Electrostatic simulation methodology for capacitive touch-screen panels

摘要

In recent years, projected capacitance has become by far the most used method of touchscreen sensing in the consumer electronics industry. Touch screen panels (TSPs) consist of varying transparent layers of lens, substrate, adhesive and indium-tin-oxide (ITO) electrodes. ITO has become the material of choice for manufacturing TSPs due to its high conductivity and high transparency. A touch is detected when there is a change in mutual capacitance between transmitting and receiving electrodes embedded the touch-screen. As a core feature in all aspects of modern electronics, there is a constant need to reevaluate and customize existing designs. Utilizing computer simulations allows a designer to predict the behavior of a design without building the physical sensor. Simulations have 3 main uses for touch-screen developers. ud(1) Building and testing new prototype designs, (2) optimization of existing designs and (3) testing the linearity and uniformity of existing designs due to vendor process variation. This thesis asks the questions: “What key metrics characterize a good TSP?” and “How can TSP designs be optimized usingudcomputer simulations?” This thesis contains a literature review of recent simulation approaches and review of the rise of projected capacitance technology in the touch-screen industry. The main focus of this thesis is the electrostatic simulation of touch-screen sensors. The relevant physics of electromagnetism is introduced and the dominant mathematical methods of simulation are reviewed and compared - namely the Finite Element Method (FEM) and the Method of Moments (MOM). Both these methods are used in experimental studies. The operation of a typical sensor and the mechanism of mutual capacitance is explained and accompanied by an equivalent circuit diagram. Important features of sensor design are introduced such as typical patterns, stack-ups, and trace routes. udSimulations produce a capacitance matrix. From this matrix critical parameters which characterize sensor performance are derived such as signal-to-noise ratio (SNR) and change in mutual capacitance (ΔCm). Several experimental studies of contrasting pattern designs and stack-ups are conducted in order to demonstrate optimization of touch-screen designs. Within each simulation, features of the design are paramaterized in order to perform parametric sweeps. These sweeps can include layer thickness, relative permittivity of a layer, sensor pitch and size of a specific geometric feature. In each case, several parameters of the design are varied and the effect on the capacitances are recorded. From these values the critical parameters of the sensor are determined along with the overall performance of the sensor. A design-of-experiments (DOE) methodology is also described in order to demonstrate the optimal simulation for a touch-screen design with an exhaustive number of variable parameters.ududThis thesis also examines some of the implications of limited computational resources and its effect on solution time and convergence. Methods of decreasing the computational load will also be discussed. In summary, this body of work serves as a complete guide in the designing, running and analysis of electromagnetic simulations for modern TSPs.