Consumer electronic devices are undergoing a revolution in form-factor as there is increasing interest and market demand for flexible, foldable/rollable devices, and the display technologies which enable them. These new form factors enable new ways for users to interact with their devices - for example, a phone-sized device may open up to a larger tablet, which allows stylus input. The addition of force-sensing to these devices is incredibly advantageous, as this provides depth-of-touch information which can vastly improve user experience. It also permits more elegant ways for false-touch elimination, simplified or augmented HMI’s, and even back-side sensing on larger displays, which has real value for improving gaming experience.
To enable both touch and location sensing, we have developed a solution where an ultra-thin force sensing array is integrated behind the display. The typical stack up thickness for an OLED display is in the order of ~450-500µm, and force must be effectively transferred through this to the sensor underneath. Effective force transfer was confirmed by extensive mechanical simulations that considered the properties of the various layers and also the utilization of different cover lens material (e.g. polymer versus UTG cover lens). However, even though force transfer is occurring efficiently, the force applied on a specific location will tend to spread over several sensels. At low forces (below 20-30gr), this distributed signal can be hard to effectively process over the background noise using traditional algorithms.
To accomplish effective force transfer we have designed and fabricated a high-resolution matrix sensor consisting of 146x126 sensing elements (sensels) at 1.0 mm pitch. When integrated under a 420um thick OLED, we demonstrate excellent sensitivity and dynamic range from 10-600gr. The raw signal capture was further processed through an AI-based algorithm with a very high efficacy in distinguishing true signals in a high S/N ratio background. Overall, we measured very good linearity and accuracy for both stylus ( < 0.4mm) and finger ( < 0.8mm) touch. Repeatability was ±5% for force < 100gr and uniformity was ±25% across the panel area, all close to or within the targeted specifications.