  A zero-temperature magnetic-field-driven superconductor to insulator
transition (SIT) in quasi-two-dimensional superconductors is expected to occur
when the applied magnetic-field crosses a certain critical value. A fundamental
question is whether this transition is due to the localization of Cooper pairs
or due to the destruction of them. Here we address this question by studying
the SIT in amorphous WSi. Transport measurements reveal the localization of
Cooper pairs at a quantum critical field B_c^1 (Bose-insulator), with a product
of the correlation length and dynamical exponents zv~4/3 near the quantum
critical point (QCP). Beyond B_c^1, superconducting fluctuations still persist
at finite temperatures. Above a second critical field B_c^2>B_c^1, the Cooper
pairs are destroyed and the film becomes a Fermi-insulator. The different
phases all merge at a tricritical point at finite temperatures with zv=2/3. Our
results suggest a sequential superconductor to Bose insulator to Fermi
insulator phase transition, which differs from the conventional scenario
involving a single quantum critical point.
