Chemically Amplified, Positive Tone, Polynorbornene Dielectric for Microelectronics Packaging

Abstract

A low permittivity, positive tone, polynorbornene dielectric has been developed that exhibits excellent lithographic and electrical properties. The polymer resin is a random copolymer of a norbornene hexafluoroalcohol (NBHFA) and a norbornene tert-butyl ester (NBTBE). High optical sensitivity and contrast were achieved using a chemically amplified solubility switching mechanism through the acid-catalyzed deprotection of the tert-butyl ester functionality. After developing in aqueous base, the film was thermally cured through a Fischer esterification reaction, resulting in a cross-linked permanent dielectric. The effect of the photoacid generator (PAG) concentration on the lithographic patterning and curing reactions was studied. Higher PAG loading was favorable for both sensitivity and dielectric constant. The sensitivity of a formulation was measured as low as 8.09 mJ/cm 2 . The molar ratio of the two monomers composing the polymer was varied. A higher NBHFA content was favorable because it resulted in a lower modulus, lower shrinkage, and lower dielectric constant and loss. A formulation with 70 mol% of the NBHFA had a modulus of 2.60 GPa, a 12.2% volume decrease during cure, and a dielectric constant of 2.23. The direction-dependent coefficient of thermal expansion was measured, and it was found that the anisotropy of the PNB films decreased with higher NBTBE content. Permanent dielectric materials are used in microelectronic devices and packages to separate and insulate components and interconnect. Low dielectric constant (low-k) insulators are desirable for on-chip, chip-to-chip, chip-to-package, and on-package wiring to avoid electrical delay and lower the energy consumed in the signal interconnect. [1][2][3] Polymers are widely used for these purposes because they offer a low-stress, easily processed alternative to inorganic dielectrics. [2][3][4][5] In addition, photosensitive dielectrics are desirable because they can be directly patterned by photolithographic means. Lithographically printed dielectrics do not require the use of photoresist or additional pattern transfer steps, which can be costly and expose the device to aggressive wet or dry etch process steps.Positive tone dielectrics are especially attractive because they result in sloped sidewalls that are generally favored for void-free electroplating of copper. Positive tone materials also exhibit better process yield because exposures through dark-field masks are less susceptible to particle defects. The ability to develop the latent image in an aqueous base developer mitigates the need for environmentally unfriendly organic solvent developers. High sensitivity is very important since it directly leads to higher throughput and tool utilization since less optical energy is required for the solubility switching reaction. The sensitivity of the photopatternable material is coupled to the quantum efficiency of the photoactive compound. Poor quantum efficiency often requires a high loading of the photoactive compounds to effect the inhibition or solu...

Improved Mechanical Properties of Chemically Amplified, Positive Tone, Polynorbornene Dielectric

“…The sensitivity (D 100 ) of this formulation was measured to be 175 mJ/cm 2 , which is significantly higher than the same PNB dielectric without plasticizer at 248 nm (8.09 mJ/cm 2 ). 6 This can be attributed to two factors. First, 248 nm radiation directly activates the PAG, whereas exposure at 365 nm requires an inefficient energy transfer between the CPTX and PAG.…”

“…The 70/30 polymer produced the lowest modulus and dielectric constant of the copolymers investigated in a previous work. 6 Mechanical properties of the TMPEO-loaded films were measured by nanoindentation. As seen in Figure 5, the elastic modulus of the film decreased from 2.47 to 1.80 GPa and the hardness decreased from 0.20 to 0.14 GPa with the addition of 20 pphr TMPEO.…”

“…6 This dielectric was composed of a copolymer of hexafluoroisopropanol norbornene (HFANB) and tert-butyl ester norbornene (TBENB). The chemical structure of this polymer is shown in Figure 1.…”

“…Previously, a chemically amplified, positive tone, permanent polynorbornene (PNB) dielectric was reported. 6 This dielectric was composed of a copolymer of hexafluoroisopropanol norbornene (HFANB) and tert-butyl ester norbornene (TBENB). The chemical structure of this polymer is shown in Figure 1.…”

Thermodynamic Control in the Catalytic Insertion Polymerization of Norbornenes as Rationale for the Lack of Reactivity of Endo-Substituted Norbornenes

“…The catalytic polymerization of olefins has proved to be a formidable tool for the efficient, cost-effective, and sustainable conception of organic materials with an unsurpassed degree of control on microstructure. − Recently, significant progress has been made to expand the scope of polymerization catalysts to functional monomers. − Among all polyolefins, cyclic olefin polymers such as insertion polynorbornenes (PNBEs) offer outstanding thermomechanical properties which resemble those of costly ultrahigh-performance polymers. − These polymers are utilized as 157 nm photoresists, − CO 2 separation membranes, or ultrahigh- T g thermosets . These desirable properties originate from the chemically inert saturated backbone and from the rigidity of the chain imposed by the restricted rotation between two neighboring monomers .…”