, which requires a high temperature stabilization bake during it’s processing.
Two other common barrier layers that are solderable and do not suffer this weakness are Nickel (Ni) and Palladium (Pd). Both of these metals are deposited under the gold and on top of the Ti/W or Cr adhesion layer to a thickness of 10-40 u-inches. Both are resistant to high temperature excursions and can be used in a system that integrates a TaN resistive layer. Both solder very well. Pd is the preferred because it is easier to process in terms of etch selectivity, it does not diffuse through the top gold layer during high temperature exposure as easily as Ni, and unlike Ni, it is not a magnetic metal.
The above represent by far the majority of the thin film work and are tried and proven systems. When delivery is a prime concern, sticking with the standard systems will usually reduce cycle time. Depending on the specific needs however, there are other metals available and the thickness of the layers can be adjusted for a specific purpose. As an example the thickness of a gold conductor can be increased up to 500 u-inches to carry a high current, or TaN can be deposited at 250 ohms-per-square for a specific need.
Line widths and spaces are fabricated on the surface of the substrates using carefully controlled photolithography processes. Diablo production photolithographic processes offer the ability to pattern lines and spaces as small as .0006” with tolerances as tight as .0001. When designing a circuit, never design smaller than necessary as it will unnecessarily reduce yields and increase costs. The finest lines can be obtained using a metal system without a solderable barrier. If fine lines are required in a solderable system, we recommend using a Pd barrier. The majority of the work at Diablo is done in an etch-back process but we are well equipped to do pattern plating as well.
Sheet resistors are etched onto the substrate surface using a secondary patterning step after the conductor layer has been defined. Sheet resistors offer extremely high reliability. The design of sheet resistors is easy to understand and is summarized in the illustration below. After etching, resistors are stabilized at 450-475 degrees Celsius to form an oxide on the metal surface which serves to passivate the resistor and prevent drifting through later process steps. The temperature coefficient of resistance (TCR) of the resistive film is between zero and –200 parts-per-million (PPM) per degree Celsius of temperature change.
Resistor tolerances for most microwave work is normally adequate at +/- 10%, and this can easily be achieved without laser trimming. Stabilizing to 5% is also done routinely at a slight increase in cost. If tighter tolerances are required, laser trimming is necessary. Laser trimming to tolerances of less than 1% is done routinely, and if the circuit is built in volumes sufficient to justify the set-up, this is the preferred method. As a guideline, about 75% of the work we do is at 10%, another 10% is non-trimmed to 5%, and the remainder laser trimmed to tighter tolerances. Perhaps the main reason for this trend is the concern microwave designers have regarding the effect of laser trimming on microwave performance. It is believed that the physical size of the resistor element, which remains constant until laser trimmed, is more important than trimming to a more precise value.
Laser trimming, Plunge Cut versus Scan Cut
When laser trimming is required, it is highly recommended to design a circuit that can be scan cut as opposed to plunge cut. Scan cutting removes an equal amount of material from both sides of a resistor, keeping the physical size as large as possible, hence meeting the goal of a minimal physical change. A plunge cut, on the other hand, quickly trims a resistor to its DC value, but leaves a single deep cut into the resistor interior that can change the RF signature significantly.
Do not specify resistor tolerances tighter than necessary.
This is often the most important factor in determining cost of a circuit. Because the fabrication cost of a circuit is mainly calculated by dividing the cost to process a substrate by the number of circuits that the substrate yields, minimizing the size of the circuit to increase the yield from a substrate offers an easy method to decreasing circuit cost. Keep the substrate sizes in mind when laying out a circuit and don't forget to add .006 or .010 dicing streets in between all circuits when calculating circuit yield on the substrate. When the circuit is close to finalization, we will be happy to review how manufacturable it is and recommend possible changes that might reduce costs.
Separating the individual circuits form the larger substrate is accomplished with a diamond dicing saw. The general rule of thumb is that on substrate materials up to .015 in thickness, we use a .006” wide blade. On thicker substrates, a .010” blade is used. Of course, this can be altered and we have many blades available, but these are good guidelines and the quality of the cut is excellent. When designing a circuit, it is highly recommended that the design leave a minimum of .001” between a circuit element and the saw kerf to avoid the possibility of chipping into the active area of the circuit. When possible, a .003” setback will ensure good circuit yields.
