6.
如图,在棱长为\(2\)的正方体\(ABCD-{A}_{1}{B}_{1}{C}_{1}{D}_{1} \)中,\(E,F,M,N \)分别是棱\(AB,AD,{A}_{1}{B}_{1},{A}_{1}{D}_{1} \)的中点,点\(P,Q \)分别在棱\(D{D}_{1} \),\(B{B}_{1} \)上移动,且\(DP=BQ=\lambda \left( 0 < \lambda < 2 \right)\).
\((1)\)当\(\lambda =1\)时,证明:直线\(B{{C}_{1}}\parallel \) 平面\(EFPQ;\)
\((2)\)是否存在\(\lambda \),使平面\(EFPQ\)与面\(PQMN\)所成的二面角为直二面角?若存在,求出\(\lambda \)的值;若不存在,说明理由.