A screen printing method for applying a polyimide coating for wafer surface protection has shown significant advantages in efficiency and cost over standard spin-etch methods. Only two steps, called print and cure, are needed to screen print wafer surface protection, as opposed to seven or more steps for spin-etch techniques.
The screen printable polyimide EPO-TEK 600 can be used as a passivation coating for environmental and mechanical protection on any type of device. The quality of the surface protection achieved is exceptional, especially during thermal excursions. The screen printing method also produces highly effective alpha particle barriers for memory devices, and it enhances electrical properties for power devices. It is also possible to use screen printing as a more efficient method of applying interlayer dielectric coatings.
Wafer surface protection is needed for almost all semiconductor devices. It protects against alpha emissions from packaging materials, surface leakage of electrical current, moisture, contamination and handling during test-probe assembly, processing damage during wafer fabrication, cracking of die caused by mold compound induced thermal stress, and various other eventualities and conditions that may impinge upon the integrity or functionality of the device.
Comparison of Methods
While screen printing the passivation layer is a totally additive process, the extensively used spin-etch technique is a subtractive technique that needs chemical etching in order to selectively remove the spun-on polyimide from the bond pads. Residual etchants remain on the chip surface, necessitating additional cleaning steps, which can lead to more contamination. Two applications of the polyimide are required to obtain the minimum alpha protection coating of 40 microns. A minimum of seven extra process steps are needed: spin coat, gel polyimide, spin coat, gel polyimide, expose to light mask/source, etch away uncured polyimide, and remove chemical etchant.
The screen printing technique employs a polyimide densely filled with a low CTE (Coefficient of thermal expansion) filler material. This results in a low shrinkage, a flat wafer and low stress coating. By contrast, the unfilled polyimides used for high RPM spin techniques result in high shrinkage during cure, high stress because of CTE mismatch with the silicon wafer, and wafer "bowing."
The screen printing method needs less polyimide solution per wafer, and the material is considerably lower in cost than spin-etch polyimides, resulting in an order-of-magnitude difference in process cost and combined materials. While spin-etch polyimide solutions are normally 85% solvents, the printable polyimide EPO-TEK 600 is 73% solids. In addition, 70% of spin-etch solutions are discarded during the high RPM spin process.
Additionally, the screen printable polyimide allows memory IC protection; spin-etch polyimides do not, since the applied layers are normally too thin.
Passivating polyimides may also be dot dispensed or sprayed onto the wafer surface. As in the screen printing technique, spraying and dispensing polyimides needs only two additional process steps. Dispensing and spraying techniques also have the benefits of selectivity - only good die are coated - and the capability of coating the sides of the die. This alternative also has major drawbacks, including: CTE mismatch between silicon die and polyimide, leading to wire lift or broken wire; planarity difficulties, brought about by the requirement to raise the height of the dome to increase edge coverage, which intensifies wire problems; and shifting rheology of the polyimides, due to high vapor pressure. It is necessary to add solvent during production, which changes percent solids and leads to shrinkage and expansion problems.
Another alternative, the use of alpha particle free mold compounds, requires no additional process steps. However, this method does not protect against die cracking because of high stress induced by the curing of the mold compound. Additionally, the mold compound usually contains "tramp metal" from metal wear on grinding blades during manufacturing, on steel die during preforming and in the encapsulation mold (due to abrasive fillers). The cost of low alpha-mold compounds is usually five to seven times greater than the cost of standard mold compound.
Similarly, low alpha emitting ceramic packages, while producing reliable alpha protection with no extra process steps, needs an added expenditure of $10 to $15 per package when compared to the use of standard ceramic.
Screen Printing: Processing Characteristics
Highly precise and repeatable printing is, of course, a vital requirement for successful implementation of the screen printable wafer coating technique. The polyimide designed for this process EPO-TEK 600 has been cautiously formulated in order to optimize printing in micrometer resolution with minimal post-print flow. High print quality is also promoted by the potential of the material to maintain stable viscosity over time (see "Rheology," under Material Characteristics).
Adequate surface preparation (cleaning) must be carried out to allow successful adhesion of the coating even before the printing process. A wide range of surface preparation techniques have been employed with success, including UV ozone treatment and chemical cleaning, which is followed by the use of a silane coupling agent, and plasma cleaning. The EPO-TEK 600 polyimide does not need the wafer surface to be primed with adhesion promoters.
The choice of the stencil or print screen should be made carefully. The screen emulsion selected must be of a type that is not vulnerable to the solvents present in the polyimide. A long-lasting metal stencil mask could be used instead of a screen, but this technique has its boundaries, since complex wafer surface patterns do not easily lend themselves to application through stencil.
The screen printing equipment itself must be provided with the precision optics and various other capabilities essential for aligning and printing on wafers with micrometer accuracy and exceptional repeatability. This needs sophisticated optics. The choice of squeegee material should also be made carefully after the printer is selected. The squeegee must not be vulnerable to attacks by the solvents in the polyimide, and it must develop sharp print patterns. The precise squeegee shape, pressure and material will differ according to the precise viscosity and thickness of the polyimide application.
The polyimide is printed on the wafer with one pass of the squeegee across the screen, so that the whole wafer is covered, leaving open just the bond pads. A wafer coated with EPO-TEK 600 is displayed in Figure 1, together with an enlargement displaying the open lands. For bond pads that are smaller than 5 mil2, it is essential to coat the whole wafer area, cure the polyimide and then reopen the bond pad lands by using excimer laser ablation techniques.
The printer setup must be checked to make sure that the screen, squeegee and print table are parallel. Table 1 shows two sets of printing parameters in the development of the screen printable polyimide wafer coating.
Table 1. Screen print parameters.
||325 Mesh stainless steel:
70 Micron emulsion
|Squeegee push pressure
|Squeegee push length
||250 Mesh stainless steel:
85 Micron emulsion
|Squeegee push length
Infrared curing of the screen printed polyimide may be used to obtain good results after printing. This is because, if the coating is cured in a convection oven, the solvents and various other volatiles expelled from the material will recondense on the film, leading to sag and loss of resolution. Additionally, some of this residue will condense on the open areas of the print pattern, thus severely compromising wire soldering or bonding.
Moreover, convection curing of the coating will take place from the outside in, causing the cured outer layer of the film to become a barrier to the escape of the volatiles. The result is that it is majorly difficult to achieve a complete cure of the material using standard methods. Experimentation with different IR curing systems has indicated that gradient, broadband IR belt systems offer the best cure to the polyimide in the shortest period of time (10 to 20 minutes).
It is been proved that screen printing passivation increases yield, when compared to both not coating the wafers and other coating methods. Yield in probe and test experiments at one leading fabricator have confirmed a 15% yield improvement for screen printing compared to spin-etch.
Back-lapping operation is a particular area in which the benefits of screen printable polyimide have been proved. Usually, this step decreases yields because of the active side of the wafers moving in the wax as the back side is lapped to the preferred thickness. This movement can result in smeared metalization and nonfunctional or electrically unstable devices. The EPO-TEK 600 polyimide barrier is capable of protecting the active devices from smearing during the back-lapping process.
Figure 2 shows this function of the wafer coating, comparing the average yields attained after back-lapping with and without the polyimide. While the uncoated wafer yielded 53.7% electrically active devices, the yield was 88% for the coated wafer.
Another area where EPO-TEK 600 brings about a positive contribution to yield is related to the issue of pattern or phase shift during the transfer mold process. When the molding compound cures, its shrinkage puts stress on the metalized lines. This can cause them to shift out of alignment and create electrical discontinuity. The low stress polyimide coating is capable of protecting the surface of the device from these stresses, thus protecting against this source of yield loss.
The EPO-TEK 600 polyimide coating is also considered to be effective in decreasing surface current leakage and enhancing the electrical stability of power devices. (See "Electrical Properties".)
Screen Printing: Material Characteristics
The thixotropic index (> 5.0) and high viscosity (> 300,000 cps) of EPO-TEK 600 guarantees high-resolution screen printing with minimum sag and flow of the polyimide into the device’s open wire bond pads during the print and cure stages. In addition, the high thixotropy of the material stops filler separation in response to shear forces experienced during printing and curing.
EPO-TEK 600 low CTE filler is extremely homogeneous, with smaller-sized particles that move easily through extremely fine screen mesh and improve the application of a continuous, uniform passivation film across the whole wafer surface.
The rheological properties of the polyimide remain stable over time, allowing a screen life in excess of 8 hours, which is necessary to achieve successful screen printing. The initial viscosity of the material changed too quickly, achieving consistently satisfactory print quality would prove difficult.
Another vital characteristic of the EPO-TEK 600 wafer coating is the use of fillers and polyimides that contain very low levels of alpha-radiating uranium and thorium and ionic contaminants. The material comprises of <5 ppb (typically, <1 ppb) of uranium and thorium, which is the intensity level considered essential to prevent soft errors in memory cells and allow reliable passivation of logic, memory and MPU devices.
A 45 micron thick cured film of EPO-TEK 600 on a wafer was observed to emit just 0.001 net α/cm2/hour in one test of alpha particle emissions. Data obtained from testing a 4 mil thick coating on a ceramic substrate showed alpha emissions of about 0.025 net α/cm2/hour. (Generally, thicker films are needed for application of EPO-TEK 600 as a passivation material on ceramic, as ceramic substrates emit higher amounts of alpha particles compared to silicon wafers.)
In alpha particle testing performed by a leading fabricator, memory chips coated with EPO-TEK 600 polyimide and enclosed in plastic packages recorded 0.05 strikes per hour for a chip with a 3.0 mil coating and 55 to 72 strikes per hour for chips with a 1.2 mil coating. Alpha strikes above 550 per hour were recorded for uncoated chips.
EPO-TEK 600 contains low levels of ionic contamination as well. This reduces the development of corrosive acids (through the reaction of mobile ions and water vapor) which can damage the aluminum metallization on the surface of the active device. The levels of hydrolyzable ions observed in a refluxed solution of EPO-TEK 600 are indicated in Table 2. Performance and analytical testing has proved that the polyimide has low levels of ionic contaminants and does not cause corrosive failures.
Table 2. Anion-cation Analysis of Reflux Solution of Screen-Printable Polyimide.
EPO-TEK 600 is prepared with nonvolatile, high boiling point solvents in order to allow a long screen life. The result is a screen life of more than one day, and the complete removal of pinholing or skimming over the wafer coating surface.
EPO-TEK 600 shows excellent thermal stability at high temperatures. As shown in Figure 3, post-cure thermogravimetric analysis points out a decomposition temperature above 600 °C, without any outgassing below 500 °C.
In one set of tests, wafers with cured polyimide coatings were exposed to a belt furnace CERDIP seal temperature profile of 20 minutes duration, with 5 to 8 minutes of exposure to the peak temperature of 450oC. No outgassing, degradation of the polyimide coating, or adverse outcomes on the electrical performance of devices were recorded, nor was there any bending of the wafer due to thermal stress.
As shown in Figure 3, the EPO-TEK 600’s glass transition temperature is 250 °C, which is high enough to enable thermocompression wire-bonding at temperatures above 300 °C.
Assembled plastic packages that are die coated with EPO-TEK 600 have been under thermal shock and thermal cycling tests. Temperatures that range from -50 °C to +125 °C caused no harmful effects on the performance or coating of the assembled devices.
Low CTE Filler
Larger IC devices emphasize the requirement for wafer passivation polyimides that exert the lowest possible amount of thermal stress. Since unfilled polyimides shrink volumetrically up to 85% during the curing process, they produce a great amount of stress on the device. Huge wafers coated with unfilled polyimides display considerable "bowing" after cure. The high coefficient of thermal extension of unfilled polyimides may also lead to breaking of devices during post-mold thermal excursions.
In order to prevent this problem, the EPO-TEK 600 is densely filled with a low CTE filler. This leads to a polyimide film that shows low shrinkage during cure and a very low post cure CTE (10x10-6 ppm/°C) that reduces the CTE differential with silicon. This reduces the potential for cracking and wafer stress in response to thermal challenges.
Testing of unfilled and filled polyimide films on plastic sheets has showed a bowing effect on the sheets that are coated with the unfilled polyimide. Wafers protected with EPO-TEK 600 show perfect flatness after cure, without any bowing.
Test results have pointed out that wafer coatings created by the EPO-TEK 600 polyimide are very effective as moisture barriers. Plastic packages that consist of chips screen-printed with EPO-TEK 600 were subjected to PTHB (121 °C, 15 psi steam autoclave, 250 hours) and THB (85 °C, 85% RH, 1000 hours) conditions. No electrical failures occurred from polyimide degradation or delamination, nor were there any corrosive failures. The lack of corrosion under PTHB conditions is due to the excellent adhesion of the coating to the wafer surface, which blocks capillary flow of water and ionic particles along the interface of the coating and the device.
EPO-TEK 600 films of different thicknesses were applied to silicon and glass in another set of experiments. The cured films were immersed in water for up to 90 minutes. As shown in Figure 5, these experiments demonstrated that the polyimide absorbs low levels of water even while immersed for an extended period. They were designed to test the behavior of the wafer coating in order to absorb water vapor from the air during pre-mold processing. The achieved results point out that by eliminating electrical leakage and surface corrosion, EPO-TEK 600 could promote the reliability of active devices.
The screen printable polyimide has showed excellent insulating properties and suitability for protecting power devices. The bulk electrical properties of the material include dielectric constant of 2.4, dielectric strength of 430 V/mil and volume resistivity of 1015 ohm-cm.
In a test of the protective coating value on power devices, EPO-TEK 600 was screen-printed on the surface of bipolar transistors and then the surface leakage current was measured for coated and uncoated wafers. Uncoated devices averaged leakage of 150 picoamps; however, polyimide-coated devices averaged leakage of just 14 picoamps, as shown in Figure 6. This result is attributed to the elimination of moisture from the surface of the EPO-TEK 600.
This information has been sourced, reviewed and adapted from materials provided by Epoxy Technology, Inc.
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