Positive‐tone, aqueous‐developable, polynorbornene dielectric: Lithographic, and dissolution properties

Sutlief

et al. 2014

Self Cite

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...

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Chemically Amplified, Positive Tone, Polynorbornene Dielectric for Microelectronics Packaging

Sutlief

et al. 2014

Self Cite

“…The contrast and sensitivity of the optical exposure were measured by exposing the film through a variable density optical mask (Optoline International Inc.). The contrast (γ) was calculated by plotting the normalized film thickness after an aqueous base develop vs. the logarithmic exposure dose and fitting a line to the points nearest D 100 , Equation 9. D 100 is the lowest dose at full development and D 0 is the maximum dose where no appreciable dissolution occurred.…”

Section: Methodsmentioning

confidence: 99%

“…It is noted that other PNB dielectrics with similar CTEs have been deposited crack-free on silicon wafers at much greater thicknesses. 9,10 The difference between the chemically amplified dielectric and previous PNB dielectrics is the presence of the TBENB. During polymer synthesis, the presence of TBENB slows the polymerization, and limits the obtainable molecular weight to ∼50 kg/mol.…”

mentioning

confidence: 99%

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

Sutlief

et al. 2014

Self Cite

The mechanical properties of an aqueous developed, chemically amplified, polynorbornene-based permanent dielectric have been investigated. The previously reported hexafluoroisopropanol norbornene and tert-butyl ester norbornene copolymer has been modified via two routes to improve the mechanical properties of the polymer and enable thick-film deposition. First, a third monomer, butyl norbornene (ButylNB) was added to the polymer backbone. The inclusion of 24 mol% ButylNB lowered the elastic modulus from 2.64 to 2.35 GPa and raised the dielectric constant from 2.78 to 3.48. The second approach added a low molecular weight, plasticizing additive in the copolymer formulation. Many additives were immiscible with the resin or did not affect the mechanical properties. Trimethyololpropane ethoxylate (TMPEO) was found to be a miscible additive that improved mechanical properties and could participate in crosslinking the final dielectric material. TMPEO interacted with the PAG, lowering its decomposition temperature. An optimal formulation and processing scheme were determined. A formulation with 10 pphr TMPEO was measured to have a dielectric constant of 2.94, an elastic modulus of 1.95 GPa, a sensitivity at 365 nm of 175 mJ/cm 2 , and a contrast of 4. Photo-definable, permanent, low-k dielectrics are widely used in the fabrication of microelectronic devices and packages.1-5 These dielectrics electrically isolate the interconnect and mechanically stabilize the structures for the life of the device. On integrated circuits, organic dielectrics can be used for interlayer isolation and/or the stress buffer layer on top of the device. Stress buffer layers protect the top surface of the chip and can mitigate mechanical failures that arise from a mismatch of the coefficient of thermal expansion between the chip and package during thermal cycling. In microelectronics packages, low-k dielectrics can be used in the buildup layers, electrically separating the electrical conductors. Low permittivity is critical for use in these applications, as it affects device performance, energy loss, and signal integrity.Photo-definability is an attractive property for permanent polymeric dielectrics. The ability to directly pattern the dielectric by photolithographic techniques mitigates the need for a separate photoresist and pattern transfer steps. This can lower the overall fabrication cost and reduce the number of individual process steps.2 For stress buffer and redistribution applications, positive tone photo-definable dielectrics are more desirable than negative tone ones because the lithographic mask is mostly opaque mask and less prone to transfer particle defects. Also, holes and lines in positive tone materials exhibit sloped sidewalls (opening wider at the top) which is beneficial for void-free plating of the copper interconnects. Aqueous developability is another attractive property, as it mitigates the need for environmentally harmful organic solvent developers. Further, the dielectric should exhibit high sensitivity and contrast ...

“…8 The DNQ additive in the PNB film inhibits dissolution by formation of a hydrogen bonded complex. Ultraviolet exposure causes the DNQ to undergo the Wolff rearrangement to form an indene carboxylic acid.…”

mentioning

confidence: 99%

Positive Tone, Polynorbornene Dielectric Crosslinking

Elce

et al. 2014

Self Cite

The processing and properties of a positive-tone, aqueous develop, epoxy crosslinked permanent dielectric based on a polynorbornene (PNB) backbone and bis(diazonaphthoquinone) (DNQ) photosensitive compound were investigated. The developing and cure properties of the films were studied as a function of cure temperature, epoxy crosslinker loading and DNQ loading. Reduced modulus measurements showed that crosslinking of the polymer film occurred via reaction of the polymer with DNQ. The final modulus of the DNQ-crosslinked film was 4.0 GPa. Swelling measurements for a UV exposed film showed material leaching from the film. Residual solvent from swelling measurements was analysed by gel permeation chromatography which showed the indene carboxylic acid form of DNQ leached out of the polymer film. The unexposed film did not exhibit material loss through leaching. When developed, films showed a decline in modulus to 2.6 GPa, likely due to the reaction of DNQ with the aqueous base developer forming nonreactive byproducts that did not contribute to crosslinking. An epoxy crosslinker was added to the formulation which helped crosslink the polymer film by inhibiting uptake of the aqueous base during developing. The epoxy inhibition of the base uptake was confirmed by quartz crystal microbalance, where an increase in epoxy loading led to a decrease in base uptake of the film during developing. Microelectronics packaging faces a continuing challenge to accommodate scaling of electronic components to smaller size and higher performance. A higher density of electronic components requires superior dielectrics, such as in the form of photo-definable dielectrics to insulate the components electrically and mechanically support them. 1 Polynorbornene (PNB) has shown promise for use as a dielectric because of its low dielectric constant and good mechanical properties. 2The photo-definability can be achieved with a negative tone or positive tone chemistry. Negative tone materials become less soluble in a developer when exposed to UV radiation, whereas positive tone materials become more soluble in a developer when exposed to UV irradiation. Negative tone PNB dielectrics have been well studied. [3][4][5][6][7] A positive tone chemistry is desirable for packaging applications, because the film is less sensitive to mask defects or particulates during exposure.Positive tone photo-definability has previously been demonstrated with a bis(diazonaphthoquinone) (DNQ) added to a polynorbornene polymer. 8 The DNQ additive in the PNB film inhibits dissolution by formation of a hydrogen bonded complex. Ultraviolet exposure causes the DNQ to undergo the Wolff rearrangement to form an indene carboxylic acid. Unlike DNQ, the indene carboxylic acid does not extensively hydrogen bond to the PNB, leading to the solubility switch of the PNB in aqueous base.9 DNQ-based photochemistry is compatible with an aqueous developer which is more environmentally friendly than solvent-based developers.A permanent dielectric can be achieved with an epoxy-based cross...