Flexible Electronics, Volume 1: Mechanical background, materials and manufacturing by Vinod Kumar Khanna

Author:Vinod Kumar Khanna
Language: eng
Format: epub
ISBN: 9780750314626
Publisher: IOP Publishing
Published: 2019-07-14T16:00:00+00:00

8.5 Controlling internal compressive strain in a film

For compressive internal strain, equation (8.9) is written as

(8.12)

Substituting for εInternal,SiNx from equation (8.4) into equation (8.12) we get

(8.13)

Thus will increase if εg,SiNx is raised, i.e. more strain is generated in SiNx film during/after fabrication. The key to controlling internal compressive strain lies in adjusting the strain in SiNx film during its deposition.

Chen et al (2006) studied the SiOx/SiNx stacks as the barrier structure in which both the films are deposited by plasma-enhanced chemical vapor deposition at 80 °C in a parallel plate, capacitive-coupled reactor on a polycarbonate substrate. SiNx film is more resistive to moisture than SiOx film. Furthermore, both SiNx and SiOx films show high impermeability to oxygen. Therefore, their combination acts as a sturdy barrier to both water vapor and oxygen.

Compressive internal stress (and hence strain) can be introduced in a thin film with the help of temperature gradient and growth disorder. After the deposition temperature is fixed at a selected value, control of the RF power is the key parameter used to produce variations in stress as per requirement. By increasing the RF power, both the SiOx and SiNx films become denser and reach near stoichiometry.

There are two aspects to internal stress control, one concerning a single film deposition and another for deposition of a stack of films. Let us consider individual films first. For 70 nm thick SiOx film deposition on Si substrate, the compressive internal stress increases from −40 MPa to −110 MPa as the RF power is increased from 30 to 120 W. During deposition of 50 nm thick SiNx on Si, the compressive internal stress increases from −40 MPa to −65 MPa with RF power increase from 30 to 180 W (Chen et al 2006).

Now turning our attention to multilayer stacks, the higher internal stress may lead to failure of the coatings. The probability of film cracking rises with increasing stack number. Therefore, a trade-off must be made between the contradicting requirements of a high internal stress to face bending challenges during operation of the device and low internal stress to avoid cracking during the multilayer deposition step of device fabrication. Consequently, an optimized route is followed in which the RF power control is employed for keeping the compressive internal stress of the film high and decreasing it stack by stack in a gradual fashion. Thus, stress-induced cracks during multilayer deposition are prevented, low moisture permeability is attained (for achieving reliable barrier function) and the maintenance of high internal stress is also not compromised (for the desired bending capability).

From the above considerations, the multilayer stack process is designed by steadily decreasing the compressive internal stress of each film for preventing stress-induced cracking. In order to ensure a slow decrease of internal stress in the barrier, the RF power for deposition of SiOx film is taken successively as 80 W, 50 W and 30 W. RF power of 150 W, 125 W and 100 W is taken for SiNx film. By this slow decrease of stress, the rate of release of strain energy does not surpass the fracture energy of the film.

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