A Megillah used to read from to fulfill one’s obligation must be hand-written in Sefer Torah font and on parchment. It shares most of the same laws as writing a Sefer Torah. Although most members of the congregation do not have kosher Megillot, they nevertheless fulfill their obligation by listening to it being read from someone else from a kosher Megillah using the halachic mechanism of “one who listens is tantamount to having read.” This rule applies to hearing Sefer Torah, Megillah, and the like, as long as the reader has the congregation in mind and the congregation themselves have in mind to fulfill their obligation.
Regarding a Sefer Torah, the Gemara states (Gittin 45b) states that only a man may write a Sefer Torah, not a woman, for our Sages understood the juxtaposition of the verses “And you shall tie them as a sign onto your arm” and “You shall write them” to mean that only one who is obligated to don Tefillin on one’s arm may write a Sefer Torah. However, women, who are not obligated to do so, may not write a Sefer Torah.
With regards to Megillat Esther, however, we must analyze this dictum, for our Sages taught that women are obligated in the Mitzvah of Megillah reading (and all other Purim-related Mitzvot for that matter). Moreover, the entire Purim miracle was brought about through a woman, namely Queen Esther.
Maran Ha’Chida writes in his Sefer Birkei Yosef that a Megillah written by a woman is absolutely valid, for since they are obligated to read/listen to the Megillah, they may likewise write a Megillah. He supports this ruling from the fact that the Rambam omits the law that a Megillah written by a woman is invalid.
Likewise, Hagaon Rabbeinu Yonah Navon zt”l (rabbi of Maran Ha’Chida) writes in his Sefer Get Mekushar that a Megillah written by a woman is valid and he proceeds to support this with several proofs. Many great Acharonim rule likewise.
Hagaon Harav David Oppenheim brings a proof that a Megillah written by a woman is valid, for the verse in the Megillah states, “And Queen Esther wrote,” from which our Sages derived (Megillah 19a) that the Megillah must be written in a scroll with black ink. It would be counter-intuitive that the first Megillah in history which serves as a basis for the laws of writing a Megillah, was invalid.
Thus, halachically speaking, Maran Rabbeinu Ovadia Yosef zt”l rules (in his Chazon Ovadia-Purim, page 235) that a Megillah written by a woman is absolutely kosher. Similarly, the great Rishon Le’Zion, Hagaon Rabbeinu Yitzchak Yosef Shlit”a rules likewise in his Yalkut Yosef-Purim (page 515) for many different reasons.