STEM PhD programs in the US are not necessarily free for international students, but there are opportunities for international students to receive funding to cover the cost of tuition and living expenses. Some universities in the US offer fully-funded PhD programs, which cover tuition and provide a stipend for living expenses, including housing and healthcare. These programs often require students to work as research or teaching assistants and may include additional requirements, such as maintaining a certain GPA and making satisfactory progress towards completing the degree. However, funding for international students may be more limited than for domestic students, and some universities may require international students to demonstrate that they have the financial means to support themselves for the duration of their program. It is important to research funding opportunities for international students at the universities you are interested in and to apply for funding as early as possible, as many programs have limited funding available. Additionally, international students may be eligible for scholarships or fellowships from external organisations that support research in their field of study.