Masayuki Ohe

Variable Frequency Microwave and Convection Furnace Curing of Polybenzoxazole Buffer Layer for GaAs HBT Technology

Photosensitive polybenzoxazole (PBO) film has been used in GaAs heterojunction bipolar transistor (HBT) technology for stress buffer and mechanical protection layer applications. However, this film needs to be cured at high temperatures for a long period of time in order to obtain its desired excellent material characteristics. High-temperature curing can result in degradation to the electrical characteristics and performance of the underlying GaAs devices due to limited thermal budget. In this paper, we have characterized the effects of curing the PBO film on GaAs HBT wafers using a conventional convection furnace and using a variable frequency microwave (VFM) furnace. The results show that a VFM cure can achieve similar excellent physical, mechanical, thermal, and chemical material characteristics at a lower curing temperature and in a much shorter time, as compared to convection furnace curing, therefore resulting in minimal GaAs device degradation. Based on these results, an optimum curing condition using the VFM method can be obtained that satisfies both stress buffer layer material and device requirements for GaAs HBT technology.

Positive-tone photodefinable polyimide for low temperature cure

Photodefinable Polyimides (PI) and polybenz-oxazoles (PBO) which have been widely used for various electronic applications such as buffer coating, interlayer dielectric and protection layer usually need high temperature cure condition over 300 °C to complete the cyclization and achieve good film properties. In addition, PI and PBO are also utilized recently for re-distribution layer of wafer level package. In this application, lower temperature curability is strongly required in order to prevent the thermal damage of the semi-conductor device and the other packaging material. Then, to meet this requirement, we focused on pre-cyclized polyimide with phenolic hydroxyl groups since this polymer showed the good solubility to aqueous TMAH and there was no need to apply high temperature cure condition. As a result of our study, the positive-tone photodefinable material could be obtained by using DNQ and combination of epoxy cross-linker enabled to enhance the chemical and PCT resistance of the cured film made even at 170 °C. Furthermore, the adhesion to copper was improved probably due to secondary hydroxyl groups which were generated from reacted epoxide groups. In this report, we introduce our concept of novel photodefinable positive-tone polyimide for low temperature cure.

Novel 200 degC curable positive tone poly(benzoxazole) materials

Photo-definable polyimides and polybenzoxazoles (PBO) have been widely used as dielectrics for re-distribution layers in wafer level chip size packages. These materials can simplify the manufacturing process and ensure high reliability owing to their good mechanical properties and high thermal stability. For next generation electronic components fabricated by utilizing advanced packaging technologies such as 3D-stacking using TSV, package-on-package, fan-out WL-CSP etc., the most important requirements for dielectric materials are high lithographic performance, high adhesion to Cu RDL and high chemical resistance. The improvement in lithographic performance of conventional PBOs by re-designing the photo-definable components has already been published. In this paper, the enhancement of Cu adhesion and chemical resistance obtained by re-designing key components of the formulation that include the photo-initiator and cross-linker will be reported. As a result, strong Cu adhesion and high chemical resistance was obtained when cured at temperatures <; 200 °C.

Evaluation of fan-out wafer level package using 200 °C curable positive-tone photodefinable polybenzoxazoles

Polyimides (PI) and polybenzoxazoles (PBO) have good heat resistance, mechanical properties and electrical insulation. Therefore, they have been widely used as buffer coating and interlayer dielectrics with protection property for electric applications. Recently, fan-out wafer level package (FOWLP) has been attracting attention, because it has advantages of multi-pin, thinner and interposer-free. In this application, it is preferable that the curing temperature of PI and PBO is less than 300 °C for reducing the thermal damage of semiconductor devices and packaging materials. To meet the requirement of lower curing temperature, HD MicroSystems has developed new PBO named HD-8940 designed to satisfy 200 °C curing process. In this study, we evaluated reliabilities of HD-8940 in FOWLP, assembling two types of test element grope (TEG). We also simulated thermal stress during reliability test by finite element method. Finally, we confirmed the applicability of HD-8940 to FOWLP.

Recent progress in low temperature curable photosensitive dielectrics

Hitachi Chemical DuPont MicroSystems (HDMS) has been developing photosensitive negative-tone polyimides (PI) and positive-tone poly(benzoxazole)s (PBO) for use as protection layers in semiconductor ICs or as dielectrics for re-distribution layers in wafer level packaging. These materials offer an easy manufacturing process as well as ensuring high reliability in semiconductor packages such as fan-out wafer level packaging (FOWLP). Currently, dielectrics for FOWLP applications are cured at around 230°C. However, even lower cure temperatures are now required due to the limitations in heat resistance of packaging materials, a reduction in warpage of re-constituted wafers as well as yield improvement with embedded memory ICs. To meet these challenges, HDMS has been focusing on the development of a low temperature curable PI and PBO that can also achieve high resolution and excellent cured film properties. In this paper, recent progress in our latest PI and PBO materials will be introduced.

Reliability of 200 °C curable photodefinable PBO for re-distribution layer in WLP

The reliability enhancement of polybenzoxazoles on Cu for Fan-out wafer level package was studied using TEG1 and 2 with simple re-distribution layer in Fan-out wafer level package. In TEG1 with patterned polybenzoxazole layer on Cu layer, the reliability of polybenzoxazoles around edge of the pattern was improved due to the decrease of the thermal stress in polybenzoxazole by the thinner layer. In addition, this reliability was improved due to the increase of the adhesion strength of polybenzoxazoles by Cu2O formation on the surface of Cu layer or the change in polybenzoxazoles. In TEG2 with patterned Cu layer and polybenzoxazole cover and buffer layer, the reliability of polybenzoxazoles around the edge of Cu pattern was improved due to the decrease of the thermal stress in polybenzoxazoles by the decrease of the area ratio of Cu pattern. Moreover, this reliability was improved by the increase of the adhesion strength of polybenzoxazoles due to the Cu2O formation or the change of polybenzoxazoles. It was found that re-distribution layer structure design for the decrease of stress and the increase of the adhesion strength of polybenzoxazoles were keys for the reliability enhancement of polybenzoxazoles.

Improvement in dissolution contrast of positive-tone photo-definable poly(benzoxazole) materials

Positive-tone photo-definable poly(benzoxazole) (PBO) has been widely used as dielectric for re-distribution layer of wafer level chip size package. This material can simplify the manufacturing process and ensure the reliability owing to its good mechanical properties and high thermal stability. However, lithographic performance of conventional photo-definable PBO is not enough mainly due to low dissolution contrast, and the pattern formation on the substrate with deep gap structure is often difficult. Then, in order to improve lithographic performance by enhancing dissolution contrast, we re-designed photo-definable component. As a result, dissolution contrast could be doubled in comparison to conventional PBO by applying contrast activator or more effective diazonaphthoquinone (DNQ) and we could obtain lithographic pattern on the substrate with deep gap structure without residue at the bottom of the gap. In addition, modified PBO also showed good cured film properties even when cured at low temperature below 250 °C.

Development of low CHE, negative-tone, photo-definable polyimide for HDD suspension

A negative-tone photo-definable polyimide with a low CHE has been developed for coating suspension heads in HDD applications. In order to select the low CHE polyimide, the stabilization energy of hydration calculated from the polyimide and water by Gaussian 03 was used as an indicator. A good correlation between the stabilization energy and CHE value of the polyimide was obtained and which resulted in the development of a low CHE, negative-tone, photo-definable polyimide. The developed material has a low CHE of 9 ppm/%RH, a CTE of 17 ppm/°C (matching that of stainless steel) and good lithographic properties.