Chung Jun Young, Chastek Thomas Q, Fasolka Michael J, Ro Hyun Wook, Stafford Christopher M
Polymers Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, USA.
ACS Nano. 2009 Apr 28;3(4):844-52. doi: 10.1021/nn800853y.
Residual stress, a pervasive consequence of solid materials processing, is stress that remains in a material after external forces have been removed. In polymeric materials, residual stress results from processes, such as film formation, that force and then trap polymer chains into nonequilibrium stressed conformations. In solvent-cast films, which are central to a wide range of technologies, residual stress can cause detrimental effects, including microscopic defect formation and macroscopic dimensional changes. Since residual stress is difficult to measure accurately, particularly in nanoscale thin polymer films, it remains a challenge to understand and control. We present here a quantitative method of assessing residual stress in polymer thin films by monitoring the onset of strain-induced wrinkling instabilities. Using this approach, we show that thin (>100 nm) polystyrene films prepared via spin-coating possess residual stresses of approximately 30 MPa, close to the crazing and yield stress. In contrast to conventional stress measurement techniques such as wafer curvature, our technique has the resolution to measure residual stress in films as thin as 25 nm. Furthermore, we measure the dissipation of residual stress through two relaxation mechanisms: thermal annealing and plasticizer addition. In quantifying the amount of residual stress in these films, we find that the residual stress gradually decreases with increasing annealing time and plasticizer amounts. Our robust and simple route to measure residual stress adds a key component to the understanding of polymer thin film behavior and will enable identification of more effective processing routes that mitigate the detrimental effects of residual stress.
残余应力是固体材料加工过程中普遍存在的结果，是外力去除后仍残留在材料中的应力。在聚合物材料中，残余应力源于诸如成膜等过程，这些过程迫使聚合物链进入非平衡应力构象并将其捕获。在广泛应用于多种技术的溶剂浇铸薄膜中，残余应力会导致有害影响，包括微观缺陷形成和宏观尺寸变化。由于残余应力难以精确测量，尤其是在纳米级聚合物薄膜中，因此理解和控制它仍然是一项挑战。我们在此提出一种通过监测应变诱导的皱纹失稳的起始来评估聚合物薄膜中残余应力的定量方法。使用这种方法，我们表明通过旋涂制备的薄（>100 nm）聚苯乙烯薄膜具有约30 MPa的残余应力，接近 crazing 和屈服应力。与诸如晶圆曲率等传统应力测量技术不同，我们的技术具有测量薄至25 nm薄膜中残余应力的分辨率。此外，我们通过两种弛豫机制测量残余应力的消散：热退火和添加增塑剂。在量化这些薄膜中的残余应力量时，我们发现残余应力随着退火时间和增塑剂用量的增加而逐渐降低。我们测量残余应力的稳健且简单的方法为理解聚合物薄膜行为增添了关键要素，并将有助于识别更有效的加工路线，以减轻残余应力的有害影响。
