Oneworld airlines offer flights from San Francisco to Zurich as low as $322 round trip for travel Sunday-Tuesday outbound for departures late February to early May and then again from early September into December 2019. This is a carry on only Economy Light fare.
There are many flight options with American Airlines and British Airways. Finnair looks appealing due to a connection in Helsinki rather than Philadelphia or London, however, the 40 minute connection time in Helsinki seems like a high probability of a missed flight. Perhaps an opportunity for a free stopover in Helsinki.
Finnair San Francisco SFO – Helsinki HEL – Zurich ZRH – Helsinki HEL – SFO