Plated-through holes are laser drilled prior to sputtering, and metalized during the sputtering process to provide reliable front to back electrical connections. On the top surface, plated-through holes should always be surrounded by a .008 minimum annular ring or pad to ensure there is adequate topside metal. The rule of thumb for via size is to maintain a 1:1 ratio of substrate thickness to via diameter. This means simply that in a .010” thick substrate, the preferred via size would be .010”. This is preferred process and allows easy and reliable processing, but when smaller vias are necessary, a 1:.8 ratio can be used. This allows a via .008” to be used in a .010” thick substrate. In general, drilling vias adds 15-20% to the cost of the circuit and adds a week or so to the lead time.
Edge Wrap-arounds offer an alternative to plated vias. It has always been known that a ground wrapped around the edge of a substrate to connect front to back is a low impedance ground, but they were limited in usage to large substrates or placing a few smaller circuits around the outside perimeter of the base substrate. If an edge wrap is good solution for a design, Diablo has developed some very unique and cost effective ways to produce edge-wrapped chip resistors, and these methods work equally well for many circuits. If there is an interest in using this approach, please feel free to discuss it with our engineering staff and we will explain it in detail.
Filled (solid) Vias can be produced with several methods. The highest quality would be W/Cu and they are true hermetic vias that can be used for through-substrate RF I.O. These vias are space qualified and the very best technology, however they are done by outside specialty houses and are typically expensive and add several weeks to a turnaround. More practical solutions include Pd/Ag and specialty conductive epoxies. The Pd/Ag is a well understood and reliable thick film based material that is fired at high temperature into the via prior to sputtering. After firing, the substrate is sometimes polished to leave a smooth surface. Without polishing, it typically forms a 1-3 mil curved recessed at each via due to shrinkage, but provides an excellent thermal and electrical path and a reasonable cost. Many conductive epoxies have been developed recently that hold great promise. They are simply screen printed or otherwise forced into the vias in finished circuits and then cured at low temperatures, typically 150-250 degrees C. They have an upper limit of temperature excursion of about 250 degrees C.
Air Bridges are supported or unsupported interconnects used to connect various circuit elements on the substrate. A typical application might be the interconnecting of fingers on a Lange coupler where wire bonding is both difficult and inconsistent. Unlike wire bonding, air bridges are virtually identical, often resulting in reduced tuning times. In an unsupported air bridge, there is an air gap of about 8-10 microns between the air bridge and the underlying trace. Unsupported bridges offer increased performance, but suffer the weakness of being fragile and subject to damage by assembly personnel, rough handling, etc. An alternative is polyimide or BCB supported interconnects. The Dielectric of the polyimide of BCB is around 3, so there will be some added loss in the supported version, but the upside is the ruggedness of the structure. Supported bridges require little in extra care over a standard circuit, making them quiet popular. An unsupported air bridge requires 2 added masking steps as well as the associated photolithography and etching, whereas a supported system takes 3 layers. The added cost to add air bridges is 30% to 40%, but if there is a sufficient number of interconnects on a circuit that would have been done with individual wire bonds, it can be very cost effective.
Photomask Fabrication and Layout
As thin film circuits become more complex it is important for designers to remember some basic layout guidelines:
Etch factors: If you are going to add etch factors yourself, we prefer to work with a .00002” per side etch factor when doing an etch back process, or a .00001” per side “growth factor” when doing a plate up process. We have the capability to work with other factors, but these are generally accepted standards and have proven to give the best results.
AutoCAD is the most common format used by our customers and is an excellent choice for doing circuit design. Files should be saved as .DXF (preferred) or .DWG. We can use either, or many other common formats. When doing a design, design to the final desired dimensions, give us those dimensions and we will add etch factors for our process.
Don’t forget to tell us which resistor is a test resistor or what the critical resistor(s) are and give us their value. If the circuit is all 10%, you need only add a note and we will figure out the individual resistors. If the majority are 10% and one is critical and should be 5%, identify the critical one and leave the rest at 10%. Specifying all the resistors at 5% when they are not necessary will add cost for no gain in performance.
To keep costs down, review all specifications and be realistic. If you tell us the circuit outside dimension is +/- .0001”, we will build and measure to this tolerance. If you can actually live with +/- .002”, we will have a better yield, and the circuit will cost you less.
The most commonly left out fabrication note on thin film circuits drawings is whether the circuit is metalized on the top side only or on both sides. Please indicate one or two sides on your drawings or sketches.
Conductors:
Resistors:
Metalized Vias:
Filled Vias:
Substrates:
Dicing:
Drawing Data Format: