A semiconductor structure includes a first conductive layer coupled to a transistor. A first dielectric layer is over the first conductive layer. A second conductive layer is within the first dielectric layer, contacting a portion of a top surface of the first conductive layer. The second conductive layer includes a cap portion extending above a top surface of the first dielectric layer. A first dielectric spacer is between the first dielectric layer and the second conductive layer. A phase change material layer is above a top surface of the second conductive layer. A third conductive layer is over the phase change material layer. A second dielectric layer is over the first dielectric layer. A second dielectric spacer is on a sidewall of the cap portion, wherein a thermal conductivity of the second dielectric spacer is less than that of the first dielectric layer or that of the second dielectric layer.