weird deformation of ultra thin plate under thermal expansion

Hi, everyone,

I am using the structural mechanical module to simulate the deformation of a ultra thin plate under the temperature change. the plate size is: length 100mm, width 5mm, height 0.1mm, the material properties are: E = 3.0e9, poisson ratio is 0.3, desity is 1000 kg/m^3 , coefficient of thermal expansion is 1.2e-4. I am using linear elastic material model and adding the thermal expansion under this model. the temperature is the only load which changes from 340 to 290 K. The Boundary conditions: the bottom plane is fixed, while all other boundaries are free. the final simulation result shows the upper portion is under the bottom plane, which is weird! It should above the bottom plane. please refer to the attached pictures.

I tested, this weird problem is possibly due to the fact that the ratio of length to height is too large. I change the height to 1mm, the length to 40mm, and the simulation results shows the deformation of the upper portion is above the bottom plane, which is reasonable.

Can anybody give an explanations and any help for simulating the ultra thin plate deformation? Thanks very much!

Wei