If you meant "solid paper" in the literal engineering sense, these peer-reviewed sources are highly cited:
: Industry-standard definitions from MarketMuse regarding what makes content valuable, relevant, and engaging to an audience. On "Solid" Materials (Materials Science)
: A systematic review on quantifying "quality" through subjective and objective perception metrics.
However, if you are looking for high-quality research or resources related to the components of that phrase, you may find these professional and academic papers useful: