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  • Writer's pictureQingzhou Cui

Photosensitive Polyimide/Polybenzoxazole Part 2: Material Properties

The reason that photosensitive PI/PBO have become the material of choice for semiconductor advanced process is due to many desired material properties from them. In this blog, I will walk with you for these properties. In addition, due to the fact that these materials are used as micron-sized thin films, sample preparation and material characterization have proven to be a bit challenging. Hopefully through the discussion here, we can gain some insights on how to carry out material characterization.

High glass transition temperature (Tg) is often translated to high material decomposition temperature, great material dimension stability at high temperatures, and even small material weight loss over high temperatures for many cases. For most PI/PBO materials, Tg can be easily tailored to ≥ 300C range through structure engineering and polymer crosslinking. High glass transition temperature also warrants the dielectric material to endure various harsh fabrication processes and application conditions. Moreover, high Tg usually corelates to low material creep and high reliability over the lifetime of devices made from the materials. Material Tg can be measured by many different analytical methods, among which dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and differential scanning calorimetry (DSC) have been broadly accepted. Materials end-users need be aware of that the three techniques rely on different physical principles for Tg measurements and thus the results could vary a quite bit from each other.

Good material mechanical properties are critical for PI/PBO materials to be used in the advanced packaging process. Unlike traditional photoresist materials, photosensitive PI/PBO materials also serve as structural materials in semiconductor devices. Therefore, mechanical properties are usually first investigated at by device manufacturers when they screen materials. Material vendors are obligated to provide reliable mechanical property data. These mechanical properties usually include tensile strength, elongation, and material moduli. It has become known in the field that high tensile strength, low Young’s modulus, and high elongation would help to improve material performance over the course of devices’ lifetime. These improved performances mainly include less cracking, less stress, higher reliability, higher impact resistance, etc. All three material mechanical properties can be obtained by tensile test. Successful tensile tests would require experienced test engineers or technicians because sample setup and result interpretation are very important for a successful measurement.

Low dielectric constant (Dk) and high dielectric strength (high break voltage) are important for the PI/PBO as dielectric materials. Nowadays these dielectrics are almost exclusively used to embed copper tracks, also known as redistribution layer materials. In order to realize insulation between tiny copper tracks with fine space (down to 5/5um or even 2/2um L/S nowadays), excellent dielectric properties are always mandatory. Another property that need be investigated here is copper migration into the dielectric material under voltage bias. Physical appearance and resistivity change during biased highly accelerated stress test (bHAST) are usually used to characterize the property. Physical appearance could include dendrite formation, color change, and film delamination. Achieving real -situation measurement and test structure construction are most critical aspects for the test.

Good adhesion is a key for design engineers to choose dielectric material for advanced packaging applications. Measuring adhesion for micron-sized structures on different substrates turned out to be not easy. Several test methods have been established by different material vendors. In these test methods, repetitive measurements and statistics are applied so that a reliable picture for adhesion may be constructed. However, Standardization and alignment of these tests among companies are still challenging. In compensation to the limitation, more and more end-users chose to use a cross-hatch(cross-cut) test which were burrowed from painting industry. The straightforward test has been adopted for adhesion evaluation and test results shared across material vendors and device manufacturers.

Good chemical resistance helps these PI/PBO materials to go through different fabrication processes. In reality, PI/PBO films may experience many wet processes during fabrication. So the chemical stability data are always collected by material vendors and shared with end-users during material marketing. For different fabrication and application processes, the requirements for chemical stability may vary quite a bit. Providing such data to end-users often help them to make fast decisions to look into the materials further or not in their production lines. Chemical resistance test are usually conducted using square or circular vias as reference structure. As a general rule of thumb, high crosslink extent and high cure temperature would help to improve material chemical resistance. Most PI/PBO resins, which are obtained through poly-condensation reaction, show limitations in strong acid and base solutions but they usually show decent performance in many other solutions used by fabs.

High the material decomposition temperatures and small material weight loss over temperatures are always desired for PI/PBO materials. This is actually a characteristic virtue for most PI/PBO materials. It is a common believe that material would not function as they normally do they lose more than 3% material weight. It is also well-known that material mechanical and electrical properties would change when they absorb certain amount of moisture from surroundings. The detrimental effect could be a killer factor when choosing new materials. All the parameters mentioned in this paragraph can be obtained through thermogravimetric analysis (TGA).

Stress and wafer warpage are another set of material properties that need to be evaluated by device engineers. This is because wafer reconstitution process is used for most advanced packaging technologies. Stress and wafer warpage would lead to misalignments and processing difficulties. Stress and warpage can be easily measured through a testing tool. As we all know stress and warpage are related to material mechanical properties. Meanwhile, mismatched material CTE between film-film and film-substrate is also an important factor to contribute stress/warpage. By carefully choosing material with similar CTE’s, stress/wafer warpage can be minimized to a great extent. Thermomechanical analysis (TMA) is used for CTE tests.

All these material properties that we have discussed this far can somehow translate to material reliability performance in actual devices. Thus the ultimate material test for dielectrics is to evaluate materials through various reliability tests. These reliability tests include thermal cycle, thermal shock, temperature/moisture test, highly accelerated stress test (HAST), biased highly accelerated stress test (bHAST), impact test, etc. These tests are often time-consuming and also require substantial test equipment. Investigation material response to these tests construct the main theme for material characterization. These tests are often carried by close collaboration between material vendors and test engineers from device manufacturers. With increasing demand to mimic actual material application conditions in recent years, board-level testing for PI/PBO materials has been conducted within material vendors so that the new materials can move quickly into production.

Other test worth mentioning here is material creep test, which is often neglected by current material vendors. In my own career for material testing in this field, I have found these test methods including material creep test and time-temperature superposition (TTS) are quite useful and they may provide insights on material reliability performance at high temperatures, low temperatures, and lengthy lifetime prediction. These tests can be obtained from simple, straightforward DMA tests.

In addition to the tests discussed so far, resolution and sensitivity (photospeed) are always crucial for the PI/PBO materials as photosensitive materials in advanced packaging processes. As Si chips get smaller and I/O number continue to increase, the demand for higher resolution from photosensitive PI/PBO becomes obvious. Choosing the right photochemistry for photosensitive PI/PBO materials is always the most important aspect during material formulation. We will discuss these in the next few blogs. Other factors that are often considered for material testing include easy-to-use, material stability/shelf life, etc. which are often deemed as most important factors by end-user.

All-in-all, the development of photosensitive PI/PBO materials become more and more complicated. It is not merely a simple material formulation, which can be simply transferred from material manufacturers to clean room users. In-depth expertise and strict quality control are what I considered the most important things provided by material vendors. Successful commercializing a new material in this field would require a total package of knowledge transition, trouble shooting, and follow-up support from material vendors. Without these, even an excellent material would hardly have any chance to knock the door of material users.

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