  Nonvolatile photonic integrated circuits employing phase change materials
have relied either on optical switching mechanisms with precise multi-level
control but poor scalability or electrical switching with seamless integration
and scalability but mostly limited to a binary response. Recent works have
demonstrated electrical multi-level switching; however, they relied on the
stochastic nucleation process to achieve partial crystallization with low
demonstrated repeatability and cyclability. Here, we re-engineer
waveguide-integrated microheaters to achieve precise spatial control of the
temperature profile (i.e., hotspot) and, thus, switch deterministic areas of an
embedded phase change material cell. We experimentally demonstrate this concept
using a variety of foundry-processed doped-silicon microheaters on a
silicon-on-insulator platform to trigger multi-step amorphization and
reversible switching of Sb$_{2}$Se$_{3}$ and Ge$_{2}$Sb$_{2}$Se$_{4}$Te alloys.
We further characterize the response of our microheaters using Transient
Thermoreflectance Imaging. Our approach combines the deterministic control
resulting from a spatially resolved glassy-crystalline distribution with the
scalability of electro-thermal switching devices, thus paving the way to
reliable multi-level switching towards robust reprogrammable phase-change
photonic devices for analog processing and computing.
