Thermal oxide patterning method for compensating coating stress in silicon substrates

Abstract

We introduce a novel method for correcting distortion in thin silicon substrates caused by coating stress. Thin substrates, such as lightweight mirrors for x-ray or optical imaging, and semiconductor wafers or flat panel substrates, are easily distorted by stress in thin film coatings. We report a new method for correcting stress-induced distortion in flat silicon substrates which utilizes a micro-patterned silicon oxide layer on the back side of the substrate. Due to the excellent lithographic precision of the patterning process, we demonstrate stress compensation control to a precision of ~0.2%. The proposed process is simple and inexpensive due to the relatively large pattern features on the photomask. The correction process has been tested on flat silicon wafers that were distorted by 30 nm-thick compressively-stressed coatings of chromium, achieving RMS surface height and slope error reductions of a factor of 68 and 50, respectively.

Compensating film stress in thin silicon substrates using ion implantation

“…This process caused an average of 34 nm RMS height change of the wafers, dominated by astigmatism ( 2 2Z ) and spherical deformation ( 0 2 Z ). In previous experiments [17], we had found that additional oxidation and stripping cycles result in deformation below our metrology noise floor, leading us to stop after one cycle in this work.…”

“…We demonstrated reduction of coating stress deformation using ion implantation by a factor of over 20. While this is currently not quite as effective as the patterned oxide method [17], there are a few advantages of ion implantation. The simplicity of the process may be beneficial for X-ray telescopes like Lynx, where many mirrors must be produced.…”

“…The coated wafers were annealed at 200 °C for 2 hours in a tube furnace with nitrogen flow, to stabilize the coatings [17]. This also reduces the stress in the coatings, which others have used to reduce iridium film stress without affecting reflectivity [7].…”

“…In addition to ion implantation, there are several non-uniform integrated stress compensation (NISC) methods under development, which are intended to balance a nonuniform film stress. These methods include silicon oxide patterning [17,18], active mirrors [19], differential deposition [20,21], substrate bias during deposition [22], magneto-strictive films [23], and laser micro-stressing [24]. Currently there is no approach that, on its own, has been shown to be sufficient for Lynx, and it will likely be necessary to combine one or more stress mitigation approaches (such as adjusting deposition parameters or balancing front-and back-side film stresses) with one of the NISC methods [13].…”

“…We demonstrate that ion implantation is capable of reducing film stressinduced deformation by a factor of about 20 on flat wafers. We have demonstrated elsewhere that oxide patterning offers better film stress compensation [17], but ion implantation offers a simpler process that may be compatible with front-and back-side film stress balancing approaches, and may enable multiple cycles of correction. Further improvement to the accuracy of ion implantation may also be possible.…”