Mechanical Characterization and Constitutive Description of Thin-Film Polymeric Materials

Abstract

Mechanical properties of thin-film polymers often dictate the mechanical integrity and performance of microelectronic assemblies. For example, excessive interfacial stress between the polymer and signal via induced by thermal mismatch will lead to delamination of the interface or via cracking. This type of problem becomes more aggravated as dimensions of assemblies continue to shrink. In order to define the optimal design variables and process windows, the mechanical properties of polymer thin films throughout the complete process cycle must be carefully characterized. This paper will first review the experimental techniques commonly adopted in the mechanical measurements of thin-film polymers. Then, it will focus on the most difficult part of the mechanical characterization which is along the out-of-plane (Z) direction. Experimental set-up using the capacitance gauge will be introduced. Critical mechanical properties of polymers will be discussed. For the purpose of illustration, two polymers are chosen as examples. A BPDA-PDA polyimide is used to demonstrate mechanical characterization before melting whereas a PTFE-SiO2 composite is chosen to demonstrate the rheological characterization after melting. Before melting, mechanical behaviors such as the stress-strain curve, creep, relaxation, coefficient of thermal expansion, strain-rate dependence, and temperature dependence are discussed. After melting, the rheological behavior and the effect of aspect ratio measured by a squeezing flow experiment are addressed. Constitutive descriptions are proposed. The functional dependency between stress (pressure), strain (displacement), time, and temperature are characterized. A comparison between the numerical simulations and the experimental measurements is also presented.

Dual capacitor technique for measurement of through-plane modulus of thin polymer films

“…When compared to the through-plane modulus of BPDA/PDA reported in literature, the estimated modulus value of 2.45 GPa was found to be greater than the 0.05-1.1 GPa reported from the air-capacitance gauge measurements 12,13 but less than the 6.1 GPa estimated using the hydrostatic strain measurement technique. 30 Unlike those in this report, all reported literature measurements were made on free-standing films.…”

“…For instance, the in-plane modulus E x has been reported to range from 8.3 GPa to 13 Gpa. 12,15,30,[47][48][49][50] …”

In situ measurement of the thermal expansion behavior of benzocyclobutene films

“…Several techniques have been applied to measure the through-plane thermal expansion of thin polymer films. 2,3,9,10,[13][14][15][16][17][18] Considering the need for in situ measurements, three techniques are available: Fabry-Perot interferometry, two-beam laser interferometry, 3 and the parallel plate capacitor technique of Pasztor et al 18 The accuracy of Fabry-Perot interferometry is questionable. Tong et al reports that the through-plane CTE of PMDA-ODA polyimide films obtained by air-gap capacitance change and thermal mechanical analysis (TMA) are approximately 85 ppm/°C.…”

“…6 This result is similar to that obtained by Wu and Questad for 75 m free standing films using the air gap capacitor technique. 17 However, the presence of a metal overcoat over the polymer film in the twobeam interferometric technique changes the stress state of the polymer. This metal overcoat may be insignificant for a stiff polyimide such as BPDA-PPD, but may cause pronounced effects in less rigid materials.…”