Abstract: Some implementations described herein include systems and techniques for fabricating a stacked die product. The systems and techniques include using a supporting fill mixture that includes a combination of types of composite particulates in a lateral gap region of a stack of semiconductor substrates and along a perimeter region of the stack of semiconductor substrates. One type of composite particulate included in the combination may be a relatively smaller size and include a smooth surface, allowing the composite particulate to ingress deep into the lateral gap region. Properties of the supporting fill mixture including the combination of types of composite particulates may control thermally induced stresses during downstream manufacturing to reduce a likelihood of defects in the supporting fill mixture and/or the stack of semiconductor substrates.